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1

Electronic structure of double-perovskite transition-metal oxides  

Microsoft Academic Search

Electronic structure has been investigated by reflectivity measurement for ordered-perovskite transition-metal oxides, Sr2MMoO6 (M=Cr, Mn, Fe, and Co). The ground states of the Co and Mn compounds are paramagnetic insulators, making a sharp contrast with the ferrimagnetic metallic Sr2FeMoO6. For all the compounds, we have observed intense optical transition at ~2 eV, which has been ascribed to the charge-transfer excitation

Y. Moritomo; Sh. Xu; A. Machida; T. Akimoto; E. Nishibori; M. Takata; M. Sakata

2000-01-01

2

Transition metal oxide perovskites by photoelectron and x-ray absorption spectroscopy  

Microsoft Academic Search

X-ray and ultraviolet photoelectron spectroscopy as well as x-ray absorption spectroscopy have been employed to investigate\\u000a transition metal oxide perovskites of the general formula ABO3 (A = La or rare-earth ion, B=trivalent transition metal ion). Systematics in the core levels and in the valence bands in\\u000a the series of LaBO3 compounds have been discussed. Lanthanum chemical shifts in the x-ray

W H Madhusudan; Sheelavathi Kollali; P R Sarode; M S Hegde; P Ganguly; C N R Rao

1979-01-01

3

First-principles exploration of ferromagnetic and ferroelectric double-perovskite transition-metal oxides  

Microsoft Academic Search

Possible ferromagnetic and ferroelectric phases are explored for bismuth transition-metal oxides with double-perovskite structure A2BB?O6 on the basis of first-principles calculations within the local spin-density approximation (LSDA) and generalized gradient approximation (GGA). It is found that a lattice instability of the cubic to a non-centrosymmetric phase always happens in the all cases of lead and bismuth perovskite oxides with the

Y. Uratani; T. Shishidou; F. Ishii; T. Oguchi

2006-01-01

4

Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family  

NASA Astrophysics Data System (ADS)

Computational and experimental studies using linear muffin tin orbital methods and UV-visible diffuse reflectance spectroscopy, respectively, were performed to quantitatively probe the relationships between composition, crystal structure and the electronic structure of oxides containing octahedrally coordinated d0 transition metal ions. The ions investigated in this study (Ti4+, Nb5+, Ta5+, Mo6+, and W6+) were examined primarily in perovskite and perovskite-related structures. In these compounds the top of the valence band is primarily oxygen /2p non-bonding in character, while the conduction band arises from the ?* interaction between the transition metal t2g orbitals and oxygen. For isostructural compounds the band gap increases as the effective electronegativity of the transition metal ion decreases. The effective electronegativity decreases in the following order: Mo6+>W6+>Nb5+~Ti4+>Ta5+. The band gap is also sensitive to changes in the conduction band width, which is maximized for structures possessing linear /M-O-/M bonds, such as the cubic perovskite structure. As this bond angle decreases (e.g., via octahedral tilting distortions) the conduction band narrows and the band gap increases. Decreasing the dimensionality from 3-D (e.g., the cubic perovskite structure) to 2-D (e.g., the K2NiF4 structure) does not significantly alter the band gap, whereas completely isolating the MO6 octahedra (e.g., the ordered double perovskite structure) narrows the conduction band width dramatically and leads to a significant increase in the band gap. Inductive effects due to the presence of electropositive ``spectator'' cations (alkali, alkaline earth, and rare-earth cations) tend to be small and can generally be neglected.

Eng, Hank W.; Barnes, Paris W.; Auer, Benjamin M.; Woodward, Patrick M.

2003-10-01

5

Variation of optical gaps in perovskite-type 3d transition-metal oxides  

Microsoft Academic Search

The optical study of trivalent 3d transition-metal-oxide compounds (RMO3) with the perovskitelike structure has revealed the variation of their electronic structure with the 3d element (M) as well as the A-site rare-earth element (R). The crossover of the gap nature from the Mott type to charge-transfer (CT) type with increasing atomic number of M is observed to occur around M=Cr.

T. Arima; Y. Tokura; J. B. Torrance

1993-01-01

6

Raman study of transition-metal oxides with perovskite-like structure  

Microsoft Academic Search

Perovskite-like oxides exhibit a rich variety of properties of fundamental scientific interest and potential application value. The motivation for this work is to contribute to our knowledge of perovskite-like systems and strongly correlated systems in general. The polarized Raman spectra of single crystal and thin film CaCu3Ti4O12, single crystal and thin film CaRuO3, microcrystals of La0.5Ca 0.5MnO3, and ceramic and

Nikolay Iliev Kolev

2003-01-01

7

Doped Mott insulators in (111) bilayers of perovskite transition-metal oxides with a strong spin-orbit coupling.  

PubMed

The electronic properties of Mott insulators realized in (111) bilayers of perovskite transition-metal oxides are studied. The low-energy effective Hamiltonians for such Mott insulators are derived in the presence of a strong spin-orbit coupling. These models are characterized by the antiferromagnetic Heisenberg interaction and the anisotropic interaction whose form depends on the d orbital occupancy. From exact diagonalization analyses on finite clusters, the ground state phase diagrams are derived, including a Kitaev spin liquid phase in a narrow parameter regime for t(2g) systems. Slave-boson mean-field analyses indicate the possibility of novel superconducting states induced by carrier doping into the Mott-insulating parent systems, suggesting the present model systems as unique playgrounds for studying correlation-induced novel phenomena. Possible experimental realizations are also discussed. PMID:23432283

Okamoto, Satoshi

2013-02-06

8

Doped Mott Insulators in (111) Bilayers of Perovskite Transition-Metal Oxides with a Strong Spin-Orbit Coupling  

SciTech Connect

The electronic properties of Mott insulators realized in (111) bilayers of perovskite transition-metal oxides are studied. The low-energy effective Hamiltonians for such Mott insulators are derived in the presence of a strong spin-orbit coupling. These models are characterized by the antiferromagnetic Heisenberg interaction and the anisotropic interaction whose form depends on the $d$ orbital occupancy. From exact diagonalization analyses on finite clusters, the ground state phase diagrams are derived, including a Kitaev spin liquid phase in a narrow parameter regime for $t_{2g}$ systems. Slave-boson mean-field analyses indicate the possibility of novel superconducting states induced by carrier doping into the Mott-insulating parent systems, suggesting the present model systems as unique playgrounds for studying correlation-induced novel phenomena. Possible experimental realizations are also discussed.

Okamoto, Satoshi [ORNL

2013-01-01

9

Raman study of transition-metal oxides with perovskite-like structure  

NASA Astrophysics Data System (ADS)

Perovskite-like oxides exhibit a rich variety of properties of fundamental scientific interest and potential application value. The motivation for this work is to contribute to our knowledge of perovskite-like systems and strongly correlated systems in general. The polarized Raman spectra of single crystal and thin film CaCu3Ti4O12, single crystal and thin film CaRuO3, microcrystals of La0.5Ca 0.5MnO3, and ceramic and thin film CaMnO3 have been investigated. In close comparison to results from lattice dynamics calculations most of the Raman lines in the CaCu3Ti4O12, CaRuO3, La0.5Ca0.5MnO3 and CaMnO 3 spectra, have been assigned to definite phonon modes. The validity of the model for twin orientation in the Pnma structure for CaRuO3 and La0.5Ca0.5MnO3 is confirmed. The analysis of the CaMnO3 spectra contributed to the development of a model, based on four basic distortions of the (distorted) perovskite structure. The temperature behavior of the CaCu3Ti4O 12 spectra shows that there is no evidence for structural phase transition in the temperature range 20--600 K, so such a transition cannot be responsible for the sharp drop in the dielectric constant below 100 K. The Raman spectra indirectly support the mechanism of formation of barrier layer capacitances in CaCu3Ti4O12.The observation of additional Raman mode of nominal Ag symmetry is discussed in terms of coexistence of domains of different atomic arrangement, or alternatively of non-stoichiometry (Cu deficiency). In the case of the thin film, the tetragonal distortions could be responsible for the greater separation of the additional Ag line. No anomalies in the temperature behavior of the Raman lines of CaRuO3 is observed, which is an indirect evidence for its lack of long-range magnetic ordering at low temperatures (depending on whether this ordering would be observable by Raman spectroscopy through spin-phonon coupling). In La0.5Ca0.5MnO 3 the appearance of several Raman lines below TN is analyzed in terms of ordering and freezing of the Jahn-Teller distortions in a superstructure. Polarized Raman spectra confirmed their usefulness in studying thin films and their properties.

Kolev, Nikolay Iliev

10

SYNTHESIS, CRYSTAL STRUCTURE AND MAGNETISM OF PEROVSKITE-BASED TRANSITION METAL OXIDES  

Microsoft Academic Search

A series of layered perovskite-based compounds were synthesized and studied as follows.\\u000aLa5Mo2.76(4)V1.25(4)O16 is a new pillared-perovskite synthesized by solid state chemistry method. It has layers of corner-sharing octahedra separated by dimers of edge-sharing octahedra, and is the first Mo-based pillared-perovskite whose magnetic structure was determined by neutron diffraction.\\u000aCa2FeMnO5 is an oxygen-deficient-perovskite with a brownmillerite-type ordering of oxygen vacancies,

Farshid Ramezanipour

2011-01-01

11

Cathode materials based on perovskite-like transition metal oxides for intermediate temperature solid oxide fuel cells  

NASA Astrophysics Data System (ADS)

The main requirements to cathode materials for intermediate temperature solid oxide fuel cells are considered with regard to the mechanism of oxygen reduction on the cathode. Factors that affect the oxide-ion and electronic conductivity and also the thermal expansion coefficient of compounds with the perovskite structure are discussed. The crystal structure, high-temperature properties and electrochemical characteristics of the most promising cathode materials based on oxygen-deficient complex oxides of cobalt and layered perovskite-like oxides of nickel and copper are analyzed. The bibliography includes 156 references.

Istomin, S. Ya; Antipov, E. V.

2013-07-01

12

High-pressure synthesis of the 4d and 5d transition-metal oxides with the perovskite and the perovskite-related structure and their physical properties  

NASA Astrophysics Data System (ADS)

A Walker-type multianvil high-pressure facility is capable of high-pressure syntheses and measurements beyond 10 GPa and has been utilized in my research to synthesize the 4d Ruthenium and Rhodium and the 5d Iridium oxides with the perovskite-related structures. Under high-pressure and high-temperature conditions, these families of oxides can be enlarged to a great extent so that enables us not only to address the long-standing problem about ferromagnetism in the perovskite ruthenates but also explore new phenomena associated with the structural and electronic properties in the iridates and rhodates. In the perovskite ruthenates ARuO3 (A= Ca, Sr, and Ba), a systematic study of the variations of the ferromagnetic transition temperature Tc and the critical isothermal magnetization as a function of the average A-site cation size and the size variance as well as external high pressures reveals explicitly the crucial role of the local lattice strain and disorder on Tc and the nature of the localized-electron ferromagnetism. However, such a steric effect is dominated by the electronic effect in another perovskite ruthenate PbRuO3, which is a paramagnetic metal down to 1.8 K and undergoes a first-order structural transition to a low-temperature Imma phase at Tt ? 90 K. Bandwidth broadening due to orbital hybridization between Pb-6s and Ru-4d plays an important role in suppressing the ferromagnetism in the Sr1-zPbzRuO3 system. The high-pressure sequence of the 9R-BaIrO3 was explored and three more polytypes, i.e. 5H, 6H and 3C, were identified under 10 GPa. With increasing fraction of the corner- to face-sharing IrO6/2 octahedra, the ground states of BaIrO3 evolve from a ferromagnetic insulator with Tc ? 180 K in the 9R phase to a ferromagnetic metal with Tc ? 50 K in the 5H phase, and finally to an exchange-enhanced paramagnetic metal near a quantum critical point in the 6H phase. In addition to the perovskite SrRhO3, a new 6H polytype was synthesized for the first time under high pressure and a pressure-temperature phase diagram was given for the 6H-perovskite transformation. Restoration of the Curie-Weiss behavior in the high-temperature magnetic susceptibility chi(T) of the perovskite SrRhO3 resolves the puzzle about unusual dependence of chi -1 ? T2 reported earlier and highlights the importance of spin-orbit coupling in the 4d and 5d transition-metal oxides.

Cheng, Jinguang

13

A comparison in activity between transition-metal oxides and transition metals  

NASA Astrophysics Data System (ADS)

Transition-metal oxides are widely used materials in catalysis as substrates and promoters, but also as the active catalyst materials themselves. We compare the reactivity of transition-metal oxides with the one of transition metals. The comparison is exemplified for the ammonia synthesis reaction. First we show that there exist characteristic Brnsted-Evans Polanyi (BEP) relations (linear relations between transition state and dissociation energies) for dissociation of molecules on transition-metal oxides in the rutile and perovskite structure. It is well-known that the (211) metal surface is several orders of magnitude more reactive than the (111) metal surface due to the lower BEP line for the 211 facet. We find that both rutiles and perovskites follow BEP relations that are lower than the one of the 211 facet. Second we utilize the established BEP relations together with calculated adsorption energetics in a micro-kinetic model to obtain a volcano plot for the catalytic activity. We find that oxides have a higher turn over frequency as compared with metals. Hence, oxides intrinsically have a great advantage in terms of catalytic activity which opens up for catalyst design.

Vojvodic, Aleksandra; Studt, Felix; Abild-Pedersen, Frank; Bligaard, Thomas; Nrskov, Jens

2012-02-01

14

(S)TEM analysis of functional transition metal oxides  

Microsoft Academic Search

Perovskite vanadates (AVO3) form an ideal family to study the structure-property relationships in transition metal oxides because their physical properties can easily be tailored by varying the A-site cations. (S)TEM is an ideal tool for this type of study due to its capacity for simultaneous imaging and chemical analysis. Determination of the oxidation state of vanadium in complex oxides have

Miaofang Chi

2008-01-01

15

Transition-Metal Oxides with Metallic Conductivity.  

National Technical Information Service (NTIS)

Two possible mechanisms for metallic conductivity in transition-metal oxides are discussed: band formation on the transition-meta subbary and band formation via pi bonding orbitals. This permits a general classification of metallic oxides. Two electron-tr...

J. B. Goodenough

1964-01-01

16

Thermodynamic stability of perovskites and related compounds in some alkaline earth-transition metal-oxygen systems  

Microsoft Academic Search

The thermodynamic properties of some alkaline earth (A)-transition metal (M) perovskites and K2NiF4 compounds have been collected, analyzed, and utilized to examine their stabilities by constructing the chemical potential diagrams of a log [a(A)\\/a(M)] vs log P(O2) plot. A thermodynamic analysis was performed on the dissociation reaction of K2NiF4 compounds (A2MO4) into perovskites (AMO3) and alkaline earth oxides (AO) using

Harumi Yokokawa; Natsuko Sakai; Tatsuya Kawada; Masayuki Dokiya

1991-01-01

17

Magnetic correlations in doped transition metal oxides.  

National Technical Information Service (NTIS)

The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y(sub 2(minus)x)C...

G. Aeppli W. Bao C. Broholm

1995-01-01

18

Electronic structure and orbital ordering in perovskite-type 3d transition-metal oxides studied by Hartree-Fock band-structure calculations  

Microsoft Academic Search

We have studied transition-metal 3d-oxygen 2p lattice models, where full degeneracy of transition-metal 3d and oxygen 2p orbitals and on-site Coulomb and exchange interactions between 3d electrons are taken into account, by means of a spin- and orbital-unrestricted Hartree-Fock (HF) approximation. The electronic-structure parameters deduced from the cluster-model analyses of the photoemission spectra are used as input. We have applied

T. Mizokawa; A. Fujimori

1996-01-01

19

Topological Crystalline Insulators in Transition Metal Oxides  

NASA Astrophysics Data System (ADS)

Topological crystalline insulators possess electronic states protected by crystal symmetries, rather than time-reversal symmetry. We show that the transition metal oxides with heavy transition metals are able to support nontrivial band topology resulting from mirror symmetry of the lattice. As an example, we consider pyrochlore oxides of the form A2M2O7. As a function of spin-orbit coupling strength, we find two Z2 topological insulator phases can be distinguished from each other by their mirror Chern numbers, indicating a different topological crystalline insulators. We also derive an effective kp Hamiltonian, similar to the model introduced for Pb1-xSnxTe, and discuss the effect of an on-site Hubbard interaction on the topological crystalline insulator phase using slave-rotor mean-field theory, which predicts new classes of topological quantum spin liquids.

Kargarian, Mehdi; Fiete, Gregory A.

2013-04-01

20

Rare earth-first-row transition metal perovskites as catalysts for the autothermal reforming of hydrocarbon fuels to generate hydrogen  

Microsoft Academic Search

Perovskite oxides (ABO3) containing rare earth elements on the A-site and first-row transition metal elements on the B-site were studied as catalysts for autothermal reforming of liquid hydrocarbon fuels to produce hydrogen for fuel cell systems. Experiments were conducted in a fixed bed microreactor at temperatures of 600800C and gas-hourly space velocities (GHSV) ranging from 4600 to 28,000h?1 using 2,2,4-trimethylpentane

Jennifer R. Mawdsley; Theodore R. Krause

2008-01-01

21

Transition metal mixed oxides as combustion catalysts: preparation, characterization and activity mechanisms  

Microsoft Academic Search

A number of transition metal mixed oxides with spinel-type, corundum-type and perovskite-type structures have been prepared and characterized. Attempts have been made to improve their morphological properties and their stability. Some of them have been tested in the catalytic combustion of methane, CO and H2 (perovskites), of propane and of phenantrene. FT-IR experiments allowed to obtain a quite complete picture

G. Busca; M. Daturi; E. Finocchio; V. Lorenzelli; G. Ramis; R. J. Willey

1997-01-01

22

Electric-field-induced resistance switching universally observed in transition-metal-oxide thin films  

Microsoft Academic Search

We show that polarity-dependent, nonvolatile resistance switching by electric field occurs in the thin film of various transition-metal oxides in almost the same manner. This result indicates that, contrary to the general acceptance, perovskite manganite is by no means a special compound for this phenomenon. It is also suggested that the resistance switching is not dominated by a detailed electronic

M. Hamaguchi; K. Aoyama; S. Asanuma; Y. Uesu; T. Katsufuji

2006-01-01

23

Magnetic correlations in doped transition metal oxides  

SciTech Connect

The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y{sub 2{minus}x}Ca{sub x}BaNiO{sub 5}, the two-dimensional cuprate superconductors La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3} O{sub 6+x}, and the classical three-dimensional ``Mott-Hubbard`` system V{sub 2{minus}y}O{sub 3}.

Aeppli, G. [AT and T Bell Labs., Murray Hill, NJ (United States); Bao, W.; Broholm, C. [Johns Hopkins Univ., Baltimore, MD (United States)] [and others

1995-02-15

24

Chemisorption and the electronic structure of transition metal oxides and transition metals bonded to oxide surfaces  

Microsoft Academic Search

Some simple transition metal oxides such as NiO have incomplete atomic subshells and yet are highly insulating. This implies a high degree of electronic localization and also indicates that the chemical bonds are saturated. In this respect these oxides are similar to alkaline earth oxides such as MgO. The analogy extends to their surface properties. MgO and NiO, for example,

A. Barry Kunz

1985-01-01

25

Quantum Criticality in Transition-Metal Oxides  

NASA Astrophysics Data System (ADS)

We report on experiments of the bulk susceptibility ?( T), heat capacity C( T)/ T, resistivity ?( T) and nuclear resonances (NMR and NQR) in order to review evidence of quantum critical behaviour in some metallic transition-metal oxides. In analogy to the conventional 4 f- and 5 f-electron based heavy-fermion compounds, the prerequisites of quantum criticality, i.e. a magnetic phase transition at T=0 accompanied by non-Fermi liquid behaviour, are observed in LiV2O4 which was the first transition-metal oxide showing heavy-fermion properties. Furthermore, we investigate the large group of copper-ruthenates of the composition ACu3Ru x Ti4- x O12 which also demonstrate heavy-fermion and non-Fermi liquid properties. For LaCu3Ru x Ti4- x O12 we establish a magnetic phase diagram where the substitution of titanium by ruthenium cations suppresses long-range magnetic order in favour of a magnetic quantum critical point, which comes along with a metal-to-insulator transition. Our experiments elucidate open questions concerning the nature of the heavy-fermion properties, intermediate valence, and the origin of the metal-to-insulator transition. Specifically, the role of the ruthenium and copper d-electrons is addressed: the metal-to-insulator transition turns out to correlate with the increasing itinerancy of local moments at the Cu site in our doping experiments.

Bttgen, N.; Krug von Nidda, H.-A.; Kraetschmer, W.; Gnther, A.; Widmann, S.; Riegg, S.; Krimmel, A.; Loidl, A.

2010-10-01

26

Thermodynamic stability of perovskites and related compounds in some alkaline earth-transition metal-oxygen systems  

NASA Astrophysics Data System (ADS)

The thermodynamic properties of some alkaline earth (A)-transition metal (M) perovskites and K2NiF4 compounds have been collected, analyzed, and utilized to examine their stabilities by constructing the chemical potential diagrams of a log [a(A)/a(M)] vs log P(O2) plot. A thermodynamic analysis was performed on the dissociation reaction of K2NiF4 compounds (A2MO4) into perovskites (AMO3) and alkaline earth oxides (AO) using empirical correlations between stabilization energy and tolerance factor. It has been found that the softness of calcium ions, which shrink markedly with decreasing coordination number from 12 to 9, makes the calcium K2NiF4 compounds (Ca2MO4) relatively less stable with increasing radius of the transition metal ions, r(M4+). This destabilization related to the coordination-number-dependent radii implies that when compared with the strontium perovskites, the calcium analogous perovskites may have a smaller number of oxygen vacancies, because the formation of oxygen vacancies should be accompanied with a decrease in coordination number of A-site ions.

Yokokawa, Harumi; Sakai, Natsuko; Kawada, Tatsuya; Dokiya, Masayuki

1991-09-01

27

On the behavior of Bronsted-Evans-Polanyi Relations for Transition Metal Oxides  

SciTech Connect

Versatile Broensted-Evans-Polanyi (BEP) relations are found from density functional theory for a wide range of transition metal oxides including rutiles and perovskites. For oxides, the relation depends on the type of oxide, the active site and the dissociating molecule. The slope of the BEP relation is strongly coupled to the adsorbate geometry in the transition state. If it is final state-like the dissociative chemisorption energy can be considered as a descriptor for the dissociation. If it is initial state-like, on the other hand, the dissociative chemisorption energy is not suitable as descriptor for the dissociation. Dissociation of molecules with strong intramolecular bonds belong to the former and molecules with weak intramolecular bonds to the latter group. We show, for the prototype system La-perovskites, that there is a 'cyclic' behavior in the transition state characteristics upon change of the active transition metal of the oxide.

Vojvodic, Aleksandra

2011-08-22

28

(S)TEM analysis of functional transition metal oxides  

NASA Astrophysics Data System (ADS)

Perovskite vanadates (AVO3) form an ideal family to study the structure-property relationships in transition metal oxides because their physical properties can easily be tailored by varying the A-site cations. (S)TEM is an ideal tool for this type of study due to its capacity for simultaneous imaging and chemical analysis. Determination of the oxidation state of vanadium in complex oxides have been carried out by electron energy loss spectroscopy. SrVO3/LaAlO3 is then studied both experimentally and theoretically as a prototype system. Extra electrons have been detected on the interface layer, and further proven to originate mainly from a change in the local bonding configuration of V at the La-O terminated substrate surface. Cr-containing stainless steel deposited with a LaCrO3 thin-film layer is a promising interconnect material of Solid Oxide Fuel Cells (SOFC). Our investigation on its microstructural evolution reveals that the LaCrO 3 thin film plays a role in inhibiting the growth of an oxide layer on the metal surface and thus protects the surface of the stainless steel. Ca-doped LaCoO3 is a promising SOFC cathode material. The domain structures and the oxidation state of Co in Ca-doped LaCoO3, which are directly related to its mechanical properties and electronic conductivity, are investigated by in-situ TEM and EELS. The formation of microcracks is observed during thermal cycles. Ca-doping in LaCoO3 is shown to not only improve the electronic conductivity of the material, but is also likely to strengthen the grain boundaries. The realization of its application in SOFCs depends on depressing the ferroelastisity to reduce strain formation during thermal cycles. The application of the (S)TEM techniques used for studying the perovskite systems are further extended to other compounds containing transition metal elements. The refractory minerals from Comet 81 P/Wild-2 are studied to investigate the formation of the early solar system. A relatively high Ti3+/Ti 4+ ratio in fassaite and the presence of osbornite indicate that the Comet refractory minerals formed in the inner solar nebula and were later transported to the outer solar system where the comet formed. This implies a much more dynamic and perhaps more violent solar nebula than was previously suspected.

Chi, Miaofang

29

Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films  

NASA Astrophysics Data System (ADS)

The electron occupancy of 3d-orbitals determines the properties of transition metal oxides. This can be achieved, for example, through thin-film heterostructure engineering of ABO3 oxides, enabling emerging properties at interfaces. Interestingly, epitaxial strain may break the degeneracy of 3d-eg and t2g orbitals, thus favoring a particular orbital filling with consequences for functional properties. Here we disclose the effects of symmetry breaking at free surfaces of ABO3 perovskite epitaxial films and show that it can be combined with substrate-induced epitaxial strain to tailor at will the electron occupancy of in-plane and out-of-plane surface electronic orbitals. We use X-ray linear dichroism to monitor the relative contributions of surface, strain and atomic terminations to the occupancy of 3z2-r2 and x2-y2 orbitals in La2/3Sr1/3MnO3 films. These findings open the possibility of an active tuning of surface electronic and magnetic properties as well as chemical properties (catalytic reactivity, wettability and so on).

Pesquera, D.; Herranz, G.; Barla, A.; Pellegrin, E.; Bondino, F.; Magnano, E.; Snchez, F.; Fontcuberta, J.

2012-11-01

30

Dielectric response of sputtered transition metal oxides  

NASA Astrophysics Data System (ADS)

We have investigated the dielectric properties of thin layers of five oxides of transition metals (Ta2O5, HfO2, ZrO2, (ZrO2)0.91(Y2O3)0.09, and Sn0.2Zr0.2Ti0.6O2) sputtered from ceramic targets at different pressures. We find that layers deposited at low pressure behave as expected from literature, whereas layers deposited at high pressure all exhibit an anomalous dielectric response similar to that reported for the so-called ``colossal'' dielectric constant materials. The characterization of the thickness, frequency, and temperature dependence of the capacitance, as well as the comparison of film properties before and after annealing show that the anomalous dielectric response is due to quenched-in vacancies that act as dopants and cause the insulating layers to behave as semiconductors. An increase in quenched-in vacancies concentration with sputtering pressure results in a transition from normal to anomalous dielectric response and gradual increase in layer conductivity. In contrast, the refractive index does not depend on sputtering pressure. This observation indicates the possible application of these materials as transparent coatings with a tunable electrical conductivity.

Iosad, N. N.; Ruis, G. J.; Morks, E. V.; Morpurgo, A. F.; van der Pers, N. M.; Alkemade, P. F. A.; Sivel, V. G. M.

2004-06-01

31

Interface engineering of quantum Hall effects in digital transition metal oxide heterostructures  

Microsoft Academic Search

Topological insulators are characterized by a non-trivial band topology driven by the spin-orbit coupling. To fully explore the fundamental science and application of topological insulators, material realization is indispensable. Here we predict, based on tight-binding modelling and first-principles calculations, that bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are potential candidates for two-dimensional topological insulators. The topological

Wenguang Zhu; Ying Ran; Naoto Nagaosa; Di Xiao; Satoshi Okamoto

2011-01-01

32

Transition metal-catalyzed oxidations of bishomoallylic alcohols  

Microsoft Academic Search

In recent years, new transition metal-catalyzed reactions for chemo- and stereoselective oxidations of bishomoallylic alcohols have been developed. The role of transition metals in this catalysis is connected with (i) activation of a primary oxidant (e.g. molecular oxygen, hydrogen peroxide, or tert-butyl hydroperoxidereactivity), (ii) direction of the alkenol oxidation into a specific reaction channel (chemoselectivity), and (iii) control of the

Jens Hartung; Marco Greb

2002-01-01

33

Catalytic Oxidation by Transition Metal Ions in Zeolites.  

National Technical Information Service (NTIS)

The regular cavities of zeolites form a unique environment for carrying out both stoichiometric and catalytic oxidation reactions. Transition metal ions such as molybdenum and cobalt may be introduced into the zeolite, and, as well, the acidity may be var...

J. H. Lunsford

1988-01-01

34

Fingerprints of spin-orbital entanglement in transition metal oxides.  

PubMed

The concept of spin-orbital entanglement on superexchange bonds in transition metal oxides is introduced and explained on several examples. It is shown that spin-orbital entanglement in superexchange models destabilizes the long-range (spin and orbital) order and may lead either to a disordered spin-liquid state or to novel phases at low temperature which arise from strongly frustrated interactions. Such novel ground states cannot be described within the conventionally used mean field theory which separates spin and orbital degrees of freedom. Even in cases where the ground states are disentangled, spin-orbital entanglement occurs in excited states and may become crucial for a correct description of physical properties at finite temperature. As an important example of this behaviour we present spin-orbital entanglement in the RV O(3) perovskites, with R = La,Pr,,Y b,Lu, where the finite temperature properties of these compounds can be understood only using entangled states: (i) the thermal evolution of the optical spectral weights, (ii) the dependence of the transition temperatures for the onset of orbital and magnetic order on the ionic radius in the phase diagram of the RV O(3) perovskites, and (iii) the dimerization observed in the magnon spectra for the C-type antiferromagnetic phase of Y V O(3). Finally, it is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduce topological constraints for the hole propagation and will thus radically modify the transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations. PMID:22776856

Ole?, Andrzej M

2012-07-09

35

Chromite\\/titanate based perovskites for application as anodes in solid oxide fuel cells  

Microsoft Academic Search

Perovskites containing lanthanides, partially substituted by alkaline-earth elements and transition metals like Cr, Ti, Fe or Co show a very broad range of physical properties. Therefore several perovskite materials, based on lanthanum chromite and strontium titanate were synthesised and investigated with regard to their application as anodes in solid oxide fuel cells. The perovskite powders were studied by thermogravimetric and

G. Pudmich; B. A. Boukamp; M. Gonzalez-Cuenca; W. Jungen; W. Zipprich; F. Tietz

2000-01-01

36

Methanol oxidation over nonprecious transition metal oxide catalysts  

SciTech Connect

This paper reports methanol oxidation over nonprecious transition metal oxide catalysts studied in a gradient less external recycle reactor. The catalysts (oxides of Cr, Mn, Fe, Co, Ni, Cu), which were prepared by using the incipient wetness technique, were supported on 1/8 in.{gamma}-Al{sub 2}O{sub 3} tablets. The catalysts were characterized by using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis techniques. All the catalysts exhibited similar activities for methanol conversion, but the Cu catalyst was found to be considerably more selective to CO{sub 2}. The order of the reaction was 1.2 with respect to methanol concentration and appeared to range between 0.5 and 0 with respect to oxygen concentration.

Ozkan, U.S.; Kueller, R.F.; Moctezuma, E. (Ohio State Univ., Columbus, OH (USA). Dept. of Chemical Engineering)

1990-07-01

37

Catalysis by low oxidation state transition metal (carbonyl) clusters  

Microsoft Academic Search

This review seeks to evaluate the role of low oxidation state transition metal clusters (metal carbonyl clusters) in catalysis. The review commences with a discussion of the cluster surface analogy and then proceeds to discuss whether catalysis takes place by intact molecular clusters or mononuclear fragments; both these topic have been fiercely contested in the literature. Next, the application of

Paul J. Dyson

2004-01-01

38

Thiosulfate oxidation: Catalysis of synthetic sphalerite doped with transition metals  

Microsoft Academic Search

The rate of thiosulfate oxidation to tetrathionate by dissolved molecular oxygen was measured in aqueous suspensions of synthetic sphalerites doped with first row transition metals (Mn, Fe, Co, Ni, Cu). Pure sphalerite and Mn-doped sphalerite show no catalytic activity for this reaction. Catalytic activity increases from Fe to Ni, which has maximum activity, with a slight decrease moving to Cu.

Yong Xu; Martin A. A. Schoonen; Daniel R. Strongin

1996-01-01

39

Catalytic oxidations using transition-metal carbonyls  

Microsoft Academic Search

During the past year we have been working primarily on four separate projects, all of which are related to our goal of discovering new science which will allow development of new homogeneous catalysts for selective oxidations of hydrocarbons. These four projects are: the Re(CO) photocatalyzed decomposition of hydrogen peroxide, the oxidative decarbonylation of an iridium carbonyl complex having a phosphine

Roundhill

1981-01-01

40

Valence structure of alkaline and post-transition metal oxides  

Microsoft Academic Search

As a stepping point to predicting band gaps and electronic structure of more complicated materials based on alkaline and post-transition metal oxides, we examine the valence structure of these simple binary oxides using both high resolution X-ray emission spectroscopy and a variety of density functional theory calculations. We confirm that the new modified Becke-Johnson (mBJ) extension of the Perdew-Burke-Ernzerhoff (PBE96)

John A. McLeod; Robert J. Green; Nikolay A. Skorikov; L. D. Finkelstein; Mahmoud Abu-Samak; Ernst Z. Kurmaev; Alexander Moewes

2011-01-01

41

High-temperature and electrochemical oxidation of transition metal silicides  

Microsoft Academic Search

High-temperature and electrochemical oxidation of transition metal silicides, which are widely used in microelectronics as\\u000a ohmic contacts and protective coatings for high-temperature alloys, are discussed in this review. The process of oxide film\\u000a formation during annealing or anodizing is extremely important for both applications of silicides. It is discussed for three\\u000a disilicides: MoSi2, WSi2, and TiSi2. It has been shown

A. D. Chirkin; V. O. Lavrenko; V. M. Talash

2009-01-01

42

Catalytic Oxidations Using Transition-Metal Carbonyls.  

National Technical Information Service (NTIS)

During the past year we have been working primarily on four separate projects, all of which are related to our goal of discovering new science which will allow development of new homogeneous catalysts for selective oxidations of hydrocarbons. These four p...

D. M. Roundhill

1981-01-01

43

Colossal dielectric constants in transition-metal oxides  

NASA Astrophysics Data System (ADS)

Many transition-metal oxides show very large (colossal) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is todays most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2-xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant.

Lunkenheimer, P.; Krohns, S.; Riegg, S.; Ebbinghaus, S. G.; Reller, A.; Loidl, A.

2009-12-01

44

Framework Materials Composed of Transition Metal Oxide Clusters  

Microsoft Academic Search

Transition metal oxide clusters and their derivatives, which are important in such diverse fields as analytical chemistry,\\u000a biochemical and geochemical processes, chemical sensing, catalysis, materials science, and medicine offer an unmatched variety\\u000a of attractive building block units for the design and development of new materials whose properties could possibly be correlated\\u000a with their constituent units at the molecular level. For

M. Ishaque khan

45

NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and other interstitial compounds  

Microsoft Academic Search

Owing to their unique physical and chemical properties, transition metal compounds, especially transition metal oxides, nitrides, carbides and sulfides, have been the subject of many surface science investigations. In this article we will review applications of the near-edge X-ray absorption fine structure (NEXAFS) technique in the investigations of electronic and structural properties of transition metal compounds. This review covers NEXAFS

J. G. Chen

1997-01-01

46

Thiosulfate oxidation: Catalysis of synthetic sphalerite doped with transition metals  

NASA Astrophysics Data System (ADS)

The rate of thiosulfate oxidation to tetrathionate by dissolved molecular oxygen was measured in aqueous suspensions of synthetic sphalerites doped with first row transition metals (Mn, Fe, Co, Ni, Cu). Pure sphalerite and Mn-doped sphalerite show no catalytic activity for this reaction. Catalytic activity increases from Fe to Ni, which has maximum activity, with a slight decrease moving to Cu. The catalytic activity of the doped sphalerites is proportional to the concentration of dopant. This trend in catalytic activity from pure sphalerite to Cu-doped sphalerite can be explained using the band structures of these solids.

Xu, Yong; Schoonen, Martin A. A.; Strongin, Daniel R.

1996-12-01

47

Perovskite-Type Oxides  

NASA Astrophysics Data System (ADS)

Redox properties of high-surface-area LaM1-xCuxO3 (M=Mn or Co) perovskites prepared by the citrate method were studied by H2 TPR and O2 TPD techniques. We have found that the reduction of Mn4+ occurs in all La-Mn-Cu perovskites at temperatures lower than that of Co3+ in La-Co-Cu samples. The presence of copper affects the reduction of both Mn4+ and Co3+, increasing the temperature of reduction. Furthermore, Mn-based perovskites with a low Cu substitution (x< 0.4) release a great amount of O2 at high temperature. Catalytic activity toward methane combustion was investigated under diluted conditions (0.4% CH4 and 10% O2, N2 as balance) in the temperature range 673-1073 K using a fixed bed reactor with a space velocity of 40,000 N cm3 h-1 g-1. All samples catalyze methane oxidation in the temperature range 673-773 K, giving complete methane conversion below 1073 K and producing 100% CO2. No deactivation phenomena were observed. Substitution with copper decreases catalytic activity. The higher activity of Mn-based perovskites compared with that of the corresponding Co samples with the same composition was attributed to the stronger oxidative properties of LaM1-xCux O3. An apparent activation energy of 23 kcal mol-1 was evaluated on the basis of a first-order kinetic equation for all samples.

Lisi, Luciana; Bagnasco, Giovanni; Ciambelli, Paolo; de Rossi, Sergio; Porta, Piero; Russo, Gennaro; Turco, Maria

1999-08-01

48

Interface engineering of quantum Hall effects in digital transition metal oxide heterostructures.  

PubMed

Topological insulators are characterized by a non-trivial band topology driven by the spin-orbit coupling. To fully explore the fundamental science and application of topological insulators, material realization is indispensable. Here we predict, based on tight-binding modelling and first-principles calculations, that bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are potential candidates for two-dimensional topological insulators. The topological band structure of these materials can be fine-tuned by changing dopant ions, substrates and external gate voltages. We predict that LaAuO(3) bilayers have a topologically non-trivial energy gap of about 0.15?eV, which is sufficiently large to realize the quantum spin Hall effect at room temperature. Intriguing phenomena, such as fractional quantum Hall effect, associated with the nearly flat topologically non-trivial bands found in e(g) systems are also discussed. PMID:22186892

Xiao, Di; Zhu, Wenguang; Ran, Ying; Nagaosa, Naoto; Okamoto, Satoshi

2011-12-20

49

X-ray absorption to determine the metal oxidation state of transition metal compounds  

NASA Astrophysics Data System (ADS)

We present three examples where x-ray absorption at the transition metal L2,3 edges is used to investigate the valence states of various strongly correlated (SC) and technological relevant materials. Comparison with ligand field multiplet calculations is needed to determine the metal oxidation states. The examples are CrF2, the La1-xSrxCoO3 family and YVO3. For CrF2 the results indicate a disproportionation reaction that generates Cr+, Cr2+ and Cr3+ in different proportions that can be quantified directly from the x-ray spectra. Additionally, it is shown that Co2+ is present in the catalytic La1-xSrxCoO3 perovskite family. Finally, surface effects that change the vanadium valence are also found in YVO3 nanocrystals.

Jimnez-Mier, J.; Olalde-Velasco, P.; Carabal-Sandoval, G.; Herrera-Prez, G.; Chavira, E.; Yang, W.-L.; Denlinger, J.

2013-07-01

50

Interface engineering of quantum Hall effects in digital transition metal oxide heterostructures  

SciTech Connect

Topological insulators (TIs) are characterized by a non-trivial band topology driven by the spin-orbit coupling. To fully explore the fundamental science and application of TIs, material realization is indispensable. Here we predict, based on tight-binding modeling and first-principles calculations, that bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are potential candidates for two-dimensional TIs. The topological band structure of these materials can be fine-tuned by changing dopant ions, substrates and external gate voltages. We predict that LaAuO$_3$ bilayers have a topologically non-trivial energy gap of about 0.15~eV, which is sufficiently large to realize the quantum spin Hall effect at room temperature. Intriguing phenomena, such as fractional quantum Hall effect, associated with the nearly flat topologically non-trivial bands found in $e_g$ systems are also discussed.

Xiao, Di [ORNL; Zhu, Wenguang [University of Tennessee, Knoxville (UTK); Ran, Ying [Boston College, Chestnut Hill; Nagaosa, Naoto [University of Tokyo, Tokyo, Japan; Okamoto, Satoshi [ORNL

2011-01-01

51

Spin-Orbit Interaction Rediscovered in Transition Metal Oxides  

NASA Astrophysics Data System (ADS)

The 5d-transition metal oxides are a class of novel materials that exhibit nearly every collective state known for solids. It is commonly expected that iridium oxides should be more metallic and less magnetic than their 3d and 4f counterparts due to the extended nature of the 5d orbitals. In marked contrast, many iridates are magnetic insulators that exhibit a large array of phenomena seldom or never seen in other materials. We review the anomalous physical properties of several iridates and address potential underlying mechanisms, which include strong orbital magnetism, the Jeff = .5ex1 -.1em/ -.15em.25ex2 insulating state, and spin-orbit coupling; the latter strongly competes with other interactions to create an unusual balance between relevant degrees of freedom in this class of materials.

Cao, Gang

2011-03-01

52

Resonant Ultrasound Studies of Complex Transition Metal Oxides  

SciTech Connect

Department of Energy EPSCoR The University of Mississippi Award: DE-FG02-04ER46121 Resonant Ultrasound Spectroscopy Studies of Complex Transition Metal Oxides The central thrust of this DOE funded research program has been to apply resonant ultrasound spectroscopy (RUS), an elegant and efficient method for determining the elastic stiffness constants of a crystal, to the complex and poorly understood class of materials known as transition metal oxides (TMOs). Perhaps the most interesting and challenging feature of TMOs is their strongly correlated behavior in which spin, lattice, and charge degrees of freedom are strongly coupled. Elastic constants are a measure of the interatomic potentials in a crystal and are thus sensitive probes into the atomic environment. This sensitivity makes RUS an ideal tool to study the coupling of phase transition order parameters to lattice strains. The most significant result of the project has been the construction of a high temperature RUS apparatus capable of making elastic constant measurements at temperatures as high as 1000 degrees Celsius. We have designed and built novel acoustic transducers which can operate as high as 600 degrees Celsius based on lithium niobate piezoelectric elements. For measurement between 600 to 1000 C, a buffer rod system is used in which the samples under test and transducers are separated by a rod with low acoustic attenuation. The high temperature RUS system has been used to study the charge order (CO) transition in transition metal oxides for which we have discovered a new transition occurring about 35 C below the CO transition. While the CO transition exhibits a linear coupling between the strain and order parameter, this new precursor transition shows a different coupling indicating a fundamentally different mechanism. We have also begun a study, in collaboration with the Jet Propulsion Laboratory, to study novel thermoelectric materials at elevated temperatures. These materials include silicon germanium with various doping and Zintl phase materials. Such materials show promise for increased figures of merit, vital to making thermolectrics competitive with traditional power generation mechanisms.

Dr. Henry Bass; Dr. J. R. Gladden

2008-08-18

53

Sonochemical preparation and characterization of nanoporous transition metal oxides for environmental catalysis  

Microsoft Academic Search

A series of single and multi-component nanoporous transition metal oxides were prepared by sonochemical processing. This is based on a new idea that the controlled condensation and agglomeration of monodispersed sol particles would form a nanoporous structure with a narrow pore size distribution under ultrasound irradiation. Therefore, nanoporous transition metal oxides may be obtained without using surfactant templates. X-ray diffraction

Lizhi Zhang

2003-01-01

54

Transition-metal-oxide coated titanium electrodes for redox batteries  

NASA Astrophysics Data System (ADS)

The production of porous electrodes with reproducible surface activity for experiments to corroborate predictions of porous electrode theory to ascertain if simple one step outer sphere electron transfer mechanism applies to the ferric-ferrous redox reaction at high reactant/product concentrations in a strong acid chloride medium was investigated. Kinetic rate expressions of this couple under these conditions at the metal oxide surfaces were sought. Coatings of the conductive and active rutile structure form of selected transition metal oxides were desired. Both IrO2 and RuO2 coatings having the rutile structure were prepared by thermal decomposition of their respective hydrated chloride salts. Low over potential exchange current densities of the ferric-ferrous couple were measured in concentrated solution at Pt, RuO2 and IrO2 RDF's. Significant corrections were necessary for ohmic effects, mass transfer effects, and nonuniform current distribution. It is shown that the electrocatalytic activity of RuO2 is comparable to that of Pt while the activity of IrO2 is approximately an order of magnitude less than Pt. The measured free energy of activation was similar for all three substrates and a nonbinding interaction between the couple and the surfaces is suggested. The variation in exchange currents among the different substrates can be attributed to some extent on double layer effects but primarily to crystal structure differences.

Savinell, R. F.

1982-12-01

55

Transition metals  

PubMed Central

Transition metals such as Iron (Fe) and Copper (Cu) are essential for plant cell development. At the same time, due their capability to generate hydroxyl radicals they can be potentially toxic to plant metabolism. Recent works on hydroxyl-radical activation of ion transporters suggest that hydroxyl radicals generated by transition metals could play an important role in plant growth and adaptation to imbalanced environments. In this mini-review, the relation between transition metals uptake and utilization and oxidative stress-activated ion transport in plant cells is analyzed, and a new model depicting both apoplastic and cytosolic mode of ROS signaling to plasma membrane transporters is suggested.

Rodrigo-Moreno, Ana; Poschenrieder, Charlotte; Shabala, Sergey

2013-01-01

56

High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: First principles calculations of large anisotropic chemical shielding  

NASA Astrophysics Data System (ADS)

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, ??, in prototypical transition metal oxide ABO3 perovskite crystals. Our principal findings are (1) a large anisotropy of ?? between deshielded ?x~=?y and shielded ?z components (z along the Ti-O bond); (2) a nearly linear variation, across all the systems studied, of the isotropic ?iso and uniaxial ?ax components, as a function of the B-O-B bond asymmetry. We show that the anisotropy and linear variation arise from large paramagnetic contributions to ?x and ?y due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic ?iso and uniaxial ?ax chemical shifts are in good agreement with recent BaTiO3 and SrTiO3 single crystal 17O NMR measurements. In PbTiO3 and PbZrO3, calculated ?iso are also in good agreement with NMR powder spectrum measurements. In PbZrO3, ?iso calculations of the five chemically distinct sites indicate a correction of the experimental assignments. The strong dependence of ?? on covalent O(2p)-B(nd) interactions seen in our calculations indicates that 17O NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to study the local structure in complex perovskite alloys.

Pechkis, Daniel L.; Walter, Eric J.; Krakauer, Henry

2009-11-01

57

Oxidative desulfurization of benzene fraction on transition metal oxides  

NASA Astrophysics Data System (ADS)

It is established that molecular oxygen is able to oxidize thiophene selectively in a mixture with benzene on V2O5 MoO3. The introduction of thiophene inhibits the oxidation of benzene. It is shown that the conversion of thiophene during operation of the catalyst is reduced at first and then increases until it reaches its initial value.

Boikov, E. B.; Vishnetskaya, M. V.

2013-02-01

58

Hubbard U and Hund exchange J in transition metal oxides: Screening versus localization trends from constrained random phase approximation  

NASA Astrophysics Data System (ADS)

In this work, we address the question of calculating the local effective Coulomb interaction matrix in materials with strong electronic Coulomb interactions from first-principles. To this purpose, we implement the constrained random phase approximation into a density functional code within the linearized augmented plane-wave framework. We apply our approach to the 3d and 4d early transition metal oxides SrMO3 (M= V, Cr, Mn) and (M= Nb, Mo, Tc) in their paramagnetic phases. For these systems, we explicitly assess the differences between two physically motivated low-energy Hamiltonians: The first is the three-orbital model comprising the t2g states only, which is often used for early transition metal oxides. The second choice is a model where both metal d and oxygen p states are retained in the construction of Wannier functions, but the Hubbard interactions are applied to the d states only (d-dp Hamiltonian). Interestingly, since (for a given compound) both U and J depend on the choice of the model, so do their trends within a family of these compounds. In the 3d perovskite series SrMO3, the effective Coulomb interactions in the t2g Hamiltonian decrease along the series due to the more efficient screening. The inverse, generally expected, trend, increasing interactions with increasing atomic number, is however recovered within the more localized d-dp Hamiltonian. Similar conclusions are established in the layered 4d perovskites series Sr2MO4 (M= Mo, Tc, Ru, Rh). Compared to their isoelectronic and isostructural 3d analogs, the 4d perovskite oxides SrMO3 (M= Nb, Mo, Tc) exhibit weaker screening effects. Interestingly, this leads to an effectively larger U on 4d than on 3d shells when a t2g model is constructed.

Vaugier, Log; Jiang, Hong; Biermann, Silke

2012-10-01

59

Single crystal particles of a mesoporous mixed transition metal oxide with a wormhole structure.  

PubMed

A new type of mesoporous mixed transition metal oxide of Nb and Ta (NbTa-TIT-1) has been prepared through a two-step calcination, which consists of single crystal particles with wormhole mesoporous structure. PMID:12240191

Lee, B; Lu, D; Kondo, J N; Domen, K

2001-10-21

60

Fabrication, Structure, and Magnetism of Transition Metal and Oxide Nanoclusters  

NASA Astrophysics Data System (ADS)

Nanoclusters display unusual properties due to the high surface-to-volume atom ratios. Our ability to fabricate nanoclusters with various sizes, structures, compositions and morphologies with a cluster-deposition system provides unique ways to investigate several nanoscale phenomena. In this work, we have investigated the structural and magnetic properties of various transition-metal and oxide clusters. One example is bimetallic MnAu nanoclusters. Annealing induces size dependences for the lattice parameters, tetragonal-distortion ratios, composition, as well as the morphologies. The size dependence for lattice parameters agrees well with density-functional-theory calculations. The size-dependent composition and formation dynamics are in good agreement with previous thermodynamics calculations for similar bimetallic nanoclusters. One especially interesting issue is the formation of L10 MnAu-fcc Mn core-shell clusters. After annealing, with core-shell formation the average magnetic moment is significantly enhanced from 0.17 microB/Mn for the L10 MnAu clusters to 2.1 microB/Mn. The origin of this high magnetic moment is discussed in terms of previous theoretical and experimental work on nanoscale ferromagnetic and ferrimagnetic Mn structures. This size dependence results from Ostwald ripening at high temperature and likely exists for all bimetallic nanoparticles with different mobilities for the two constituent elements. Another class of interesting material is dilute magnetic oxides, the room-temperature ferromagnetism of which has been suggested to be defect-related, especially involving oxygen vacancies. For TiO clusters, both experimental and theoretical work is carried to investigate the role of hydroxyl ions for the ferromagnetism. By utilizing the water dissociative adsorption and creation of hydroxyls at oxygen vacancies, we have shown that the magnetic moment increases initially linearly with increasing exposure time in moisture. Reducing the humidity level or destroying oxygen vacancies by oxygen annealing induces an exponential decay for the magnetic moment. Besides reproducing the magnetic moment creation upon hydroxyl adsorption, density-functional-theory calculations also reveal crucial information regarding the contribution of two types of nonequivalent hydroxyls, the long-term stability, and location of the magnetic moment.

Wei, Xiaohui

61

Perovskite catalysts for oxidative coupling  

DOEpatents

Perovskites of the structure A[sub 2]B[sub 2]C[sub 3]O[sub 10] are useful as catalysts for the oxidative coupling of lower alkane to heavier hydrocarbons. A is alkali metal; B is lanthanide or lanthanum, cerium, neodymium, samarium, praseodymium, gadolinium or dysprosium; and C is titanium.

Campbell, K.D.

1991-06-25

62

Perovskite Oxides: Oxygen Electrocatalysis and Bulk Structure.  

National Technical Information Service (NTIS)

Perovskite type oxides were considered for use as oxygen reduction and generation electrocatalysts in alkaline electrolytes. Perovskite stability and electrocatalytic activity are studied along with possible relationships of the latter with the bulk solid...

R. E. Carbonio C. Fierro D. Tryk D. Scherson E. Yeager

1987-01-01

63

Spectroscopic Studies of Diatomic Transition Metal Oxides and Fluorides  

Microsoft Academic Search

Wavelength selected fluorescence excitation spectroscopy (WSFES) techniques and ligand field theory (LFT) calculations have been applied to the following transition metal diatomics: CeO, UO, LaF, YF, ScF, HfO, TiO, and ZrO. All of the rotational spectra recorded for these molecules were at a resolution of 0.03 cm^{-1}, and, with few exceptions, Omega assignments for electronic states were unambiguously determined from

John Edward McCord

1996-01-01

64

Natural media with negative index of refraction: Perspectives of complex transition metal oxides (Review Article)  

Microsoft Academic Search

The capabilities of perovskite-like compounds with the effect of colossal magnetoresistance (CMR) and some other complex oxides to have a negative index of refraction (NIR) are considered. Physical properties of these compounds are also analyzed from the standpoint of designing tunable metamaterials on their base. Of particular interest are temperature and magnetic field driven first-order transformations in oxides with perovskite

E. L. Fertman; A. B. Beznosov

2011-01-01

65

High sensitivity of ^17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding  

Microsoft Academic Search

First principles calculations are used to show that O chemical shielding tensors, sigma, are a sensitive indicator of local structure in transition metal ABO3 perovskites, due to their strong dependence on covalent O(2p)-B(nd) interactions.footnotetextPechkis et al., JCP 131, 184511 (2009); references therein. This indicates that ^17O NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to

Daniel L. Pechkis; Eric J. Walter; Henry Krakauer

2010-01-01

66

Orbital ordering under reduced symmetry in transition metal perovskites: Oxygen vacancy in SrTiO3  

NASA Astrophysics Data System (ADS)

By using a combination of density function theory and model Hamiltonian analysis, we explain the general electronic structure features induced by an oxygen vacancy (OV) in SrTiO3. We show that the most important effect caused by an oxygen vacancy is the direct on-site coupling between the 3d3z2-r2 and 4s,4p orbitals of Ti atoms adjacent to the vacancy caused by lifting of the local cubic symmetry. This would be the case for any transition metal perovskite under symmetry-reduced environments such as an interface, surface, or a defect. We find that the OV-induced localized state is highly one dimensional and is mainly composed of Ti 3d3z2-r2 orbitals along the Ti-OV-Ti axis (defined as the z axis) and Ti 4s,4p orbitals at the OV site. The oxygen vacancy does not lead to Ti t2g-based localized states.

Lin, Chungwei; Mitra, Chandrima; Demkov, Alexander A.

2012-10-01

67

DEVELOPMENT OF TRANSITION METAL OXIDE-ZEOLITE CATALYSTS TO CONTROL CHLORINATED VOC AIR EMISSIONS  

EPA Science Inventory

The paper discusses the development of transition metal oxide (TMO)-zeolite oxidation catalysts to control chlorinated volatile organic compound (CVOC) air emissions. esearch has been initiated to enhance the utility of these catalysts by the development of a sorption-catalyst sy...

68

A new nonhydrolytic single-precursor approach to surfactant-capped nanocrystals of transition metal oxides  

SciTech Connect

The advent of new methods to prepare semiconductor and metal nanocrystals, specifically the injection of molecular precursors into hot organic surfactants, has yielded markedly improved samples with good size control, narrow size distribution, and good crystallinity of individual and dispersable nanocrystals. It is of considerable interest to apply these methods to the synthesis of transition metal oxide nanoparticles, which typically are prepared by methods involving water as solvent or reactant. The authors report the first results of a new nonhydrolytic single-precursor approach to the synthesis of dispersable nanocrystals of transition metal oxides.

Rockenberger, J.; Scher, E.C.; Alivisatos, A.P.

1999-12-15

69

Phase composition of anodic oxide films on transition metals: a thermodynamic approach  

Microsoft Academic Search

The phase composition of anodic oxide films on transition metals is analysed using thermodynamic calculations. The model of an oxide film represents a two-layer system MkOl\\/MmOn, where the outer layer MkOl is the highest oxide, and the inner layer MmOn is the lower oxide of the metal. The changes of the Gibbs energy ?Gr are considered for the reactions of

A. L Pergament; G. B Stefanovich

1998-01-01

70

Nanophase transition metal oxides show large thermodynamically driven shifts in oxidation-reduction equilibria.  

PubMed

Knowing the thermodynamic stability of transition metal oxide nanoparticles is important for understanding and controlling their role in a variety of industrial and environmental systems. Using calorimetric data on surface energies for cobalt, iron, manganese, and nickel oxide systems, we show that surface energy strongly influences their redox equilibria and phase stability. Spinels (M(3)O(4)) commonly have lower surface energies than metals (M), rocksalt oxides (MO), and trivalent oxides (M(2)O(3)) of the same metal; thus, the contraction of the stability field of the divalent oxide and expansion of the spinel field appear to be general phenomena. Using tabulated thermodynamic data for bulk phases to calculate redox phase equilibria at the nanoscale can lead to errors of several orders of magnitude in oxygen fugacity and of 100 to 200 kelvin in temperature. PMID:20929770

Navrotsky, Alexandra; Ma, Chengcheng; Lilova, Kristina; Birkner, Nancy

2010-10-01

71

Restructuring Transition Metal Oxide Nanorods for 100% Selectivity in Reduction of Nitric Oxide with Carbon Monoxide.  

PubMed

Transition metal oxide is one of the main categories of heterogeneous catalysts. They exhibit multiple phases and oxidation states. Typically, they are prepared and/or synthesized in solution or by vapor deposition. Here we report that a controlled reaction, in a gaseous environment, after synthesis can restructure the as-synthesized, transition metal oxide, nanorods into a new catalytic phase. Co3O4 nanorods with a preferentially exposed (110) surface can be restructured into nonstoichiometric CoO1-x nanorods. Structure and surface chemistry during the process were tracked with ambient pressure X-ray photoelectron spectros-copy (AP-XPS) and environmental transition electron mi-croscopy (E-TEM). The restructured nanorods are highly active in reducing NO with CO, with 100% selectivity for the formation of N2 in temperatures of 250oC-520oC. AP-XPS and E-TEM studies revealed the nonstoichiometric CoO1-x nanorods with a rock-salt structure as the active phase re-sponsible for the 100% selectivity. This study suggests a route to generate new oxide catalysts. PMID:23731229

Zhang, Shiran; Shan, Jun-Jun; Zhu, Yuan; Nguyen, Luan; Huang, Weixin; Yoshida, Hideto; Takeda, Seiji; Tao, Franklin Feng

2013-06-01

72

Solubility Behavior and Phase Stability of Transition Metal Oxides in Alkaline Hydrothermal Environments  

SciTech Connect

The solubility behavior of transition metal oxides in high temperature water is interpreted by recognizing three types of chemical reaction equilibria: metal oxide hydration/dehydration, metal oxide dissolution and metal ion hydroxocomplex formation. The equilibria are quantified using thermodynamic concepts and the thermochemical properties of the metal oxides/ions representative of the most common constituents of construction metal alloys, i.e., element shaving atomic numbers between Z = 22 (Ti) and Z = 30 (Zn), are summarized on the basis of metal oxide solubility studies conducted in the laboratory. Particular attention is devoted to the uncharged metal ion hydrocomplex, M{sup Z}(OH){sub Z}(aq), since its thermochemical properties define minimum solubilities of the metal oxide at a given temperature. Experimentally-extracted values of standard partial molal entropy (S{sup 0}) for the transition metal ion neutral hydroxocomplex are shown to be influenced by ligand field stabilization energies and complex symmetry.

S.E. Ziemniak

2000-05-18

73

Types of single particle symmetry breaking in transition metal oxides due to electron correlation  

NASA Astrophysics Data System (ADS)

Very accurate wave functions are calculated for small transition metal oxide molecules. These wave functions are decomposed using reduced density matrices to study the underlying correlation of electrons. The correlation is primarily of left-right type between the transition metals and the oxygen atoms, which is mediated by excitations from the nominal single Slater ground state into antibonding and d-type orbitals. In a localized representation, this correlation manifests itself in a 2-electron hopping term that is off-diagonal. This term is of similar magnitude to the commonly considered Hubbard-type on-site interaction.

Wagner, Lucas K.

2013-03-01

74

Types of single particle symmetry breaking in transition metal oxides due to electron correlation.  

PubMed

Very accurate wave functions are calculated for small transition metal oxide molecules. These wave functions are decomposed using reduced density matrices to study the underlying correlation of electrons. The correlation is primarily of left-right type between the transition metals and the oxygen atoms, which is mediated by excitations from the nominal single Slater ground state into antibonding and d-type orbitals. In a localized representation, this correlation manifests itself in a 2-electron hopping term that is off-diagonal. This term is of similar magnitude to the commonly considered Hubbard-type on-site interaction. PMID:23485276

Wagner, Lucas K

2013-03-01

75

Extended Hubbard Models for Transition Metal Oxides and Halides: Importance of Spin and Charge Fluctuations in Charge Transfer Metals  

Microsoft Academic Search

The atomic parameters used in the extended Hubbard (EHB) models for transition metal oxides were determined so as to reproduce ab initio results and experimental results. The EHB model was investigated in relation to classification of transition metal oxides into the Mott-Hubbard and charge transfer (CT) semiconductors. The model calculations indicate that both spin and charge fluctuations are of importance

Kizashi Yamaguchi; Masayoshi Nakano; Hideo Namimoto; Takayuki Fueno

1988-01-01

76

The correlation of structure and electronic properties near the surface of transition metal oxides  

Microsoft Academic Search

The strong couplings between different degrees of freedom are believed to be responsible for novel and complex phenomena discovered in transition metal oxides (TMOs). The physical complexity is directly responsible for their tunability. Creating surfaces\\/interfaces add an additional ' man-made' twist, approaching the quantum phenomena of correlated materials. The dissertation focused on the structural and electronic properties in proximity of

Chenxi Lu

2005-01-01

77

Activation of flue gas nitrogen oxides by transition metal complexes  

Microsoft Academic Search

Sulfur and nitrogen oxides are major flue gas pollutants released by coal-fired electric power plants. In the atmosphere these oxides are converted to sulfuric and nitric acids, which contribute to the acid rain problem. Most of the nitrogen oxides (90%-95%) present in coal-derived flue gas exist as the relatively inert and water-insoluble nitric oxide (NO), thus presenting a difficult removal

M. E. Miller; D. H. Finseth; H. W. Pennline

1987-01-01

78

Emerging catalytic applications of transition metal oxide nanomaterials under microwave and conventional heating  

NASA Astrophysics Data System (ADS)

Heterogeneous transition metal oxide catalysts have advantages over homogeneous catalysts, such as easy separations and efficient recycling and minimization of metal traces in the products. Transition metal oxide nanomaterials with different properties such as shapes and particle size were synthesized by hydrothermal, solvothermal, solvent-free and by energy efficient microwave heating methods and characterized using X-Ray and microscopic techniques. The synthesized catalysts were tested for tandem reactions to form quinoxalines, oxidations of hydrocarbons to form alcohols, aldehydes and ketones, epoxidation, epoxide ring opening, and N-aryl coupling reactions. The kinetics and energy consumption associated with these reactions were compared for both microwave and conventionally heated reactions. Further, Synchrotron radiation-based time-resolved XRD experiments under a wide variety of temperature and pressure conditions were conducted to study the reactions under working conditions. EXAFS and XANES data collections were performed to determine inter-atomic distances and oxidation states of the catalysts.

Sithambaram, Shanthakumar

79

Reminiscences on the discovery of electrochromic phenomena in transition metal oxides  

Microsoft Academic Search

It has long been known that transition metal oxides of the types WO3, MoO3, TiO2, V2O5, etc., show considerable variation in stoichiometry, and that very often the non-stoichiometric compounds are characterized by strong coloration. This observation led to the fabrication of highly disordered thin films of some of these oxidesones that showed strong coloration upon optical excitation. Back in the

Satyen K Deb

1995-01-01

80

Structure formation upon reactive direct current magnetron sputtering of transition metal oxide films  

SciTech Connect

A comparative study of reactive direct current magnetron sputtering for different transition metal oxides reveals crystalline films at room temperature for group 4 and amorphous films for groups 5 and 6. This observation cannot be explained by the known growth laws and is attributed to the impact of energetic particles, originating from the oxidized target, on the growing film. This scenario is supported by measured target characteristics, the evolution of deposition stress of the films, and the observed backsputtering.

Ngaruiya, J.M.; Kappertz, O.; Mohamed, S.H.; Wuttig, M. [I. Physikalisches Institut der RWTH Aachen, D-52056 Aachen, Germany and Jomo Kenyatta University of Agriculture and Technology, Box 62000 Nairobi (Kenya); I. Physikalisches Institut der RWTH Aachen, D-52056 Aachen (Germany)

2004-08-02

81

Chemical synthesis of transition metal oxide nanotubes in water using an iced lipid nanotube as a template.  

PubMed

An iced lipid nanotube has been employed as a template to perform sol-gel transcription of transition metal alkoxides in the absence of solution catalysts, giving titania, tantalum oxide and vanadium oxide nanotubes in aqueous dispersions. PMID:16136234

Ji, Qingmin; Shimizu, Toshimi

2005-08-05

82

Characterization of nanometer scale wear and oxidation of transition metal dichalcogenide lubricants by atomic force microscopy  

SciTech Connect

Atomic force microscopy has been used to characterize wear and oxidation of transition metal dichalcogenide surfaces. Sequential images recorded on molybdenum disulfide (MoS{sub 2}) and niobium diselenide (NbSe{sub 2}) surfaces show that wear proceeds at defects, and that MoS{sub 2} wears at least five times more slowly than NbSe{sub 2}. Images of thermally treated MoS{sub 2} and NbSe{sub 2} further demonstrate that oxidation creates surface defects on both materials. However, for similar oxidation conditions, NbSe{sub 2} surfaces show extensive degradation, while MoS{sub 2} surfaces only exhibit isolated defects. The implications of these results to understanding the tribological properties of the transition metal dichalcogenides are discussed.

Kim, Y.; Huang, J.; Lieber, C.M. (Division of Applied Sciences, Harvard University, Cambridge, Massachusetts (United States). Department of Chemistry)

1991-12-23

83

Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors.  

PubMed

Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr0.55(Ca1?-?ySry)0.45MnO3 (y = 0.25), SrTiO3, and Ti1?-?xCoxO2 (x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons. PMID:23092248

Mizokawa, Takashi

2012-10-23

84

Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors  

NASA Astrophysics Data System (ADS)

Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr0.55(Ca1 - y Sr y )0.45MnO3 ( y = 0.25), SrTiO3, and Ti1 - x Co x O2 ( x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons.

Mizokawa, Takashi

2012-10-01

85

Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors  

PubMed Central

Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr0.55(Ca1???ySry)0.45MnO3 (y = 0.25), SrTiO3, and Ti1???xCoxO2 (x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons.

2012-01-01

86

Pathways for Carboxylic Acid Decomposition on Transition Metal Oxides  

Microsoft Academic Search

The concept of structure sensitivity is well established for reactions catalyzed by metals as it has been generally demonstrated by the use of supported metal catalysts exhibiting different particle size [l-71. The con-cept of structure sensitivity in catalysis by metal oxides is considerably less well developed than in catalysis by metals, in spite of the growing number of examples of

Sivanandi Rajadurai

1994-01-01

87

Catalytic oxidation reactions of aromatic diamines by transition metal complexes  

Microsoft Academic Search

The catalytic reactions of M(TPP)Cl (M = Fe (1), Mn (2)), Fe(TMP)Cl (3), and Fe(TDCPP)Cl (4) (H2TPP = 5,10,15,20-tetraphenylporphyrin; H2TMP = 5,10,15,20-tetramesitylporphyrin; H2TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin) with 2,4-diaminotoluene (5) and p-phenylenediamine (6) were carried out using t-BuOOH (3 M isooctane solution) as oxidant. With the former substrate, the nitro derivatives 2-amino-4-nitrotoluene (5a) and 2,4-dinitrotoluene (5b) (total yield of 5a + 5b

Francesca Porta; Fabio Colonna; Francesca Arciprete; Stefano Banfi; Fausta Coppa

1996-01-01

88

A critical examination of room temperature ferromagnetism in transition metal-doped oxide semiconductors  

Microsoft Academic Search

The current situation with the transition metal-doped oxide semiconductors, TiO2 and ZnO, regarding room temperature ferromagnetism is critically examined. It is provisionally concluded that the apparent ferromagnetism found in some samples stems from secondary phases. There is an indication that the room temperature ferromagnetism may be achieved by adding additional Cu to Fe-doped ZnO.

Y. H. Jeong; S.-J. Han; J.-H. Park; Y. H. Lee

2004-01-01

89

Synthesis and characterization of reduced transition metal oxides and nanophase metals with hydrazine in aqueous solution  

Microsoft Academic Search

Reduction of the first row (3d) transition metal complexes in aqueous solution using hydrazine as the reducing agent has been investigated systematically. Nanoscale reduced metal oxides such as VO2(B), Cr2O3 were obtained through hydrazine reduction followed by proper thermal treatments. Nanocrystalline ?-Mn2O3 was synthesized directly through aqueous phase reduction at room temperature. Nanoclusters of cobalt, copper and the one-dimensional single

Zhou Gui; Rong Fan; Weiqin Mo; Xianhui Chen; Ling Yang; Yuan Hu

2003-01-01

90

Negative ion photoelectron spectroscopy of alkyl peroxides, alkoxides, and group VIII transition metal oxides  

Microsoft Academic Search

I employed negative ion photoelectron spectroscopy to investigate the structure and energetics of three groups of anions and their corresponding neutrals: alkyl peroxides ROO-, (R = H, D, CH3, CD3, and CH3CH2); alkoxides RO- (R=CH3, CD3, CH3CH2, CD3CD2, (CH3)2CH, and (CH 3)3C); and Group VIII transition metal oxides XO- and OXO- (X = Ni, Pd, Pt). The peroxides and the

Tanya Michelle Ramond

2001-01-01

91

A critical examination of room temperature ferromagnetism in transition metal-doped oxide semiconductors  

NASA Astrophysics Data System (ADS)

The current situation with the transition metal-doped oxide semiconductors, TiO2 and ZnO, regarding room temperature ferromagnetism is critically examined. It is provisionally concluded that the apparent ferromagnetism found in some samples stems from secondary phases. There is an indication that the room temperature ferromagnetism may be achieved by adding additional Cu to Fe-doped ZnO.

Jeong, Y. H.; Han, S.-J.; Park, J.-H.; Lee, Y. H.

2004-05-01

92

Quantum Spin Hall Effect in a Transition Metal Oxide Na2IrO3  

Microsoft Academic Search

We study theoretically the electronic states in a 5d transition metal oxide Na2IrO3, in which both the spin-orbit interaction and the electron correlation play crucial roles. A tight-binding model analysis together with the first-principles band structure calculation predicts that this material is a layered quantum spin Hall system. Because of the electron correlation, an antiferromagnetic order first develops at the

Atsuo Shitade; Hosho Katsura; Jan Kunes; Xiao-Liang Qi; Shou-Cheng Zhang; Naoto Nagaosa

2009-01-01

93

Quantum spin Hall effect in a transition metal oxide Na2IrO3.  

PubMed

We study theoretically the electronic states in a 5d transition metal oxide Na2IrO3, in which both the spin-orbit interaction and the electron correlation play crucial roles. A tight-binding model analysis together with the first-principles band structure calculation predicts that this material is a layered quantum spin Hall system. Because of the electron correlation, an antiferromagnetic order first develops at the edge, and later inside the bulk at low temperatures. PMID:19659103

Shitade, Atsuo; Katsura, Hosho; Kunes, Jan; Qi, Xiao-Liang; Zhang, Shou-Cheng; Nagaosa, Naoto

2009-06-24

94

Quantum spin Hall effect in a transition metal oxide Na2IrO3  

Microsoft Academic Search

We study theoretically the electronic states in a 5d transition metal oxide NaI{sub r}O, in which both the spin-orbit interaction and the electron correlation play crucial roles. Tight-binding model analysis together with the fisrt-principles band structure calculation predicts that this material is a layered quantum spin Hall system. Due to the electron correlation, an antiferromagnetic order first develops at the

Shitade; Atsuo

2010-01-01

95

Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts  

Microsoft Academic Search

A series of transition metal oxides promoted titania catalysts (MO\\u000a x\\u000a \\/TiO2; M=Cr, Mn, Fe, Ni, Cu) were prepared by wet impregnation method using dilute solutions of metal nitrate precursors. The\\u000a catalytic activity of these materials was evaluated for the selective catalytic reduction (SCR) of NO with CO as reductant\\u000a in the presence of excess oxygen (2vol.%). Among various promoted

Pavani M. Sreekanth; Panagiotis G. Smirniotis

2008-01-01

96

The low-temperature reduction of Pd-doped transition metal oxide surfaces with hydrogen  

Microsoft Academic Search

The reaction of hydrogen with a series of polyvalent metal oxides (Fe2O3, WO3, MoO3, V2O5, Sb2O3, PbO2, Cr2O3, NiO, CuO, Co3O4, MnO2, PdO, Ag2O) was investigated at low temperatures (77320K) and pressures (0.0010.7kPa). Pd-doped (0.10.5wt.%) transition metal oxides can be reduced by hydrogen at 77320K whereas the onset of the reduction of the pure oxides occurs at temperatures higher than

V. M. Belousov; M. A. Vasylyev; L. V. Lyashenko; N. Yu. Vilkova; B. E. Nieuwenhuys

2003-01-01

97

Wide bandgap tunability in complex transition metal oxides by site-specific substitution  

SciTech Connect

Fabricating complex transition metal oxides with a tunable bandgap without compromising their intriguing physical properties is a longstanding challenge. Here we examine the layered ferroelectric bismuth titanate and demonstrate that, by site-specific substitution with the Mott insulator lanthanum cobaltite, its bandgap can be narrowed by as much as 1 eV, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, a split-off state responsible for the bandgap reduction is created just below the conduction band of bismuth titanate. This provides a route for controlling the bandgap in complex oxides for use in emerging oxide optoelectronic and energy applications.

Choi, Woo Seok [ORNL; Chisholm, Matthew F [ORNL; Singh, David J [ORNL; Choi, Taekjib [ORNL; Jellison Jr, Gerald Earle [ORNL; Lee, Ho Nyung [ORNL

2012-01-01

98

Screening study of mixed transition-metal oxides for use as cathodes in thermal batteries  

SciTech Connect

Over 100 candidates were examined, including commercial materials and many that were synthesized in house. The mixed oxides were based on Ti, V, Nb, Cr, Mo, W, Mn, Fe, Co, Ni, and Cu doped with other transition metals. A number of individual (single-metal) oxides were included for comparison. The candidates were tested in single cells with Li(Si) anodes and separators based on LiCl-KCl eutectic. Screening was done under constant-current conditions at current densities of 125 me/cm{sup 2} and, to a lesser extent, 50 me/cm{sup 2} at 500 C. Relative performance and limitations of the oxide cathodes are discussed.

Guidotti, R.A.; Reinhardt, F.W.

1996-05-01

99

Synthesis of transition-metal phosphides from oxidic precursors by reduction in hydrogen plasma  

NASA Astrophysics Data System (ADS)

A series of transition metal phosphides, including MoP, WP, CoP, Co2P, and Ni2P, were synthesized from their oxidic precursors by means of hydrogen plasma reduction under mild conditions. The effects of reduction conditions, such as metal to phosphorus molar ratio, power input, and reduction time, on the synthesis of metal phosphides were investigated. The products were identified by means of XRD characterization. It is indicated that metal phosphides were readily synthesized stoichiometrically from their oxides in hydrogen plasma under mild conditions.

Guan, Jie; Wang, Yao; Qin, Minglei; Yang, Ying; Li, Xiang; Wang, Anjie

2009-06-01

100

Hydrothermal synthesis and crystal structures of two novel vanadium oxides containing interlamellar transition metal complexes  

Microsoft Academic Search

Two new vanadium oxide compounds, which contain interlamellar transition metal complexes bound to vanadium oxide sheets, were synthesized hydrothermally. The crystal structure of each reveals unusual coordination geometries and interlayer bonding. [Ni(2,2?-bipyridine)]2[V12O32] (1) was prepared from the reaction of V2O5, NiO, H2O and 2,2?-bipyridyl in the molar ratio 4:8:3312:0.5. The isomorphous compounds [M(2,2?-bipy)]2[V12O32] (where M is Co (1a) and Cu

Patricia J. Ollivier; Jeffrey R. D. DeBoard; Pamela J. Zapf; Jon Zubieta; Linda M. Meyer; Chwan-Chin Wang; Thomas E. Mallouk; Robert C. Haushalter

1998-01-01

101

Method of synthesizing a plurality of reactants and producing thin films of electro-optically active transition metal oxides  

SciTech Connect

A method is described of synthesizing thin films of electro-optically active transition metal oxides from a plurality of reactants in a reaction chamber, the process comprising the steps of: (a) inserting a first reactant into the chamber; (b) imparting second transition metal-bearing and third oxygen-bearing reactants into the chamber, the vapor pressures of the first, second and third reactants being effective for establishing a system pressure substantially less than atmospheric pressure; and (c) inducing a plasma reaction among the reactants by plasma depositions, which plasma reaction consolidates the reactants, oxidizes the transition metal-bearing reactants and produces the thin films of electro-optically active transition metal oxides.

Tracy, C.E.; Benson, D.K.; Ruth, M.R.

1987-08-18

102

Reduction of aqueous transition metal species on the surfaces of Fe(II)-containing oxides  

USGS Publications Warehouse

Experimental studies demonstrate that structural Fe(II) in magnetite and ilmenite heterogeneously reduce aqueous ferric, cupric, vanadate, and chromate ions at the oxide surfaces over a pH range of 1-7 at 25??C. For an aqueous transition metal m, such reactions are 3[Fe2+Fe3+2]O4(magnetite) + 2/nmz ??? 4[Fe3+2]O3(maghemite) + Fe2+ + 2/nmz-n and 3[Fe2+Ti]O3(ilmenite) + 2/nmz ??? Fe3+2Ti3O9(pseudorutile) + Fe2+ + 2/nmz-n, where z is the valance state and n is the charge transfer number. The half cell potential range for solid state oxidation [Fe(II)] ??? [Fe(III)] is -0.34 to -0.65 V, making structural Fe(II) a stronger reducing agent than aqueous Fe2+ (-0.77 V). Reduction rates for aqueous metal species are linear with time (up to 36 h), decrease with pH, and have rate constants between 0.1 and 3.3 ?? 10-10 mol m-2 s-1. Iron is released to solution both from the above reactions and from dissolution of the oxide surface. In the presence of chromate, Fe2+ is oxidized homogeneously in solution to Fe3+. X-ray photoelectron spectroscopy (XPS) denotes a Fe(III) oxide surface containing reduced Cr(III) and V(IV) species. Magnetite and ilmenite electrode potentials are insensitive to increases in divalent transition metals including Zn(II), Co(II), Mn(II), and Ni(II) and reduced V(IV) and Cr(III) but exhibit a log-linear concentration-potential response to Fe(III) and Cu(II). Complex positive electrode responses occur with increasing Cr(VI) and V(V) concentrations. Potential dynamic scans indicate that the high oxidation potential of dichromate is capable of suppressing the cathodic reductive dissolution of magnetite. Oxide electrode potentials are determined by the Fe(II)/Fe(III) composition of the oxide surface and respond to aqueous ion potentials which accelerate this oxidation process. Natural magnetite sands weathered under anoxic conditions are electrochemically reactive as demonstrated by rapid chromate reduction and the release of aqueous Fe(III) to experimental solution. In contrast, magnetite weathered under oxidizing vadose conditions show minimum reactivity toward chromate ions. The ability of Fe(II) oxides to reduce transition metals in soils and groundwaters will be strongly dependent on the redox environment.

White, A. F.; Peterson, M. L.

1996-01-01

103

Mesoscale Charge-Ordering in Transition Metal Oxides: Formation and Signatures  

SciTech Connect

The authors briefly outline the value of an inhomogeneous (unrestricted) Hartree-Fock plus Random Phase approach for understanding the types and properties of mesoscopic patterns of localized small polarons in transition metal oxides. Using a multiband Peierls-Hubbard model for a hole-doped CuO{sub 2} layer as an illustrative example, they demonstrate the appearance of correlated high-energy (electronic) and low-energy (localized phonon and spin-wave) signatures of various vertical, diagonal, metal-centered, and oxygen-centered mesoscopic stripe patterns of localized holes (small polarons).

Bishop, A.R.; Yu, Z.G.

1998-06-01

104

Spectroscopic Studies of Electronically Active Defects in Transition Metal Oxides for Advanced Si Devices  

SciTech Connect

Based on spectroscopic studies, and guided by ab initio theory, the electron and hole traps in HfO2 and other transition metal elemental oxides e.g., TiO2, are assigned to O-atom divacancies, clustered at internal grain boundaries. Engineering solutions for defect reduction are identified: (i) deposition of ultra-thin, < 2 nm HfO2 and phase separated Hf silicate dielectrics, in which grain boundary formation is suppressed by effectively eliminating inter-primitive unit cell {pi}-bonding interactions, and (ii) non-crystalline Zr/Hf Si oxynitrides without nanocrystalline grain boundaries.

Lucovsky, G.; Luning, J.

2009-06-04

105

Electron enrichment in 3d transition metal oxide hetero-nanostructures.  

PubMed

Direct experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heterostructure with nanoscale dimensionality is reported. Aqueous chemical synthesis and vapor phase deposition are combined to fabricate oriented arrays of high-interfacial-area hetero-nanostructures comprised of titanium oxide and iron oxide nanomaterials. Synchrotron-based soft X-ray spectroscopy techniques with high spectral resolution are utilized to directly probe the titanium and oxygen orbital character of the interfacial region's occupied and unoccupied densities of states. These data demonstrate the interface to possess electrons in Ti 3d bands and an emergent degree of orbital hybridization that is absent in parent oxide reference crystals. The carrier dynamics of the hetero-nanostructures are studied by ultrafast transient absorption spectroscopy, which reveals the presence of a dense manifold of states, the relaxations from which exhibit multiple exponential decays whose magnitudes depend on their energetic positions within the electronic structure. PMID:21834542

Kronawitter, Coleman X; Bakke, Jonathan R; Wheeler, Damon A; Wang, Wei-Cheng; Chang, Chinglin; Antoun, Bonnie R; Zhang, Jin Z; Guo, Jinghua; Bent, Stacey F; Mao, Samuel S; Vayssieres, Lionel

2011-08-16

106

Large third-order optical nonlinearities in transition-metal oxides  

NASA Astrophysics Data System (ADS)

ADVANCES in the field of optical computing1-3 will require the development of materials that combine a large nonlinear optical response with a fast response time. For many applications, this translates into a third-order nonlinear optical susceptibility, ?(3), in excess of 10-8 e.s.u., and a response time faster than lO ps (ref. 4). Although a wide range of inorganic5-18 and organic19-21 materials have been found to exhibit a large ?(3), either the response times tend to be far too slow or the materials are not sufficiently stable for device applications. Recently, the transition-metal oxide Fe2O3 was found to have a large ?(3) (ref. 22). Here we show that oxides of several other 3d transition metals show a similarly large nonlinear optical response; moreover, we find that a significant contribution to the overall ?(3) (~10-8 e.s.u. in the case of V2O5) has a response time of the order of 35 ps.

Ando, Masanori; Kadono, Kohei; Haruta, Masatake; Sakaguchi, Toru; Miya, Masaru

1995-04-01

107

Redefining the metal/charge-transfer insulator paradigm in transition metal oxides  

NASA Astrophysics Data System (ADS)

The universality of the phase diagram in the variables of interaction strength and d-occupancy, shown for late transition metal oxides in Ref.[1], is examined for two series of early transition metal oxides: (SrVO3, SrCrO3, SrMnO3) and (LaTiO3, LaVO3, LaCrO3) using density functional theory (DFT), DFT+U and DFT+dynamical mean field theory methods. The interaction required to drive the metal-insulator transition is found to depend sensitively on the d-occupancy Nd, and beyond a threshold value of the d-occupancy an insulating state cannot be achieved for any practical value of the interaction. The critical Nd values are determined and compared to ab initio and experimental estimates where available. Additionally, the minimal model for the transition is determined and the crucial role played by the Hunds coupling is demonstrated. [4pt] [1] Xin Wang, M. J. Han, Luca de' Medici, C. A. Marianetti, and Andrew J. Millis (2011). arXiv:1110.2782

Dang, Hung; Ai, Xinyuan; Marianetti, Chris; Millis, Andrew

2012-02-01

108

Neutron diffraction studies of nickel-containing perovskite oxide catalysts exposed to autothermal reforming environments.  

SciTech Connect

Six nickel-containing perovskite oxides (La{sub 1-x}Sr{sub x})M{sub 0.9}Ni{sub 0.1}O{sub 3{+-}{delta}}, where x = 0 or 0.2 and M = Cr, Fe, or Mn were used to catalyze the autothermal reforming of isooctane (C{sub 8}H{sub 18}) into a hydrogen-rich gas during short-term tests at 700 C. To determine the phase stability of the samples in the reducing environment of the reforming reactor, characterization studies of the as-prepared and tested perovskite samples were conducted using powder X-ray diffraction, powder neutron diffraction, transmission electron microscopy, and scanning electron microscopy. We determined that the reducing conditions of the microreactor caused metallic nickel to form in all six compositions. However, the extent of the nickel loss from the perovskite lattices varied: the chromium-containing compositions lost the least nickel, compared to the manganese- and iron-containing compositions, and the strontium-free compositions lost more nickel than their strontium-containing analogs. Five of the six perovskite compositions tested showed no breakdown of the perovskite lattice despite the loss of nickel from the B-sites, producing only the third example of a B-cation-deficient, 3d transition-metal-containing perovskite.

Mawdsley, J. R.; Vaughey, J. T.; Krause, T. R.; Chemical Sciences and Engineering Division

2009-10-27

109

Fabrication and electrocatalytic application of functionalized nanoporous carbon material with different transition metal oxides  

NASA Astrophysics Data System (ADS)

In the work presented here, an attempt is made to study the effect of functionalization with different transition metal oxides on the mesostructural properties as well as electrochemical behavior of Pt/nanoporous carbon supports. In this respect, the functionalized samples have been synthesized by using CMK-3 and metallocene as transition metal sources. The platinum catalysts (5 wt% Pt) obtained through a conventional wet impregnation method. All the materials have been characterized by XRD (low and high), N2 adsorption-desorption isotherms, high-resolution transmission electron microscopy, high-resolution field emission scanning electron, EDX mapping images and cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The results showed that the mesostructural order has been destroyed by functionalization of CMK-3 with CoO, whereas it is not that much affected in NiO and CuO functionalized samples. EDX image mapping exhibited the good and uniform dispersion of functionalizing elements (Ni, Cu, Fe and Co), Pt in the carbon supports. Moreover, XRD studies revealed the formation of smaller platinum crystallite sizes in NiO and CuO functionalized samples in relative to other functionalized supports. Electrochemical measurements were performed using CV and RDE method. Kinetic analysis revealed an activity increases in the following order: CMK-3-NiO-Pt > CMK-3-CuO-Pt > CMK-3-CoO-Pt > CMK-3-Fe2O3-Pt which is showing of simultaneous effect of surface area and surface reactivity parameters.

Samiee, L.; Shoghi, F.; Vinu, A.

2013-01-01

110

Ligand interchange controls many oxidations of divalent first-row transition metal ions by free radicals  

SciTech Connect

These studies suggest that the rate-determining step in the oxidation of a large variety of transition metal complexes by free radicals is the ligand exchange step. In other words it is proposed that free radicals behave in many reactions with transition metal complexes as incoming nucleophilic ligands. It seemed of interest to investigate this speculative statement in more detail. It was decided therefore to study the interaction of various Mn(II), Fe(II), and Co(II) complexes with a variety of free radicals. These ions were selected as it was shown that Mn(II) ligand exchange processes processed usually via the I[sub a] mechanism, whereas those of Fe(II) and Co(II) proceed usually via the I[sub d] mechanism. The selected reactions are such that they were all suggested to proceed via the inner-sphere mechanism as their rates of reaction are nearly independent of the nature of the free radical and can therefore be considered as typical complex formation reactions as shown earlier. The pressure dependence of the studied reactions enables a detailed comparison with the volumes of activation for solvent exchange and complex formation reactions of these divalent first-row transition elements. Futhermore, it allows the authors to comment on the general behavior of free radicals during complex formation reactions with different metal centers.

Eldik, R. van (Univ. of Witten/Herdecke, Witten (Germany)); Cohen, H. (Nuclear Research Centre Negev, Beer-Sheva (Israel)); Meyerstein, D. (R. Bloch Coal Research Center and Chemistry Department, Beer-Sheva (Israel))

1994-03-30

111

STM Image Contrast Interpretation and its Role in Determining the Structure of Transition Metal Oxide Surfaces  

NASA Astrophysics Data System (ADS)

The surfaces of transition metal oxides play a critical role in many applications such as heterogeneous catalysis, gas detection, thermionic emission, electrolysis, and photolysis. Understanding the mechanisms of such surface processes requires a detailed knowledge of the surface microscopic structure. Since its invention in the early 1980's, scanning tunneling microscopy (STM) has come to be a popular tool for oxide surface studies. However, despite some experimental successes, interpretation of the contrast in STM images of metal oxides has remained challenging due to the numerous contributing factors such as nonstoichiometry, structural complexity, surface disorder, and uncertainties regarding the bonding and termination layers in such multicomponent systems. In this thesis work, a computer simulation scheme that explores these effects separately has been developed to assist the interpretation of atomic-scale contrast in STM images. A semiquantitative technique, based on the one -dimensional square well tunneling model, is used to simulate constant current STM images. This model provides an efficient mechanism to test and explore effects of various ill-defined experimental parameters. The method was applied to the atomic-scale resolution STM study of three transition metal oxides: {rm M_{x}WO _3} (M=Rb, Na), {rm Mo_{18}O_{52}}, and V_2{rm O}_5. Our observations include surface termination layer variations, surface ordering, surface relaxations, surface steps caused by crystallographic shear (CS) planes, surface oxygen vacancies and other defects. In each case, the competition between geometric and electronic contributions to the image contrast is evident. Tunneling spectroscopy experiments and calculations were also performed on the sodium tungsten bronzes and the implication to their electronic structures is discussed.

Lu, Weier

1995-01-01

112

Perovskite oxide nanowires: synthesis, property and structural characterization.  

PubMed

Perovskite oxide materials display a wide spectrum of functional properties, including switchable polarization, piezoelectricity, pyroelectricity, and non-linear dielectric behavior. These properties are indispensable for application in electronic devices such as non-volatile memories, sensors, microactuators, infrared detectors, microwave phase filters, and so on. Recent advances in science and technology of perovskite oxide materials have resulted in the feature sizes of perovskite oxides-based electronic devices entering into nanoscale dimensions. At nanoscale perovskite oxide materials exhibit a pronounced size effect manifesting itself in a significant deviation of the properties of low-dimensional structures from the bulk and film counterparts. In the last decade low-dimensional perovskite nanosized oxides have been received much attention because of their superior physical and chemical properties. Among them, perovskite oxide nanowires are especially attractive for nanoscience studies and nanotechnology applications. Compared to other low-dimensional perovskite oxide systems, perovskite oxide nanowires are not only used as the building blocks of future nanodevices, but also they offer fundamental scientific opportunities for investigating the intrinsic size effects of physical properties. In the recent years, much progress has been made both in synthesis and physical property testing of perovskite oxide nanowires, which have a profound impact on the nanoelectronics. In this work, an overview of the state of art in perovskite oxide nanowires is presented, which covers their synthesis, property, and structural characterization. In the first part, the recent literatures for fabricating perovskite oxide nanowires with promising features, are critically reviewed. The second part deals with the recent advances on the physical property testing of perovskite oxide nanowires. The third part summarizes the recent progress on microstructural characterizations of perovskite oxide nanowires, to improve their crystalline quality, morphology and uniformity. Finally, this review concludes with some perspectives and outlook on the future developments of perovskite oxide nanowires. PMID:21128390

Zhu, Xinhua; Liu, Zhiguo; Ming, Naiben

2010-07-01

113

High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets  

SciTech Connect

This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal metal to one in which only a single spin specie can conduct. Three main thrusts are being pursued: (i) Mott transitions in transition metal oxides, (ii) magnetism in half-metallic compounds, and (iii) large volume-collapse transitions in f-band metals.

Richard T. Scalettar; Warren E. Pickett

2005-08-02

114

High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets  

SciTech Connect

This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal metal to one in which only a single spin specie can conduct. Three main thrusts are being pursued: (1) Mott transitions in transition metal oxides, (2) magnetism in half-metallic compounds, and (3) large volume-collapse transitions in f-band metals.

Scalettar, Richard T.; Pickett, Warren E.

2004-07-01

115

Sonochemical preparation and characterization of nanoporous transition metal oxides for environmental catalysis  

NASA Astrophysics Data System (ADS)

A series of single and multi-component nanoporous transition metal oxides were prepared by sonochemical processing. This is based on a new idea that the controlled condensation and agglomeration of monodispersed sol particles would form a nanoporous structure with a narrow pore size distribution under ultrasound irradiation. Therefore, nanoporous transition metal oxides may be obtained without using surfactant templates. X-ray diffraction (XRD), nitrogen adsorption, UV-vis diffuse reflectance spectrometry, transmission electron microscopy (TEM), infrared (IR) spectrometry, thermalgravimetric analysis (TGA) and differential thermal analysis (DTA) were used to characterize the resulting materials. Compared with conventional surfactant-templated approaches, sonochemical method can avoid the collapse of the framework of nanoporous materials and the pollution arising from the removal of surfactant templates. The first part of the thesis describes the preparation of thermally stable mesoporous TiO2 under high intensity ultrasound irradiation. The photocatalytic activities of the resulting mesoporous TiO2 materials were evaluated by the degradation of volatile organic compounds. These sonochemically prepared mesoporous TiO2 showed better activities than the commercial photocatalyst P25. The reasons of the high activities of the mesoporous TiO 2 were discussed based on the characterization results in the thesis. Nanoporous ceria-zirconia solid solutions with high surface area were also prepared. It was found that the sonochemically prepared solid solutions possessed higher thermal stability than that synthesized by other conventional methods. XPS analysis showed that the molar ratios of Ce to Zr in solid solutions were quite close to that in the starting solutions. This study also confirmed that the addition of zirconia to ceria could enhance the thermal resistance of ceria. Nanoporous composites of amorphous titanium oxide and size-tunable strontium titanate nanocrystals were obtained through simply varying volume ratio of ethanol and water in reaction system. A dissolution-crystallization mechanism could be used to explain the formation of nanoporous composites under ultrasound irradiation.

Zhang, Lizhi

116

Energetics of perovskite-type materials applied in solid oxide fuel cells (SOFCs): Electrolytes, cathodes and interconnects  

NASA Astrophysics Data System (ADS)

Perovskite-type oxides (ABO3) find a great variety of applications in solid oxide fuel cells (SOFCs), including solid electrolytes, cathodes and interconnects, which are closely related to the defect chemistry involved. Thermodynamic studies are needed to systematically understand the nature of the structure-property relations and provide guidance to predict and/or select proper materials. High temperature solution calorimetry in molten oxide solvents is a powerful tool and has been applied for several perovskite systems that have simple (undoped) and complex (doped) compositions. LaBO3 perovskites (B = Al, Ga, Sc, In, Cr, Fe, Co, Ni) represent a group of excellent parent materials for electrolytes, cathodes, and interconnects in SOFCs. Their enthalpies of formation from oxides generally exhibit a relationship between stability and the major structural parameter for perovskites, the tolerance factor. As the tolerance factor deviates more from unity, the enthalpy of formation from oxides becomes less exothermic. This work verifies this general trend for A3+B3+O3 type perovskites, joining other two types, i.e., A1+B5+O 3 and A2+B4+O3. In alkaline earth doped perovskites, though structural parameters are likely to continue affecting stability, defects, which are introduced upon doping, actually play a more profound role in defining energetic trends. In the newly developed electrolyte materials, Mg, Sr, and Ba-doped LaGaO 3 perovskites, oxygen vacancies are created to compensate the charge imbalance between dopant and host ions. Oxygen vacancies have a destabilization effect on the structure due to the partial disconnection of the corner-shared BO6 octahedral framework. On the other hand, they tend to order at the short-range scale, forming vacancy-dopant clusters, as evidenced by neutron diffraction. In alkaline earth doped perovskites that contain transition metals, two charge compensation scenarios are possible: oxidation of the transition metal or creation of oxygen vacancies, which strongly depend on the oxygen partial pressure and temperature. Major defects are dopants, oxygen vacancies and electron holes. La1-xSrxFeO3-delta (cathode materials) and La1-x-AxCrO 3-delta (A = Ca or Sr) (interconnect materials) were chosen for study. The overall energetic trends in these complex perovskites are modeled by several competing factors: (1) the chemical nature of dopant ions (acidity/basicity), (2) coupled substitution involving oxygen vacancy formation, (3) redox of the transition metal in each sample series with a given dopant composition, and (4) short-range oxygen vacancy ordering in oxygen-deficient samples.

Cheng, Jihong

117

Site-specific mapping of transition metal oxygen coordination in complex oxides  

NASA Astrophysics Data System (ADS)

We demonstrate site-specific mapping of the oxygen coordination number for transition metals in complex oxides using atomically resolved electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope. Pb2Sr2Bi2Fe6O16 contains iron with a constant Fe3+ valency in both octahedral and tetragonal pyramidal coordination and is selected to demonstrate the principle of site-specific coordination mapping. Analysis of the site-specific Fe-L2,3 data reveals distinct variations in the fine structure that are attributed to Fe in a six-fold (octahedron) or five-fold (distorted tetragonal pyramid) oxygen coordination. Using these variations, atomic resolution coordination maps are generated that are in excellent agreement with simulations.

Turner, S.; Egoavil, R.; Batuk, M.; Abakumov, A. A.; Hadermann, J.; Verbeeck, J.; Van Tendeloo, G.

2012-12-01

118

Quantum Monte Carlo study of ground state properties of transition metal oxides  

NASA Astrophysics Data System (ADS)

We apply a recently developed quantum Monte Carlo method ( S. Zhang and H. Krakauer, Phys. Rev. Lett. 90, 126401 (2003).) to study the transition metal oxides TiO and MnO. Starting from a trial wavefunction, this method uses much of the same machinery of density functional methods (single particle basis, non-local pseudopotentials) to project the system ground state by random walks in the space of Slater determinants, using auxiliary Hubbard-Stratonovich complex fields. The trial wave function, which is used to control the phase/sign problem, is a single Slater determinant generated using a density functional or Hartree-Fock type calculation. Our preliminary results as well as previous results show the robustness of this method for studying atoms, molecules, and solids. (Supported by ONR and NSF)

Al-Saidi, Wissam; Krakauer, Henry; Zhang, Shiwei

2004-03-01

119

Limitations of the hybrid functional approach to electronic structure of transition metal oxides  

NASA Astrophysics Data System (ADS)

During the past decade, ab initio methods to calculate the electronic structure of materials based on hybrid functionals have increasingly become widely popular. Here we show that, in the case of small gap transition metal oxides, such as VO2, with rather subtle physics in the vicinity of the Fermi surface, such hybrid functional schemes without the inclusion of expensive fully self-consistent GW corrections fail to yield this physics and incorrectly describe the features of the wave function of states near the Fermi surface. While a fully self-consistent GW on top of a hybrid functional approach does correct these wave functions as expected, and is found to be in general agreement with the results of a fully self-consistent GW approach based on semilocal functionals, it is much more computationally demanding as compared to the latter approach for the benefit of essentially the same results.

Coulter, John E.; Manousakis, Efstratios; Gali, Adam

2013-07-01

120

Control of Octahedral Tilts and Magnetic Properties of Perovskite Oxide Heterostructures by Substrate Symmetry  

SciTech Connect

Perovskite transition-metal oxides are networks of corner-sharing octahedra whose tilts and distortions are known to affect their electronic and magnetic properties. We report calculations on a model interfacial structure which avoids chemical influences and show that the symmetry mismatch imposes an interfacial layer with distortion modes that do not exist in either bulk material, creating new interface properties driven by symmetry alone. Depending on the resistance of the octahedra to deformation, the interface layer can be as small as one unit cell or extend deep into the thin film.

He, Jun [ORNL; Borisevich, Albina Y [ORNL; Kalinin, Sergei V [ORNL; Pennycook, Stephen J [ORNL; Pantelides, Sokrates T [ORNL

2010-01-01

121

Manganese: A transition metal protects nigrostriatal neurons from oxidative stress in the iron-induced animal model of Parkinsonism  

Microsoft Academic Search

It has been suggested that transition metals such as iron and manganese produce oxidative injury to the dopaminergic nigrostriatal system, which may play a critical role in the pathogenesis of Parkinson's disease. Intranigral infusion of ferrous citrate (0 to 8.4 nmol, i.n.) acutely increased lipid peroxidation in the substantia nigra and dopamine turnover in the caudate nucleus. Subsequently, it caused

I. Szirki; K. P. Mohanakumar; P. Rauhala; H. G. Kim; K. J. Yeh; C. C. Chiueh

1998-01-01

122

Oxidative stress and DNA damage in relation to transition metals overload in Abu-Qir Bay, Egypt  

Microsoft Academic Search

The aim of the present study is to evaluate the transition metals overload in Abu-Qir Bay in Egypt, as compared to a less polluted area (reference area) through some biomarkers of oxidative stress. Catalase enzyme activity, malondialdehyde (MDA) concentration and DNA damage (number of apurinic\\/apyrimidinic sites) were the tested biomarkers. The levels of iron and copper in Mugil cephalus liver

Haiam M. Aboul-Ela; Abir A. Saad; Amany M. A. El-Sikaily; Taha I. Zaghloul

2011-01-01

123

Investigation of coordination changes in substituted transition-metal oxides by K-edge XANES: beyond the pre-edge  

SciTech Connect

Transition-metal oxides are important materials whose properties can be tuned through selective substitution of the constituent elements. For oxygen deficient materials, substitution can lead to variations in coordination number (CN). To understand these changes in CN, the pre-edge peak intensity, resulting primarily from a 1s to n-1d transition, from first-row transition-metal K-edge XANES spectra can be used. However, investigation of these peaks from spectra of second-row transition-metals can be difficult owing to lower spectral resolutions. It is shown here, through examination of Ca{sub 2}Fe{sub 2-x}Ga{sub x}O{sub 5}, SrFe{sub 1-x}Zr{sub x}O{sub 3-{delta}}, and (ZrO{sub 2}){sub x}(SiO{sub 2}){sub 1-x}, that changes in CN also lead to significant variations of the main absorption edge.

Gaultois, Michael W.; Greedan, John E.; Grosvenor, Andrew P. (Saskatchewan)

2011-10-25

124

Hydroxide oxidation and peroxide formation at embedded binuclear transition metal sites; TM = Cr, Mn, Fe, Co.  

PubMed

Key steps in electro-catalytic water oxidation on binuclear Transition Metal (TM) sites are addressed. These comprise (a) two one-electron oxidation steps of TM-OH moieties to form the corresponding two TM=O oxy-groups, and (b) a chemical step whereby the two oxy-species form a TM-O-O-TM peroxy-bridge. A test rig representing a generic low crystal field oxide support is described and employed. The energetics for homo-nuclear Cr(III-V), Mn(III-V), Fe(II-IV) and Co(II-IV) sites are compared. The uniqueness of the tyrosine/tyrosyl-radical (TyrOH/TyrO?) reference potential for driving the oxidation steps is demonstrated. The oxidation of adsorbed TM-OH moieties on binuclear Mn and Co candidates requires an overpotential of approximately 0.5 V relative to the chosen reference potential. Correspondingly, the subsequent O-O bond formation becomes strongly exothermic, of the order of 1 eV. The hydroxide oxidation steps on binuclear CrCr and FeFe systems are, in total, exothermic by 1.21 and 0.61 eV, respectively, relative to the TyrOH/TyrO? reference potential. Consequently, the chemical step for transforming the TM=O moieties to the peroxo species is found to be endothermic by the order of 0.7 eV. Based on these findings, a catalyst containing one TM from each class is suggested. The validity of this concept is demonstrated for the FeCo binuclear site. The results are discussed in the context of experimental observations, which display a preference for mixed oxide systems. PMID:21773631

Busch, M; Ahlberg, E; Panas, I

2011-07-20

125

Photo-oxidation of Sb(III) in the seawater by marine phytoplankton-transition metals-light system.  

PubMed

The photo-oxidation of Sb(III) to Sb(V) by marine microalgae (diatom, green and red algae) with or without the presence of transition metals (Fe(III), Cu(II) and Mn(II)). The influence of marine phytoplankton on the photochemistry of antimony was confirmed for the first time. The conversion ratio of Sb(III) to Sb(V) increased with increasing algae concentration and irradiation time. Different species of marine phytoplankton were found to have different photo-oxidizing abilities. The photochemical redox of transition metals could induce the species transformation of antimony. After photo-induced oxidation by marine phytoplankton and transition metals, the ratio of Sb(V) to Sb(III) was in the range of 1.07-5.48 for six algae (Tetraselmis levis, Chlorella autotrophica, Nannochloropsis sp., Tetraselmis subcordiformis, Phaeodactylum tricornutum, and Porphyridium purpureum), and only 0.92 for Dunaliella salina. The distribution of antimony in the sunlit surface seawater was greatly affected by combined effects of marine phytoplankton (main contributor) and transition metals; both synergistic and antagonistic effects were observed. The results provided further insights into the distribution of Sb(III) and Sb(V) and the biogeochemical cycle of antimony, and have significant implications for the risk assessment of antimony in the sunlit surface seawater. PMID:16735057

Li, Shun-Xing; Zheng, Feng-Ying; Hong, Hua-Sheng; Deng, Nan-Sheng; Zhou, Xia-Yi

2006-06-02

126

Spin-phonon coupling effects in transition-metal perovskites: A DFT + U and hybrid-functional study  

NASA Astrophysics Data System (ADS)

Spin-phonon coupling effects, as reflected in phonon frequency shifts between ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic CaMnO3, SrMnO3, BaMnO3, LaCrO3, LaFeO3, and La2(CrFe)O6, are investigated using density-functional methods. The calculations are carried out both with a hybrid-functional Heyd-Scuseria-Ernzerhof (HSE) approach and with a DFT + U approach using a U that has been fitted to HSE calculations. The phonon frequency shifts obtained in going from the FM to the AFM spin configuration agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational effort. We find that in the AMnO3 materials class with A= Ca, Sr, and Ba, this frequency shift decreases as the A cation radius increases for the ? phonons, while it increases for R-point phonons. In LaMO3 with M= Cr, Fe, and Cr/Fe, the phonon frequencies at ? decrease as the spin order changes from AFM to FM for LaCrO3 and LaFeO3, but they increase for the double perovskite La2(CrFe)O6. We discuss these results and the prospects for bulk and superlattice forms of these materials to be useful as multiferroics.

Hong, Jiawang; Stroppa, Alessandro; iguez, Jorge; Picozzi, Silvia; Vanderbilt, David

2012-02-01

127

Oxide ion conductivity in doped Ga based perovskite type oxide  

Microsoft Academic Search

Oxide ion conductivity of Ga based perovskite type oxides, LnGaO3, (Ln = Nd, La) and its related oxide, A3Ga2O6 (A = Ba and St) was investigated. Oxide ion transference number is almost 1.0 on Sr3Ga2O6 or Ba3Ga2O6. However, electrical conductivity of these A3Ga2O6 system oxides was lower than log (?Scm?1) = ? 3 due to the low solubility of the

Tatsumi Ishihara; Hideaki Matsuda; Mohamad Azmi bin Bustam; Yusaku Takita

1996-01-01

128

Resistive switching memory devices composed of binary transition metal oxides using sol-gel chemistry.  

PubMed

We describe a novel and versatile approach for preparing resistive switching memory devices based on binary transition metal oxides (TMOs). Titanium isopropoxide (TIPP) was spin-coated onto platinum (Pt)-coated silicon substrates using a sol-gel process. The sol-gel-derived layer was converted into a TiO2 film by thermal annealing. A top electrode (Ag electrode) was then coated onto the TiO2 films to complete device fabrication. When an external bias was applied to the devices, a switching phenomenon independent of the voltage polarity (i.e., unipolar switching) was observed at low operating voltages (about 0.6 VRESET and 1.4 VSET). In addition, it was confirmed that the electrical properties (i.e., retention time, cycling test and switching speed) of the sol-gel-derived devices were comparable to those of vacuum deposited devices. This approach can be extended to a variety of binary TMOs such as niobium oxides. The reported approach offers new opportunities for preparing the binary TMO-based resistive switching memory devices allowing a facile solution processing. PMID:19317425

Lee, Chanwoo; Kim, Inpyo; Choi, Wonsup; Shin, Hyunjung; Cho, Jinhan

2009-04-21

129

Influence of the negative oxygen ions on the structure evolution of transition metal oxide thin films  

NASA Astrophysics Data System (ADS)

The energy distributions of O- ions of magnetron sputtered Nb, Ta, Zr, and Hf in an Ar/O2 atmosphere were measured as a function of the oxygen partial pressure. Three ion populations were detected in the plasma: high, medium, and low energy ions, with energies corresponding to the target potential, half of the target potential, and <150 eV, respectively. The ion energy distribution functions were compared to distributions obtained based on Sigmund's linear collision cascade sputtering theory. If the surface binding energy is assumed to be equal to the heat of formation, good agreement between the experiment and theory was achieved. From correlating the measured ion energy distributions with previously published phase stability data [Ngaruiya et al., Appl. Phys. Lett. 85, 748 (2004)], it can be deduced that large fluxes of medium and high energy O- ions comparable to the fluxes of the low energy O- ions enable formation of crystalline transition metal oxide thin films during low temperature growth. The presented data here may be of general relevance for understanding the structure evolution of thin oxide films.

Mrz, Stanislav; Schneider, Jochen M.

2006-07-01

130

Catalytic activity of transition metal borides in the oxidation of hydrogen  

SciTech Connect

The specific catalytic activity of transition metal borides in the oxidation of hydrogen at 593/sup 0/K in excess O/sub 2/ decreases in the series: Ni/sub 2/B > Mo/sub 2/B/sub 5/ > W/sub 2/B/sub 5/ > VB/sub 2/ > CrB/sub 2/ > TaB/sub 2/ > NbB/sub 2/, ZrB/sub 2/ > TiB/sub 2/ >> Co/sub 3/B, HfB/sub 2/. The reaction rate is a function of the rate of the reaction of H/sub 2/ with adsorbed oxygen ZO and decreases with decreasing Z-O bond energy. A higher rate is found in the case of excess H/sub 2/ (Ni/sub 2/B > Co/sub 3/B > ZrB/sub 2/ > VB/sub 2/ > W/sub 2/B/sub 5/, HfB/sub 2/ > Mo/sub 2/B/sub 5/ > TaB/sub 2/ > CrB/sub 2/ > TiB/sub 2/, NbB/sub 2/) as a result of the participation of dissociatively adsorbed hydrogen. The rates of hydrogen oxidation on metal-like borides and carbides were compared and the differences found were explained.

Vlasenko, N.V.; Il'chenko, N.I.; Golodets, G.I.

1987-05-01

131

Surface structure determination of nanoscale transition metal oxide films using x-ray photoelectron diffraction and low energy electron diffraction  

Microsoft Academic Search

In this study, attempts were made to grow well-ordered chromium- and iron-oxide films on Ag(111) and Ag(001) surfaces, respectively. Two sample preparation techniques, multilayer growth and sequential growth, were used to grow these transition metal oxide films. Both sample preparation techniques result in a (&surd;3x&surd;3)R30 LEED pattern for high coverage CrOx on Ag(111). The high coverage phase is identified as

Weerasinghe Priyantha

2005-01-01

132

Heterogeneous phosphate ion-exchange complexes of transition metals in the lquid-phase oxidation of cumene  

SciTech Connect

It was shown that phosphate ion-exchange complexes of the transition metals can be used as effective heterogeneous catalysts in the liquid-phase oxidation of cumene. The process occurs according to a radical-chain heterogeneous-homogeneous mechanism with the participation of the catalyst both in the step of initiation and in steps of chain propagation and termination. Factors determining the activity of the catalysts in the oxidative process were detected.

Berentsveig, V.V. (Moscow State Univ., USSR); Dotsenko, O.E.; Kopylova, V.D.; Boiko, E.T.

1982-07-01

133

Nonpolar resistance switching of metal\\/binary-transition-metal oxides\\/metal sandwiches: Homogeneous\\/inhomogeneous transition of current distribution  

Microsoft Academic Search

Exotic features of a metal\\/oxide\\/metal sandwich, which will be the basis for a drastically innovative nonvolatile memory device, is brought to light from a physical point of view. Here the insulator is one of the ubiquitous and classic binary-transition-metal oxides (TMO), such as Fe2O3 , NiO , and CoO . The sandwich exhibits a resistance that reversibly switches between two

I. H. Inoue; S. Yasuda; H. Akinaga; H. Takagi

2008-01-01

134

High-resolution structural studies of ultra-thin magnetic, transition metal overlayers and two-dimensional transition metal oxides using synchrotron radiation  

SciTech Connect

This thesis report the surface-structure determination of three, ultra-thin magnetic transition-metal films, Fe/Au(100), Mn/Ni(100), and Mn/Cu(100) using Angle-Resolved Photoemission Extended Fine Structure (ARPEFS) and photoelectron holography. These structural studies are the first to use non-s initial states in the ARPEFS procedure. This thesis also reports an ARPEFS surface-structure determination of a two-dimensional transition-metal oxide, [(1 x 1)O/W(110)] x 12. The authors have analyzed the ARPFES signal from the Au 4f{sub 7/5} core level of the Au(1 ML)/Fe(15 ML)/Au(100) system. The analysis shows that the Fe grows layer by layer with one monolayer of gold, acting as a surfactant, remaining on top of the growing Fe layers. These surface gold atoms sit in the four-fold hollow site, 1.67 {+-} 0.02 A above the iron surface. The grown Fe layer is very much like the bulk, bcc iron, with an interlayer spacing of 1.43 {+-} 0.03 A. Analysis of the Mn 3p ARPEFS signals from c(2 x 2)Mn/Ni(100) and c(2 x 2)Mn/Cu(100) shows that the Mn forms highly corrugated surface alloys. The corrugation of the Mn/Ni(100) and Mn/Cu(100) systems are 0.24 {+-} 0.02 A and 0.30 {+-} 0.04 A respectively. In both cases the Mn is sticking above the plane of the surface substrate atoms. For the Mn/Ni(100) system the first layer Ni is contracted 4% from the bulk value. The Mn/Cu(100) system shows bulk spacing for the substrate Cu. Photoelectron holography shows that the Mn/Ni interface is very abrupt with very little Mn leaking into the second layer, while the Mn/Cu(100) case has a significant amount of Mn leaking into the second layer. A new, five-element electrostatic electron lens was developed for hemispherical electron-energy analyzers. This lens system can be operated at constant transverse or constants angular magnification, and has been optimized for use with the very small photon-spot sizes. Improvements to the hemispherical electron-energy analyzer are also discussed.

Kellar, S.A. [Univ. of California, Berkeley, CA (United States). Dept. of Chemistry]|[Lawrence Berkeley National Lab., CA (United States). Advanced Light Source Div.

1997-05-01

135

Modular construction of oxide structures--compositional control of transition metal coordination environments.  

PubMed

The effects of reaction temperature and pO2 were investigated on a series of (Ba,Ca,Nd)FeO3-delta perovskite systems in order to isolate phases containing ordered arrangements of the distinct vacancy and cation ordering patterns identified in less compositionally complex iron oxide systems. Initial synthesis in air at high temperature yields cubic perovskite phases (I) with average iron oxidation states higher than 3; selected area electron diffraction together with diffuse features observed in the synchrotron X-ray diffraction (SXRD) patterns of these materials show evidence of small domains of short-range cation and vacancy order. Annealing these materials in nitrogen or in a sealed tube in the presence of an NiO/Ni buffer yielded the Fe(3+) phase Ca2Ba2Nd2Fe6O16 (II), closely related to Sr2LaFe3O8 but with partial cation order as well as anion order present the larger Ba cations are largely present in the 12-coordinate site between the octahedral iron layers, and Ca is largely present in 10-coordinate sites between octahedral and tetrahedral sites. Further reduction of Ca2Ba2Nd2Fe6O16 using a Zr getter yields the mixed-valence phase Ca2Ba2Nd2Fe6O15.6 (III). The structure of III was solved by maximum entropy analysis of XRD data coupled with analysis of high-temperature neutron diffraction data and refined against combined SXRD and high-Q ambient-temperature neutron data. This material crystallizes in a 20-fold perovskite super cell (Imma, a approximately square root(2 x a(p), b approximately 10 x a(p), c approximately square root(x 2a(p)) and can be visualized as an intergrowth between brownmillerite (Ca2Fe2O5) and the YBa2Fe3O8 structure. There are three distinct iron coordination environments, octahedral (O), square-pyramidal (Sp), and trigonal planar (Tp, formed by distorting the tetrahedral site in brownmillerite), which form a Sp-O-Tp-O-Sp repeat. Bond valence calculations indicate that Tp is an Fe(2+) site, while the O and Sp sites are Fe(3+). The A-site cations are also partially ordered over three distinct sites: 8-coordinate between the Sp layers, 10-coordinate between Tp and O layers, and 12-coordinate between Sp and O layers. Mossbauer spectroscopy, magnetometry, and variable-temperature neutron diffraction show that the material undergoes two magnetic transitions at approximately 700 and 255 K. PMID:18505254

Tenailleau, Christophe; Allix, Mathieu; Claridge, John B; Hervieu, Maryvonne; Thomas, Michael F; Hirst, James P; Rosseinsky, Matthew J

2008-05-28

136

Density functional theory study of CO2 capture with transition metal oxides and hydroxides  

NASA Astrophysics Data System (ADS)

We have used density functional theory (DFT) employing several different exchange-correlation functionals (PW91, PBE, PBEsol, TPSS, and revTPSS) coupled with lattice dynamics calculations to compute the thermodynamics of CO2 absorption/desorption reactions for selected transition metal oxides, (TMO), and hydroxides, TM(OH)2, where TM = Mn, Ni, Zn, and Cd. The van't Hoff plots, which describe the reaction equilibrium as a function of the partial pressures of CO2 and H2O as well as temperature, were computed from DFT total energies, complemented by the free energy contribution of solids and gases from lattice dynamics and statistical mechanics, respectively. We find that the PBEsol functional calculations are generally in better agreement with experimental phase equilibrium data compared with the other functionals we tested. In contrast, the formation enthalpies of the compounds are better computed with the TPSS and revTPSS functionals. The PBEsol functional gives better equilibrium properties due to a partial cancellation of errors in the enthalpies of formation. We have identified all CO2 capture reactions that lie on the Gibbs free energy convex hull as a function of temperature and the partial pressures of CO2 and H2O for all TMO and TM(OH)2 systems studied here.

Zhang, Bo; Duan, Yuhua; Johnson, Karl

2012-02-01

137

Early Transition Metal Oxides as Catalysts: Crossing Scales from Clusters to Single Crystals to Functioning Materials  

SciTech Connect

The proportion of chemical industry processes using catalysts exceeds 80%. Current commercial heterogeneous catalysts are structurally and chemically complex and data gathered from them can seldom be interpreted with atomic-level precision. We seek to reduce the complexity of TMO catalysts to levels addressable and controllable at the atomic level, while maintaining intimate linkages with practical catalysis and catalytic materials. The focus of the proposed work is to gain a fundamental understanding of chemical transformations in order to design and construct new catalysts with more precise control of specific chemical reactions. We are employing an integrated experimental/theoretical approach to advance our current ability to understand, design, and control the catalytic and surface chemistry of transition metal oxides, specifically for redox and acid-base chemistries. The approach combines novel solid-state inorganic synthesis, surface science, experimental and theoretical/computational chemical physics, and mechanistic organic chemistry to address this complex and important challenge. Selected highlights from the results obtained in the last year are presented in the conference proceedings extended abstract.

Dixon, David A.; Dohnalek, Zdenek; Gutowski, Maciej S.; Hu, Jian Zhi; Iglesia, Enrique; Kay, Bruce D.; Liu, Jun; Peden, Charles HF; Wang, Lai; Wang, Yong; White, John M.; Bondarchuk, Oleksander A.; Herrera, Jose E.; Kim, J.; Kwak, Ja Hun; Stuchinskaya, T.; Zhai, Hua Jin; Chisolm, Claire N.; Macht, Josef

2007-05-20

138

Improved layered mixed transition metal oxides for Li-ion batteries  

NASA Astrophysics Data System (ADS)

Recent work in our laboratory has been directed towards development of mixed layered transition metal oxides with general composition Li[Ni, Co, M, Mn]O2 (M=Al, Ti) for Li ion battery cathodes. Compounds such as Li[Ni1/3Co1/3Mn1/3]O2 (often called NMCs) are currently being commercialized for use in consumer electronic batteries, but the high cobalt content makes them too expensive for vehicular applications such as electric vehicles (EV), plug-in hybrid electric vehicles (PHEVs), or hybrid electric vehicles (HEVs). To reduce materials costs, we have explored partial or full substitution of Co with Al, Ti, and Fe. Fe substitution generally decreases capacity and results in poorer rate and cycling behavior. Interestingly, low levels of substitution with Al or Ti improve aspects of performance with minimal impact on energy densities, for some formulations. High levels of Al substitution compromise specific capacity, however, so further improvements require that the Ni and Mn content be increased and Co correspondingly decreased. Low levels of Al or Ti substitution can then be used offset negative effects induced by the higher Ni content. The structural and electrochemical characterization of substituted NMCs is presented in this paper.

Doeff, Marca M.; Conry, Thomas; Wilcox, James

2010-04-01

139

A DFT-based method of calculating optical properties of transition metal oxide materials  

NASA Astrophysics Data System (ADS)

As part of an ongoing investigation of optical properties of transition metal oxide materials, we have examined the optical properties of Vanadium Dioxide using an ab-initio method. Starting from hybrid DFT, we apply the GW approximation and solve the Bethe-Salpeter Equation (BSE) on the wavefunctions obtained from the DFT starting point. We find that the hybrid functional is not fully satisfactory for description of the optical spectrum of VO2, and that corrections are required. The hybrid functional results may be a good starting place for many-body perturbation theory. We apply the GW approximation and then solve the BSE from that starting point. We show that including single particle-hole quasiparticles is not sufficient for the optical spectrum, and that two-particle-two-hole effects must be included via the BSE to give agreement between the integrated strength of the optical spectrum at low energies and the experimental spectrum. We also find that a large number of high energy states must be included for a convergent description of the low energy optical spectrum.

Coulter, John E.; Gali, Adam; Efstratios, Manousakis

2013-03-01

140

Probing the Reactivity and Radical Nature of Oxidized Transition Metal-Thiolate Complexes by Mass Spectrometry  

NASA Astrophysics Data System (ADS)

Transition metal thiolate complexes such as [PPN]+[RuL3]- (PPN = bis(triphenylphosphoranylidene) ammonium and L = diphenylphosphinobenzenethiolate) are known to undergo addition reactions with unsaturated hydrocarbons via the formation of new C-S bonds in solution upon oxidation. The reaction mechanism is proposed to involve metal-stabilized thiyl radical intermediates, a new type of distonic ions such as [RuL3]+ ion in the case of [PPN]+[RuL3]-. This study presents the reactivity and structure investigation of [RuL3]+ by mass spectrometry (MS) in conjunction with ion/molecule reactions. The addition reactions of [RuL3]+ with alkenes or methyl ketones in the gas phase are indeed observed, in agreement with the proposed mechanism. Such reactivity is also maintained by several fragment ions of [RuL3]+, indicating the preserved thiyl diradical core structure is responsible for the addition reaction. The thiyl radical nature of [RuL3]+ was further verified by the ion/molecule reaction of [RuL3]+ with dimethyl disulfide, in which the characteristic CH3S transfer occurs, both at atmospheric pressure and also at low pressure (~mTorr). These results provide, for the first time, clear mass spectrometric evidence of the radical nature of [RuL3]+ (i.e., the distonic ion structure of [RuL3]+), arising from the oxidation of non-innocent thiolate ligands of the complex [PPN]+[RuL3]-. Similar thiolate complexes, including ReL3 and NiL2, were also examined. Although reactions of oxidized ReL3 or NiL2 with CH3SSCH3 take place at atmospheric pressure, the corresponding reaction did not occur in vacuum. Consistent with these data, the addition of ethylene was not observed either, indicating lower reactivities of [ReL3]+ and [NiL2]+ in comparison to [RuL3]+.

Lu, Mei; Campbell, J. Larry; Chauhan, Rajat; Grapperhaus, Craig A.; Chen, Hao

2013-04-01

141

Probing the reactivity and radical nature of oxidized transition metal-thiolate complexes by mass spectrometry.  

PubMed

Transition metal thiolate complexes such as [PPN](+)[RuL3](-) (PPN = bis(triphenylphosphoranylidene) ammonium and L = diphenylphosphinobenzenethiolate) are known to undergo addition reactions with unsaturated hydrocarbons via the formation of new C-S bonds in solution upon oxidation. The reaction mechanism is proposed to involve metal-stabilized thiyl radical intermediates, a new type of distonic ions such as [RuL3](+) ion in the case of [PPN](+)[RuL3](-). This study presents the reactivity and structure investigation of [RuL3](+) by mass spectrometry (MS) in conjunction with ion/molecule reactions. The addition reactions of [RuL3](+) with alkenes or methyl ketones in the gas phase are indeed observed, in agreement with the proposed mechanism. Such reactivity is also maintained by several fragment ions of [RuL3](+), indicating the preserved thiyl diradical core structure is responsible for the addition reaction. The thiyl radical nature of [RuL3](+) was further verified by the ion/molecule reaction of [RuL3](+) with dimethyl disulfide, in which the characteristic CH3S transfer occurs, both at atmospheric pressure and also at low pressure (~mTorr). These results provide, for the first time, clear mass spectrometric evidence of the radical nature of [RuL3](+) (i.e., the distonic ion structure of [RuL3](+)), arising from the oxidation of non-innocent thiolate ligands of the complex [PPN](+)[RuL3](-). Similar thiolate complexes, including ReL3 and NiL2, were also examined. Although reactions of oxidized ReL3 or NiL2 with CH3SSCH3 take place at atmospheric pressure, the corresponding reaction did not occur in vacuum. Consistent with these data, the addition of ethylene was not observed either, indicating lower reactivities of [ReL3](+) and [NiL2](+) in comparison to [RuL3](+). PMID:23315345

Lu, Mei; Campbell, J Larry; Chauhan, Rajat; Grapperhaus, Craig A; Chen, Hao

2013-01-12

142

Water adsorption and hydrolysis on molecular transition metal oxides and oxyhydroxides.  

PubMed

Addition of water to molecular transition metal oxides (TiO2(g) and CrO3(g)) and oxyhydroxides (ScO(OH)(g), VO2(OH)(g), and MnO3(OH)(g)) was studied by means of quantum chemistry. In the investigated reactions, each reaction step comprised the breaking of one M=O bond and the formation of two OH groups. Exothermicity was observed when the product had tetrahedral or lower oxygen coordination. The reactions were found to involve stable water complexes as intermediates. The stabilities of such complexes were accentuated in the addition reaction Sc(OH)3(g) + H2O(g), in which the formation of a tetrahedral complex was found exothermic. For VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g), water addition to the remaining M=O bonds was found endothermic, whereas the formation of water complexes, using hydrogen bonds and preserving the oxyhydroxide kernel, was preferred. Thus, the sequence of such kernels for water clustering in the investigated reactions was found to be Sc(OH)3.H2O(g), Ti(OH)4(g), VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g). These stability considerations are important, as CrO2(OH)2(g) is believed to be the product of water-induced degradation of the protective chromium oxide scale on stainless steel at elevated temperatures. PMID:11196853

Johnson, J R; Panas, I

2000-07-24

143

Electrochromism in some thin-film transition-metal oxides characterized by x-ray electron spectroscopy  

Microsoft Academic Search

Qualitative results of an x-ray photoelectron and optical investigation of three thin-film transition-metal oxides, namely, WO3, MoO3, and V2O5 are reported. Data were obtained on films that were not colored and colored by an electrochromic process. The particular electrochromic coloration process used is electrolytic in nature and employs a sandwich-structure electrochromic cell and electrolyte pool. The optical spectra show increased

Richard J. Colton; Alberto M. Guzman; J. Wayne Rabalais

1978-01-01

144

Electrochromism in some thin-film transition-metal oxides characterized by x-ray electron spectroscopy  

Microsoft Academic Search

Qualitative results of an x-ray photoelectron and optical investigation of three thin-film transition-metal oxides, namely, WO3, MoO3, and V2O5 are reported. Data were obtained on films that were notcolored and colored by an electrochromic process. The particular electrochromic coloration process used is electrolytic in nature and employs a sandwich-structure electrochromic cell and electrolyte pool. The optical spectra show increased absorption

Richard J. Colton; Alberto M. Guzman; J. Wayne Rabalais

1978-01-01

145

Synthesis, Characterization and Magnetic Property Studies of Copper-Based Phosphate and Silicate Compounds Containing Nanosize Transition Metal Oxide Frameworks  

Microsoft Academic Search

The search for novel compounds with nanosize transition metal oxide frameworks is important for the study and understanding of materials with interesting and important magnetic and electronic properties. These low-dimensional structural features have become associated with unusual physical phenomena such as charge density waves, magnetic ordering, and superconductivity. Exploratory synthesis of copper(II) and copper(I\\/II) phosphates containing oligomers, chains and layers

Kristen Miriam Sharon Gronberg Etheredge

1996-01-01

146

Energetics of lithium transition metal oxides applied as cathode materials in lithium ion batteries  

NASA Astrophysics Data System (ADS)

In this work, high temperature oxide melt solution calorimetry was employed to investigate the thermodynamic properties of several representative lithium transition metal oxide systems applied as cathode materials in lithium ion batteries: layered alpha-NaFeO2 structure LiNi1-x COxO2 solid solution, Li1+xMn 2-xO4 spinel, LiMO2 (M = Mn, Fe and Co), and chemically delithiated LixCoO2. The enthalpies of formation and enthalpies of mixing for LiNi1-x COxO2 solid solution series (0 ? x ? 1) were determined using high temperature oxide melt solution calorimetry. The enthalpies of formation at 298 K from binary oxides (Li2O, NiO and CoO) and O2, and the lattice parameters decrease approximately linearly with the Co content x in LiNi1-xCOxO 2. The solid solution deviates slightly positively from ideality. Lithium substituted spinel Li1+xMn2-xO 4 (0 ? x ? 1/3) samples synthesized are stoichiometric at x < 0.25, and oxygen deficient at x ? 0.25. The enthalpy of formation from binary oxides (Li2O, Mn2O3 and MnO2 ) becomes more exothermic with x in Li1+xMn2-x O4 for stoichiometric compounds, and deviates endothermically from this trend for oxygen deficient compounds. This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn3+ to Mn4+ versus formation of oxygen vacancies). LiMO2 samples (M = Mn, Fe, and Co) with different structures were synthesized and their enthalpies of formation from oxides (Li2O and M2O3) or oxides (Li2O and MO) plus oxygen at 298 K were determined. The relative stability of the polymorphs of LiMO 2 was established based on their enthalpies of formation. Phase transformations in LiFeO2 were investigated by differential scanning calorimetry and high temperature oxide melt solution calorimetry. The gamma phase is the stable form of LiFeO2 at room temperature and the a phase is stable at high temperature. Energetics of chemically delithiated LixCoO2 samples (0.5 ? x ? 1.0) with the alpha-NaFeO2 structure (O 3 type) was studied. The enthalpies of formation from binary oxides (Li2O and CoO) and oxygen vary linearly with x and LixCoO 2 becomes less stable with decreasing x. The enthalpies of formation from oxides and oxygen for monoclinic Li0.5CoO2 and O3-CoO2 were estimated by extrapolation from the linear trend.

Wang, Miaojun

147

Insights on Hydrogen Liberation from Water Using Anionic Transition Metal Oxide Clusters: a Combined Computational and Spectroscopic Study  

NASA Astrophysics Data System (ADS)

Transition Metal Oxides (TMOs) offer widespread applications in catalysis and the generation of alternate sources of energy. Cluster models are useful to model the defect sites in these TMO surfaces which are responsible for their catalytic activities. In this combined computational (DFT) and spectroscopic (PES/MS) study, we present the interesting features in the chemical reactions of molybdenum oxide and tungsten oxide clusters with water. The results obtained provide valuable insights on the roles played by differing metal-oxygen bond strengths, the initial electrostatic complex formed and the geometric factors involved in the liberation of hydrogen gas from water.

Ramabhadran, Raghunath O.; Mann, Jenniffer E.; Waller, Sarah. E.; Rothgeb, David W.; Jarrold, Caroline C.; Raghavachari, Krishnan

2013-06-01

148

Charge Density Wave Properties of Quasi Low-Dimensional Transition Metal Oxide Bronzes  

NASA Astrophysics Data System (ADS)

Several families of transition metal oxide bronzes including the quasi one-dimensional molybdenum blue bronzes A{0.3}MoO3, the quasi two-dimensional (2D) Mo purple bronzes AMo6O{17} and the quasi 2D monophosphate tungsten bronzes (PO2)4(WO3){2m} show Peierls transitions leading to commensurate or incommensurate charge density wave (CDW) states. The properties of the monophosphate tungsten bronzes (PO2)4(WO3){2m} are reviewed. These series of compounds provide a model system where both the low-dimensional character and the average electron concentration are function of the m parameter. The low m compounds (m=4, 6) show conventional CDW instabilities as well as giant positive magnetotransport and quantum oscillations at low temperature. Detailed information on the Fermi surface in the CDW state is obtained from transport data, the size of the pockets left by the CDW gap openings being found of the order of 1% of the high temperature two-dimensional Brillouin zone. The m=5 compound, which may be considered as an intergrowth of m=4 and 6, shows also instabilities with slightly different properties. The absence of CDW long range order affects the properties of the m=8 member which shows low temperature properties possibly due to weak localisation effects. Finally, studies on the 4leq m leq 14 compounds show that the Peierls temperatures are increasing with m, reaching high values, up to 550 K, for m=13. This is attributed to a 2D character increasing with m. The increase of the room temperature resistivity with m is discussed in terms of enhanced electron-phonon or electron-electron interactions.

Schlenker, C.; Hess, C.; Le Touze, C.; Dumas, J.

1996-12-01

149

Inverse layer capacitance in perovskite oxide superlattices  

NASA Astrophysics Data System (ADS)

Ferroelectricity is one of the most important functionalities that can be tuned in perovskite oxide superlattices. At fixed displacement field D, the overall polar instability can be accessed by the inverse of the capacitance per basal area as C-1=V / D, where V is the potential drop across the supercell.ootnotetextM. Stengel, D. Vanderbilt, and N.A. Spaldin, Nature Mater. 8, 392 (2009). Here we propose that C-1 can be further rigorously decomposed into contributions from individual AO or BO2 layers, giving an layer inverse capacitance defined as cj-1=?0-1(hj+Dhj/D - pj/D), where hj and pj are the layer height and Wannier-based layer polarizationootnotetextX. Wu, O. Di'ehuez, K.M. Rabe and D. Vanderbilt, Phys. Rev. Lett. 97, 107602 (2006). of layer j, respectively. We compute the cj-1 in several typical multicomponent perovskite superlattices such as CaTiO3/BaTiO3 and PbTiO3/SrTiO3, and demonstrate that they satisfy a locality principle: their behavior depends mainly on the local chemical environment (i.e., the identities of neighboring layers). Thus, we show that the cj-1 can provide an insightful local analysis of the ferroelectric tendency at interfaces in functional oxide superlattices.

Wu, Xifan; Stengel, Massimiliano; Vanderbilt, David

2011-03-01

150

Facile synthesis of transition-metal oxide nanocrystals embedded in hollow carbon microspheres for high-rate lithium-ion-battery anodes.  

PubMed

Charged up: A general soft-template route for the synthesis of uniform hollow carbon microspheres embedded with transition-metal oxide nanocrystals (OHCMs) has been developed (see figure). The obtained OHCMs possess a microsized spherical shape, embedded transition-metal oxide nanocrystals, and fully encapsulating conductive carbon shells, which endow the resulting anode materials with high specific capacities, rate capabilities, electrode densities, and cycle stabilities. PMID:23788047

Liu, Jun; Liu, Wei; Chen, Kunfeng; Ji, Shaomin; Zhou, Yichun; Wan, Yanling; Xue, Dongfeng; Hodgson, Peter; Li, Yuncang

2013-06-20

151

Transition-metal-oxide coated titanium electrodes for redox batteries. Final report  

SciTech Connect

A primary motivation for this work was to produce porous electrodes (of titanium) with reproducible surface activity for experiments to corroborate predictions of porous electrode theory. An additional goal of this research program was to ascertain if simple one-step outer-sphere electron transfer mechanism applies to the ferric-ferrous redox reaction at high reactant/product concentrations in a strong acid chloride medium. Kinetic rate expressions of this couple under these conditions at the metal oxide surfaces were sought. Coatings of the conductive and active rutile structure form of selected transition metal oxides (namely RuO/sub 2/, IrO/sub 2/, OsO/sub 2/) were desired. Both IrO/sub 2/ and RuO/sub 2/ coatings having the rutile structure (as confirmed by x-ray diffraction) were prepared by thermal decomposition of their respective hydrated chloride salts. In the case of OsO/sub 2/ only amorphous coatings have been achieved. Low over potential exchange current densities of the ferric-ferrous couple were measured in concentrated solution (0.5 M Fe/sup 2 +/, 0.5 M Fe/sup 3 +/, 1 M HCl) at Pt, RuO/sub 2/ and IrO/sub 2/ RDE's. Significant corrections were necessary for ohmic effects, mass transfer effects, and nonuniform current distribution. The data show that the electrocatalytic activity of RuO/sub 2/ (ks = 1.92 x 10/sup -3/ cm/s) is comparable to that of Pt (ks = 1.42 x 10/sup -3/ cm/s) while the activity of IrO/sub 2/ (ks = 1.21 x 10/sup -4/ cm/s) is approximately an order of magnitude less than Pt. Since the measured free energy of activation was similar for all three substrates (5.4 to 5.8 Kcal/mole), a nonbinding interaction between the couple and the surfaces is suggested. The variation in exchange currents among the different substrates can be attributed to some extent on double layer effects but primarily to crystal structure differences.

Savinell, R.F.

1982-12-01

152

Synthesis and characterization of transition-metal-doped zinc oxide nanocrystals for spintronics  

NASA Astrophysics Data System (ADS)

Spintronics (spin transport electr onics), in which both spin and charge of carriers are utilized for information processing, is believed to challenge the current microelectronics and to become the next-generation electronics. Nanostructured spintronic materials and their synthetic methodologies are of paramount importance for manufacturing future nanoscale spintronic devices. This thesis aims at studying synthesis, characterization, and magnetism of transition-metal-doped zinc oxide (ZnO) nanocrystals---a diluted magnetic semiconductor (DMS)---for potential applications in future nano-spintronics. A simple bottom-up-based synthetic strategy named a solvothermal technique is introduced as the primary synthetic approach and its crystal growth mechanism is scrutinized. N-type cobalt-doped ZnO-based DMS nanocrystals are employed as a model system, and characterized by a broad spectrum of advanced microscopic and spectroscopic techniques. It is found that the self-orientation growth mechanism, imperfect oriented attachment, is intimately correlated with the high-temperature ferromagnetism via defects. The influence of processing on the magnetic properties, such as compositional variations, reaction conditions, and post-growth treatment, is also studied. In this way, an in-depth understanding of processing-structure-property interrelationships and origins of magnetism in DMS nanocrystals are obtained in light of the theoretical framework of a spin-split impurity band model. In addition, a nanoscale spinodal decomposition phase model is also briefly discussed. Following the similar synthetic route, copper- and manganese-doped ZnO nanocrystals have been synthesized and characterized. They both show high-temperature ferromagnetism in line with the aforementioned theoretical model(s). Moreover, they display interesting exchange biasing phenomena at low temperatures, revealing the complexity of magnetic phases therein. The crystal growth strategy demonstrated in this work not only provides a more convenient approach to directly tailor magnetic properties of advanced multifunctional spintronic materials on a nanometer scale but also contributes to a deeper insight into the microscopic origin of magnetism in wide-band-gap oxide DMSs.

Wang, Xuefeng

153

Comparison of Transition Metal-Mediated Oxidation Reactions of Guanine in Nucleoside and Single-Stranded Oligodeoxynucleotide Contexts  

PubMed Central

As the most readily oxidized of DNAs four natural bases, guanine is a prime target for attack by reactive oxygen species (ROS) and transition metal-mediated oxidants. The oxidation products of a modified guanosine nucleoside and of a single-stranded oligodeoxynucleotide, 5?-d(TTTTTTTGTTTTTTT)-3? have been studied using oxidants that include CoII, NiII, and IrIV compounds as well as photochemically generated oxidants such as sulphate radical, electron-transfer agents (riboflavin) and singlet oxygen. The oxidized lesions formed include spiroiminodihydantoin (Sp), guanidinohydantoin (Gh), imidazolone (Iz), oxazolone (Z) and 5-carboxamido-5-formamido-2-iminohydantion (2-Ih) nucleosides with a high degree of dependence on the exact oxidation system employed. Interestingly, a nickel(II) macrocyclic complex in conjunction with KHSO5 leads to the recently reported 2-Ih heterocycle as the major product in both the nucleoside and oligonucleotide contexts.

Ghude, Pranjali; Schallenberger, Mark A.; Fleming, Aaron M.; Muller, James G.; Burrows, Cynthia J.

2011-01-01

154

Generalized trends in the formation energies of perovskite oxides.  

PubMed

Generalized trends in the formation energies of several families of perovskite oxides (ABO3) and plausible explanations to their existence are provided in this study through a combination of DFT calculations, solid-state physics analyses and simple physical/chemical descriptors. The studied elements at the A site of perovskites comprise rare-earth, alkaline-earth and alkaline metals, whereas 3d and 5d metals were studied at the B site. We also include ReO3-type compounds, which have the same crystal structure of cubic ABO3 perovskites except without A-site elements. From the observations we extract the following four conclusions for the perovskites studied in the present paper: for a given cation at the B site, (I) perovskites with cations of identical oxidation state at the A site possess close formation energies; and (II) perovskites with cations of different oxidation states at the A site usually have quite different but ordered formation energies. On the other hand, for a given A-site cation, (III) the formation energies of perovskites vary linearly with respect to the atomic number of the elements at the B site within the same period of the periodic table, and the slopes depend systematically on the oxidation state of the A-site cation; and (IV) the trends in formation energies of perovskites with elements from different periods at the B site depend on the oxidation state of A-site cations. Since the energetics of perovskites is shown to be the superposition of the individual contributions of their constituent oxides, the trends can be rationalized in terms of A-O and B-O interactions in the ionic crystal. These findings reveal the existence of general systematic trends in the formation energies of perovskites and provide further insight into the role of ion-ion interactions in the properties of ternary compounds. PMID:23579382

Zeng, ZhenHua; Calle-Vallejo, Federico; Mogensen, Mogens B; Rossmeisl, Jan

2013-04-12

155

Cooperative and frustration effects in novel perovskite-related phases  

Microsoft Academic Search

.\\u000a We report on magnetic and electronic properties of various perovskite-type oxides containing 4d- and 5d-transition metals.\\u000a The compounds under investigation crystallize in (distorted) cubic, layered, and hexagonal perovskite-related structures.\\u000a These changes in structural dimensionality are reflected by different ordering phenomena. (Pseudo-) cubic perovskites ACu3B4O12 (with A = alkali, alkaline earth or rare earth; B = Ru, Ti) possess an

S. G. Ebbinghaus; S. Riegg; T. Gtzfried; A. Reller

2009-01-01

156

Domain Structures in Perovskite Oxide Superlattices  

NASA Astrophysics Data System (ADS)

Perovskite oxides possess a wide range of technologically relevant functional properties including ferromagnetism, ferroelectricity, and superconductivity. Furthermore, the interfaces of perovskite oxides have been shown to exhibit unexpected functional properties not found in the constituent materials. These functional properties arise due to various structural and chemical changes as well as electronic and/or magnetic interactions occurring over nanometer length scales at the interfaces. In order to understand how these interfacial effects impact the ferromagnetic (FM) properties of the half metal La0.7Sr0.3MnO3 (LSMO), we have examined superlattices composed of LSMO sublayers alternating with either the antiferromagnetic (AFM) insulator La0.7Sr0.3FeO3 (LSFO) or the non-magnetic metal La0.5Sr0.5TiO3 (LSTO). A comprehensive approach consisting of bulk magnetization, magneto-transport measurements, and scanning transmission electron microscopy as well as soft x-ray magnetic spectroscopy and microscopy has been used to fully characterize the properties of the interfaces. We find that the nature of the charge transfer across the interfaces affects the FM properties of LSMO, such that at a given sublayer thickness, the LSMO/LSTO system displays a similar Curie temperature but a higher saturation magnetization than the LSMO/LSFO system. For a specific range of sublayer thicknesses, the LSMO/LSFO system displays a unique spin-flop coupling where the FM moments and the AFM spin axis maintain a perpendicular orientation relative to one another. Through this coupling mechanism, the direction of the AFM spin axis can be reoriented with an applied magnetic field. In this talk, I will discuss how these interfacial phenomena contribute to the types of FM and AFM domain patterns observed in the individual layers in the superlattices.

Takamura, Yayoi

2012-02-01

157

High-pressure Synthesis and Magnetic Properties of 4d and 5d Transition-metal Oxides  

NASA Astrophysics Data System (ADS)

The pressure effect on synthesis of oxides with perovskite ABO3 and perovskite-related structures has become more clear in recent years. The geometric tolerance factor t ? (A-O)/2(B-O) measures the structural stability. High-pressure synthesis enlarges the range of the t factor where the perovskite structure can be stabilized. For the ABO3 compounds with t > 1, high pressure reduces the t factor since the A-O bond is more compressible than the B-O bond. Therefore, perovskite would be the high-pressure phase for ambient-pressure polytype structures. However, the bonding compressibility argument is no longer valid for the ABO3 with t < 1. A dt/dP > 0 is normally obtained for t less than but very close to 1, i.e. the orthorhombic distortion becomes smaller under pressure. For those highly distorted perovskites with t factor far less than one, pressure enlarges further the orthorhombic distortion and eventually leads to a phase transition to the post-perovskite phase. As for PbRuO3, high pressure prefers the small-volume perovskite phase relative to a competitive pyrochlore phase Pb2Ru2O7. Understanding the pressure effect and the new capacity provided by a Walker-type multianvil press enabled us to expand the perovskite family and to obtain new phases of 4d and 5d oxides. Studies of these new 4d and 5d oxides allow us not only to address long-standing problems, but also to explore exotic physical properties. (1) In the perovskite ARuO3 (A= alkaline earth), we have completed the phase diagram from A= Ca to Sr and to Ba and also accounted for the A-cation size-variance effect. A systematic study of the Curie temperature Tc and the critical behavior as a function of the average A-site size and the size variance as well as external high pressures reveals explicitly the crucial role of the lattice strain on the ferromagnetism. The mean-field critical behaviour near Tc found previously in SrRuO3 is not typical of these perovskite ruthenates. Tc is completely suppressed by Pb doping in Sr1-xPbxRuO3 not due to the steric effect, but to the orbital hybridization between Pb^2+ 6s and Ru^4+ 4d. As the end member, metallic PbRuO3 undergoes a first-order phase transition to a metallic Imma phase at Tt 90 K. (2) A new polytype phase 5H has been synthesized under a narrow pressure range, which fits the structural sequence along with other polytypes 9R, 6H and 3C of BaIrO3. The ground states of these BaIrO3 polytypes evolve from a ferromagnetic insulator with Tc 180 K in the 9R phase to a ferromagnetic metal with Tc 50 K in the 5H phase, and finally to an exchange-enhanced paramagnetic metal in the 6H phase, which may be close to a quantum critical point. (3) In the Ca1-xSrxIrO3 system, high pressure stabilizes the post-perovskite structure on the Ca side (x < 0.3), but favors the perovskite structure on the Sr side (x > 0.6). Refs. J.-G. Cheng, et al. PRB 80, 104430(2009); 80, 174426 (2009); 81, 134412(2010); JACS 131, 7461(2009).

Cheng, J.-G.

2011-03-01

158

Self-interaction correction in multiple scattering theory: application to transition metal oxides  

Microsoft Academic Search

We apply to transition metal monoxides the self-interaction corrected (SIC) local spin density approximation, implemented locally in the multiple scattering theory within the Korringa-Kohn-Rostoker (KKR) band structure method. The calculated electronic structure and in particular magnetic moments and energy gaps are discussed in reference to the earlier SIC results obtained within the linear muffin-tin orbital atomic sphere approximation band structure

W M Temmerman; W Hergert; Physikalische Chemie

2009-01-01

159

Lifetime of combustion-generated environmentally persistent free radicals on Zn(II)O and other transition metal oxides.  

PubMed

Previous studies indicated that Environmentally Persistent Free Radicals (EPFRs) are formed in the post-flame, cool zone of combustion. They result from the chemisorption of gas-phase products of incomplete combustion (particularly hydroxyl- and chlorine-substituted aromatics) on Cu(II)O, Fe(III)(2)O(3), and Ni(II)O domains of particulate matter (fly ash or soot particles). This study reports our detailed laboratory investigation on the lifetime of EPFRs on Zn(II)O/silica surface. Similarly, as in the case of other transition metals, chemisorption of the adsorbate on the Zn(II)O surface and subsequent transfer of electron from the adsorbate to the metal forms a surface-bound EPFR and a reduced metal ion center. The EPFRs are stabilized by their interaction with the metal oxide domain surface. The half-lives of EPFRs formed on Zn(II)O domains were the longest observed among the transition metal oxides studied and ranged from 3 to 73 days. These half-lives were an order of magnitude longer than those formed on nickel and iron oxides, and were 2 orders of magnitude longer compared to the EPFRs on copper oxide which have half-lives only on the order of hours. The longest-lived radicals on Zn(II)O correspond to the persistency in ambient air particles of almost a year. The half-life of EPFRs was found to correlate with the standard reduction potential of the associated metal. PMID:22990982

Vejerano, Eric; Lomnicki, Slawo; Dellinger, Barry

2012-09-18

160

On dithiothreitol (DTT) as a measure of oxidative potential for ambient particles: evidence for the importance of soluble transition metals  

PubMed Central

The rate of consumption of dithiothreitol (DTT) is increasingly used to measure the oxidative potential of particulate matter (PM), which has been linked to the adverse health effects of PM. While several quinones are known to be very reactive in the DTT assay, it is unclear what other chemical species might contribute to the loss of DTT in PM extracts. To address this question, we quantify the rate of DTT loss from individual redox-active species that are common in ambient particulate matter. While most past research has indicated that the DTT assay is not sensitive to metals, our results show that seven out of the ten transition metals tested do oxidize DTT, as do three out of the five quinones tested. While metals are less efficient at oxidizing DTT compared to the most reactive quinones, concentrations of soluble transition metals in fine particulate matter are generally much higher than those of quinones. The net result is that metals appear to dominate the DTT response for typical ambient PM2.5 samples. Based on particulate concentrations of quinones and soluble metals from the literature, and our measured DTT responses for these species, we estimate that for typical PM2.5 samples approximately 80 % of DTT loss is from transition metals (especially copper and manganese), while quinones account for approximately 20 %. We find a similar result for DTT loss measured in a small set of PM2.5 samples from the San Joaquin Valley of California. Because of the important contribution from metals, we also tested how the DTT assay is affected by EDTA, a chelator that is sometimes used in the assay. EDTA significantly suppresses the response from both metals and quinones; we therefore recommend that EDTA should not be included in the DTT assay.

Charrier, J. G.; Anastasio, C.

2013-01-01

161

Structural characterization of tellurite glasses doped with transition metal oxides using Raman spectra and ab initio calculations  

Microsoft Academic Search

Systems of iron tellurite glasses were prepared by melt quenching with compositions of [85%TeO2+5%Fe2O3+10%TMO], where transition metal oxides (TMO) are TiO2, V2O5, MnO, CoO, NiO and CuO. Furthermore, the main structural units of these samples have been characterized by means of Raman spectra (1501200cm?1) as well as wavenumber predictions by means of Gaussian 98 ab initio calculations for the proposed

Tarek A. Mohamed; I. Shaltout; K. M. Al Yahyaei

2006-01-01

162

Topological Oxide Insulator in Cubic Perovskite Structure  

PubMed Central

The emergence of topologically protected conducting states with the chiral spin texture is the most prominent feature at the surface of topological insulators. On the application side, large band gap and high resistivity to distinguish surface from bulk degrees of freedom should be guaranteed for the full usage of the surface states. Here, we suggest that the oxide cubic perovskite YBiO3, more than just an oxide, defines itself as a new three-dimensional topological insulator exhibiting both a large bulk band gap and a high resistivity. Based on first-principles calculations varying the spin-orbit coupling strength, the non-trivial band topology of YBiO3 is investigated, where the spin-orbit coupling of the Bi 6p orbital plays a crucial role. Taking the exquisite synthesis techniques in oxide electronics into account, YBiO3 can also be used to provide various interface configurations hosting exotic topological phenomena combined with other quantum phases.

Jin, Hosub; Rhim, Sonny H.; Im, Jino; Freeman, Arthur J.

2013-01-01

163

Topological Oxide Insulator in Cubic Perovskite Structure  

NASA Astrophysics Data System (ADS)

The emergence of topologically protected conducting states with the chiral spin texture is the most prominent feature at the surface of topological insulators. On the application side, large band gap and high resistivity to distinguish surface from bulk degrees of freedom should be guaranteed for the full usage of the surface states. Here, we suggest that the oxide cubic perovskite YBiO3, more than just an oxide, defines itself as a new three-dimensional topological insulator exhibiting both a large bulk band gap and a high resistivity. Based on first-principles calculations varying the spin-orbit coupling strength, the non-trivial band topology of YBiO3 is investigated, where the spin-orbit coupling of the Bi 6p orbital plays a crucial role. Taking the exquisite synthesis techniques in oxide electronics into account, YBiO3 can also be used to provide various interface configurations hosting exotic topological phenomena combined with other quantum phases.

Jin, Hosub; Rhim, Sonny H.; Im, Jino; Freeman, Arthur J.

2013-04-01

164

An atomic resolution view at oxidation states in transition metal oxide heterostructures  

NASA Astrophysics Data System (ADS)

In LaMnO3/SrTiO3 (LMO/STO) superlattices the LMO/STO relative layer thickness ratio changes the degree of epitaxial strain within the layers and dramatically affects the physical properties of the system, which can be tuned from insulating, mild ferromagnetic, to metallic ferromagnets. Such behaviors are ultimately related to the oxygen - 3d metal bonds and hence to the occupancies of the partially filled 3d bands, which define their oxidation state. In this talk we will review procedures to measure oxidation states in manganites from electron energy loss spectra obtained in the aberration corrected scanning transmission electron microscope, and apply these techniques to imaging subtle O displacements and measure their effects on the electronic properties of LMO/STO interfaces. These results will be discussed and combined with density functional theory, in connection with the magnetotransport properties.

Varela, M.; Pennycook, S. J.; Pennycook, T. J.; Luo, W.; Pantelides, S. T.; Garcia-Barriocanal, J.; Bruno, F. Y.; Leon, C.; Santamaria, J.

2010-03-01

165

Control of Copper and Iron Oxidation States in Some Triple- and Double-Perovskite Oxides.  

National Technical Information Service (NTIS)

This thesis consists of eight publications and a summary of the obtained experimental results, reviewed together with the most essential literature related to the topic. The work deals with ways of tuning and analyzing the transition metal oxidation state...

K. Lehmus

2003-01-01

166

Oxide ion conductivity of double doped lanthanum gallate perovskite type oxide  

Microsoft Academic Search

Doping transition metal cation is known to enhance the electronic conduction of solid electrolytes, however, the ionic conduction\\u000a can also be improved by those dopants. In this investigation, the oxide ion conductivity of LaGaO3 based oxide doped with transition metal cations such as Fe, Co, Ni, Mn, and Cu for the Ga site was studied. It was found\\u000a that doping

T. Ishihara; H. Furutani; T. Yamada; Y. Takita

1997-01-01

167

High-fat diet induced an oxidative stress in white adipose tissue and disturbed plasma transition metals in rat: prevention by grape seed and skin extract.  

PubMed

Obesity is a public health problem characterized by increased accumulation of fat into adipose tissues leading to oxidative stress, dyslipidemia, and chronic inflammatory status. We used an experimental model of high-fat diet-induced obesity to analyze the link between dyslipidemia, oxidative stress, and fat accumulation into adipose tissue of rats, as well as the involvement of intracellular mediators such as transition metals on signal transduction. We also looked at the ability of a grape seed and skin extract (GSSE) from a Tunisian cultivar to prevent fat-induced disturbances. Data showed that a high-fat diet (HFD) provoked dyslipidemia into plasma which is linked to an oxidative stress, an accumulation of transition metals such as manganese, copper, and zinc and a depletion of iron. GSSE prevented dyslipidemia by modulating lipase activity, together with increased antioxidant capacity and depletion of transition metals as well as of free radicals such as O2 (-) and OH. These data indicated that GSSE has important preventive effects on HFD-induced obesity and oxidative stress whose transduction seems to involve transition metals. GSSE should be used as a safe anti-obesity agent that could find potential applications in metabolic disorders involving transition metals dyshomeostasis. PMID:24158847

Charradi, Kamel; Elkahoui, Salem; Limam, Ferid; Aouani, Ezzedine

2013-08-06

168

Gas Sensing with Nanocrystalline Magnetic Perovskite Oxides  

NASA Astrophysics Data System (ADS)

LaFeO3 LFO, La0.8Sr0.2FeO3 LSFO and LaMg0.1Fe0.9O3 (LMFO) thick films were prepared by screen printing from oxide powders made by the solgel citrate method. Their structural, electrical and gassensing properties at exposure to various gases, such as carbon monoxide (CO), nitrogen dioxide (NO2), methane (CH4) and ethylene (C2H4), are reported. The results of X-ray diffraction experiments show that these compounds have orthorhombic crystal structure. The substitutions of Sr2+ ion for La3+ in the A-site and Mg2+ ion for Fe3+ in the B-site in the perovskite ABO3 structure change only slightly the values of the lattice constants, but to a much larger extent electrical properties of the base material. LaFeO3 showed very good sensing performance to environmental monitoring of various gases at different operation temperatures. In addition, all LFO, LSFO and LMFO thick films were very sensitive to C2H4.

Toan, N. N.; Saukko, S.; Lantto, V.; Thu, Do Thi Anh; Hieu, Nguyen Sy

2004-01-01

169

Semiconducting transition-metal oxides based on d5 cations: Theory for MnO and Fe2O3  

NASA Astrophysics Data System (ADS)

Transition-metal oxides with partially filled d shells are typically Mott or charge-transfer insulators with notoriously poor transport properties due to large effective electron/hole masses or due to carrier self-trapping. Employing band-structure calculations and ab initio small-polaron theory for MnO and Fe2O3, we explore the potential of d5 oxides for achieving desirable semiconducting properties, e.g., in solar energy applications. The quantification of self-trapping energies and the trends with the coordination symmetry suggest strategies to overcome the main bottlenecks, i.e., the tendency for self-trapping of holes due to Mn(II) and of electrons due to Fe(III).

Peng, Haowei; Lany, Stephan

2012-05-01

170

Semiconducting Transition-Metal Oxides Based on D5 Cations: Theory for MnO and Fe2O3  

SciTech Connect

Transition-metal oxides with partially filled d shells are typically Mott or charge-transfer insulators with notoriously poor transport properties due to large effective electron/hole masses or due to carrier self-trapping. Employing band-structure calculations and ab initio small-polaron theory for MnO and Fe{sub 2}O{sub 3}, we explore the potential of d{sup 5} oxides for achieving desirable semiconducting properties, e.g., in solar energy applications. The quantification of self-trapping energies and the trends with the coordination symmetry suggest strategies to overcome the main bottlenecks, i.e., the tendency for self-trapping of holes due to Mn(II) and of electrons due to Fe(III).

Peng, H.; Lany, S.

2012-05-15

171

High temperature grown transition metal oxide thin films: tuning physical properties by MeV N+-ion bombardment  

NASA Astrophysics Data System (ADS)

In this paper, we present a systematic study on tuning the physical properties of high temperature (373 K) grown transition metal oxide thin films by the effect of 2 MeV nitrogen ion irradiation. Although we observe irradiation induced growth in crystallite sizes for both WO3 and MoO3 films, no structural phase change takes place in the films due to N+-ion beam irradiation even up to the fluence of 1 1015 N+ cm-2. On the other hand, irradiation leads to a significant increase in the optical absorption and the surface roughness of the films. These observations are corroborated by micro-Raman analysis. The results are attributed to the MeV ion-matter interaction.

Sivakumar, R.; Sanjeeviraja, C.; Jayachandran, M.; Gopalakrishnan, R.; Sarangi, S. N.; Paramanik, D.; Som, T.

2008-06-01

172

New concepts and modeling strategies to design and evaluate photo-electro-catalysts based on transition metal oxides.  

PubMed

Photocatalytic production of transportation fuels should be among our long term strategies to achieve energy and environmental sustainability for the planet, but the technology is hampered by a lack of sufficiently efficient catalysts. Although efficiency is ultimately determined by laboratory measurements, theory and computation have become powerful tools for examining underlying mechanisms and guiding avenues of inquiry. In this review, we focus on first principles calculations of transition metal oxide semiconductor photocatalysts. We discuss how theory can be applied to investigate various aspects of a photocatalytic cycle: light absorption, electron/hole transport, band edge alignments of semiconductors, and surface chemistry. Emphasis is placed on identifying accurate models for specific properties and theoretical insights into improving photocatalytic performance. PMID:23111395

Liao, Peilin; Carter, Emily A

2012-10-30

173

THE BREAKDOWN OF THE PROTECTIVE OXIDE FILM ON TRANSITION METAL ALLOYS  

Microsoft Academic Search

Initial breaks in oxidation curves of iron-chromium alloys were found to ; be associated with the ferriteaustenitic phase transformation. This ; transformation was caused by preferential oxidation of chromium at a rate much ; larger than its replenishment by diffusion the metal-oxide interface. The stress ; resulting from this transformation caused breakdown of the protective oxide film. ; Results indicated

W SMELTZER

1960-01-01

174

Carbon-hydrogen bond activation by electrophilic transition-metal compounds. Palladium(II)-mediated oxidation of arenes and alkanes including methane  

Microsoft Academic Search

The activation of carbon-hydrogen bonds by transition metals that leads to the selective functionalization of hydrocarbons, especially alkanes, is one of the most challenging problems in organometallic chemistry. Herein, the authors report on the palladium(II)-mediated oxidation of arenes and alkanes through a nonradical, electrophilic C-H activation pathway.

Efi Gretz; Thomas F. Oliver; Ayusman Sen

1987-01-01

175

Oxidation of methanol on 2nd and 3rd row group VIII transition metals (Pt, Ir, Os, Pd, Rh, and Ru): Application to direct methanol fuel cells  

Microsoft Academic Search

Using first principles quantum mechanics [nonlocal density functional theory (B3LYP)], the authors calculated the 13 most likely intermediate species for methanol oxidation on clusters of all 2nd and 3rd row Group VIII transition metals for all three likely binding sites (top, bridge, and cap). This comprehensive set of binding energies and structures allows a detailed analysis of possible reaction mechanisms

Jeremy Kua; William A. Goddard III

1999-01-01

176

Quantum Spin Hall Effect in a Transition Metal Oxide Na(sub 2)I(sub r)O(sub 3).  

National Technical Information Service (NTIS)

We study theoretically the electronic states in a 5d transition metal oxide Na(sub 2)I(sub r)O(sub 3), in which both the spin-orbit interaction and the electron correlation play crucial roles. Tight-binding model analysis together with the fisrt-principle...

H. Katsura H. Shitade J. Kunes N. Nagaosa S. Zhang X. Qi

2010-01-01

177

Synthesis, structure and magnetic properties of low dimensional spin systems in the 3d transition metal oxides and superconductivity in magnesium borate  

Microsoft Academic Search

The major part of this thesis deals with the synthesis and magnetic characterization of low dimensional spin systems in the 3d transition metal oxides. Such systems are of interest due to the simplicity of their structures, allowing theoretical modeling of their electronic and magnetic behavior. Exotic properties are also often encountered. Studies involving layered magnetic materials based on triangle lattices,

Nyrissa S. Rogado

2003-01-01

178

Studies on the interactions of transition metal oxides and sodium sulfate in the temperature range 9001200 K in oxygen  

Microsoft Academic Search

The interaction of different transition metal oxides such as TiO2, ZrO2, Nb2O5, MoO3 and WO3 with Na2SO4 in the temperature range 9001200 K in flowing oxygen has been studied. The oxides chosen for the studies are initial scaling products during the oxidation of industrial alloys and are invariably involved in hot corrosion reactions in the presence of a molten salt.

M. Mobin; A. U. Malik

1996-01-01

179

Copper- and magnesium protoporphyrin complexes inhibit oxidative modification of LDL induced by hemin, transition metal ions and tyrosyl radicals.  

PubMed

The oxidative modification of LDL may play an important role in the early events of atherogenesis. Thus the identification of antioxidative compounds may be of therapeutic and prophylactic importance regarding cardiovascular disease. Copper-chlorophyllin (Cu-CHL), a Cu(2+)-protoporphyrin IX complex, has been reported to inhibit lipid oxidation in biological membranes and liposomes. Hemin (Fe(3+)-protoporphyrin IX) has been shown to bind to LDL thereby inducing lipid peroxidation. As Cu-CHL has a similar structure as hemin, one may assume that Cu-CHL may compete with the hemin action on LDL. Therefore, in the present study Cu-CHL and the related compound magnesium-chlorophyllin (Mg-CHL) were examined in their ability to inhibit LDL oxidation initiated by hemin and other LDL oxidizing systems. LDL oxidation by hemin in presence of H(2)O(2) was strongly inhibited by both CHLs. Both chlorophyllins were also capable of effectively inhibiting LDL oxidation initiated by transition metal ions (Cu(2+)), human umbilical vein endothelial cells (HUVEC) and tyrosyl radicals generated by myeloperoxidase (MPO) in presence of H(2)O(2) and tyrosine. Cu- and Mg-CHL showed radical scavenging ability as demonstrated by the diphenylpicrylhydracylradical (DPPH)-radical assay and estimation of phenoxyl radical generated diphenyl (dityrosine) formation. As assessed by ultracentrifugation the chlorophyllins were found to bind to LDL (and HDL) in serum. The present study shows that copper chlorophyllin (Cu-CHL) and its magnesium analog could act as potent antagonists of atherogenic LDL modification induced by various oxidative stimuli. As inhibitory effects of the CHLs were found at concentrations as low as 1 mumol/l, which can be achieved in humans, the results may be physiologically/therapeutically relevant. PMID:16298745

Kapiotis, Stylianos; Hermann, Marcela; Exner, Markus; Laggner, Hilde; Gmeiner, Bernhard M K

2005-11-01

180

[Deactivation by SO2 of transition metal oxides modified low-temperature SCR catalyst for NOx reduction with NH3].  

PubMed

MnOx-CeOx/ACF catalyst was prepared by impregnation method, which exhibited high activity for low-temperature selective catalytic reduction of NOx over the temperature range 110-230 degrees C. Experiments results indicated that the catalyst yielded 80% NO conversion at 150 degrees C and 90% at 230 degrees C. The Oxides of Fe,Cu and V were added to the catalysts based on MnOx-CeOx/ACF. The additions of these transition metal oxides had a negative effect on the activity of the catalysts. Compared with MnOx-CeOx/ACF and Cu and V modified catalysts, NO conversion for Fe-MnOx-CeOx/ACF catalyst leveled off at nearly 75% in the first 6 h in the presence of SO2. Two mechanisms of catalyst deactivation by SO2 were discovered by the methods of X-ray photoelectron spectrum (XPS) and Fourier transform infrared spectra (FTIR), indicating that the catalysts were covered by ammonium sulfates and the metal oxides, acting as active components, were also sulfated by SO2 to form metal sulfates. PMID:19799275

Shen, Bo-xiong; Liu, Ting; Yang, Ting-ting; Xiong, Li-xian; Wang, Jing

2009-08-15

181

n-Type transition metal oxide as a hole extraction layer in PbS quantum dot solar cells.  

PubMed

The n-type transition metal oxides (TMO) consisting of molybdenum oxide (MoO(x)) and vanadium oxide (V(2)O(x)) are used as an efficient hole extraction layer (HEL) in heterojunction ZnO/PbS quantum dot solar cells (QDSC). A 4.4% NREL-certified device based on the MoO(x) HEL is reported with Al as the back contact material, representing a more than 65% efficiency improvement compared with the case of Au contacting the PbS quantum dot (QD) layer directly. We find the acting mechanism of the hole extraction layer to be a dipole formed at the MoO(x) and PbS interface enhancing band bending to allow efficient hole extraction from the valence band of the PbS layer by MoO(x). The carrier transport to the metal anode is likely enhanced through shallow gap states in the MoO(x) layer. PMID:21688813

Gao, Jianbo; Perkins, Craig L; Luther, Joseph M; Hanna, Mark C; Chen, Hsiang-Yu; Semonin, Octavi E; Nozik, Arthur J; Ellingson, Randy J; Beard, Matthew C

2011-06-27

182

Ion-plasma modification of the properties of anodic films of transition metal oxides  

NASA Astrophysics Data System (ADS)

We have studied the effect of high-frequency (13.56 MHz) capacitive discharge in argon on thin films of amorphous vanadium and niobium oxides grown by electrochemical oxidation in an electrolyte. The results of X-ray diffraction, spectrophotometric, and electrical measurements show that the plasma processing leads to modification of the electrical properties (growth in the electron conductivity), structure (crystallization), and optical characteristics (optical density variation in the short-wavelength range) of the oxide films.

Cheremisin, A. B.; Velichko, A. A.; Pergament, A. L.; Putrolainen, V. V.; Stefanovich, G. B.

2009-02-01

183

Crystal structure of double oxides of the perovskite type  

Microsoft Academic Search

The cell dimensions of a number of double oxides belonging to the perovskite type have been accurately determined from examination of high-angle lines on x-ray powder photographs. The structures found fall into groups, as follows:Cubic (ideal perovskite type). This includes SrTiO3, SrSnO3, SrZrO3, BaSnO3, BaZrO3, BaThO3; also BaTiO3 above 120C.Tetragonal. This includes the usual form of BaTiO3 at room temperature,

Helen D Megaw

1946-01-01

184

P-type conductive amorphous oxides of transition metals from solution processing  

NASA Astrophysics Data System (ADS)

We report a series of solution-processed p-type conductive amorphous Ln-M-O (a-Ln-M-O, where M = Ru, Ir, and Ln is a lanthanide element except Ce) having low resistivities (10-3 to 10-2 ? cm). These oxides are thermally stable to a high degree, being amorphous up to 800 C, and processable below 400 C. Their film surfaces are smooth on the atomic scale, and the process allows patterning simply by direct imprinting without distortion of the pattern after annealing. These properties have high potential for use in printed electronics. The electron configurations of these oxides are apparently different from existing p-type oxides.

Li, Jinwang; Kaneda, Toshihiko; Tokumitsu, Eisuke; Koyano, Mikio; Mitani, Tadaoki; Shimoda, Tatsuya

2012-07-01

185

Structural characterization of tellurite glasses doped with transition metal oxides using Raman spectra and ab initio calculations  

NASA Astrophysics Data System (ADS)

Systems of iron tellurite glasses were prepared by melt quenching with compositions of [85%TeO 2 + 5%Fe 2O 3 + 10%TMO], where transition metal oxides (TMO) are TiO 2, V 2O 5, MnO, CoO, NiO and CuO. Furthermore, the main structural units of these samples have been characterized by means of Raman spectra (150-1200 cm -1) as well as wavenumber predictions by means of Gaussian 98 ab initio calculations for the proposed site symmetries of TeO 44- triagonal bipyramid ( C2v) and Te 2O 76- bridged tetrahedra ( Cs and C1). Aided by normal coordinate analysis, calculated vibrational frequencies, Raman scattering activities, force constants in internal coordinates and potential energy distributions (PEDs), revised vibrational assignments for the fundamental modes have been proposed. The main structural features are correlated to the dominant units of triagonal bipyramid (tbp) or bridged tetrahedral (TeO 3+1 binds to TeO 3 through TeOTe bridge; corner sharing). Moreover, the Raman spectra of the investigated tellurites reflect a structural change from tbp (coordination number is four) to triagonal pyramidal (coordination number is three).

Mohamed, Tarek A.; Shaltout, I.; Al Yahyaei, K. M.

2006-05-01

186

X-ray absorption spectroscopy studies of spin-orbit coupling in 5d transition metal oxides.  

PubMed

In order to examine the effects of strong valence band spin-orbit coupling (SOC) in 5d transition metal oxides (TMOs), we have investigated the L(2) and L(3) edge white-line intensities of the x-ray absorption spectra of several 5d TMOs. The white-line intensities at both edges are found to decrease monotonously with increasing 5d electron occupancy, while their ratio showed anomalous behavior for late 5d TMOs (IrO(2), PtO(2), and Au(2)O(3)), deviating significantly from the theoretical value of 2 expected for the case of weak SOC. This observation serves as a clear experimental indication of strong SOC effects in 5d TMOs. We also discussed how the 5d TMOs can have charge transfer effects different from their counterpart 5d elemental metals by making comparative studies. Our works demonstrate the importance of j quantum states due to strong SOC in the 5d system. PMID:22227572

Cho, Deok-Yong; Park, Junghwan; Yu, Jaejun; Park, Je-Geun

2012-01-06

187

Epitaxial Growth and Properties of Doped Transition Metal and Complex Oxide Films  

SciTech Connect

The detailed science and technology of crystalline oxide film growth using vacuum methods is reviewed and discussed with an eye toward gaining fundamental insights into the relationships between growth process and parameters, film and interface structure and composition, and electronic, magnetic and photochemical properties. The topic is approached first from a comparative point of view based on the most widely used growth methods, and then on the basis of specific material systems that have generated very high levels of interest. Emphasis is placed on the wide diversity of structural, electronic, optical and magnetic properties exhibited by oxides, and the fascinating results that this diversity of properties can produce when combined with the degrees of freedom afforded by heteroepitaxy.

Chambers, Scott A.

2010-01-12

188

Oxygen vacancy ordering in transition-metal-oxide LaCoO3 films  

NASA Astrophysics Data System (ADS)

Oxygen vacancies in complex oxides affect the structure and the electronic and magnetic properties. Here we use atomically-resolved Z-contrast imaging, electron-energy-loss spectroscopy and densityfunctional calculations to demonstrate that ordered oxygen vacancies may act as the controlling degree of freedom for the structural, electronic, and magnetic properties of LaCoO3 thin films. We find that epitaxial strain is released through the formation of O vacancy superlattices. The O vacancies donate excess electrons to the Co d-states, resulting in ferromagnetic ordering. The appearance of Peierls-like minigaps followed by strain relaxation triggers a nonlinear rupture of the energy bands, which explains the observed insulating behavior. We conclude that oxygen vacancy ordering constitutes a degree of freedom that can be used to engineer novel behavior in complex-oxide films.

Biskup, Neven; Salafranca, Juan; Mehta, Virat; Suzuki, Yuri; Pennycook, Stephen; Pantelides, Sokrates; Varela, Maria

2013-03-01

189

Perovskites  

SciTech Connect

Less than three years ago discovered a material that was superconducting at higher temperatures had ever before been achieved. Since than other high temperature superconductors have been engineered that are efficient at temperatures of 100{degrees}K or more. Exactly why the materials are superconducting remains a mystery have shown that the new materials have this in common: they are structurally flawed members of a crystallographic family known as perovskites. Perovskites, are ceramics that have a particular atomic arrangement. They fascinate from a technological point of view as well because, as a group, natural and synthetic perovskites exhibit an array perovskites run the gamut from insulators to semiconductors, superionic conductors metal-like conductors and now high temperature superconductors. The flaws in the superconductors suggest the answer: slight modifications of the ideal perovskite architecture often result in new features. There is no one-to-one a given modification does not automatically produce a particular degree of electrical conductivity. Yet any time the ideal structure is altered, the possibility of new electrical-or other properties arises.

Hazen, R.M.

1988-06-01

190

Electrochromic phenomena in transition metal oxide thin films prepared by thermal evaporation  

Microsoft Academic Search

We present an experimental study on the electrochromic properties of MoO3, WO3 and mixed WO3-MoO3 thin films prepared by thermal evaporation. We have constructed symmetric and quasi-symmetric electrochromic cells incorporating\\u000a the evaporated oxide films as electrochromic layers. Li+ doped V2O5 films served as ion storage layers. The symmetric cells were found to exhibit significantly improved optical properties compared\\u000a to the

S. Papaefthimiou; G. Leftheriotis; P. Yianoulis

1998-01-01

191

Transition Metals and Other Forms of Oxidative Protein Damage in Renal Disease  

Microsoft Academic Search

\\u000a Oxidative and carbonyl stresses are dramatically increased in chronic renal disease, whereby an inverse relationship usually\\u000a exists between renal clearance and the accumulation of low molecular weight compounds ultimately responsible for the damage\\u000a to plasma constituents. Damage to proteins results from primary attack to protein residues by reactive oxygen species with\\u000a or without metal catalyst, or via myeloperoxidase and hypochlorous

Vincent M. Monnier; Ina Nemet; David R. Sell; Miriam F. Weiss

192

Efficient self-assembly of transition metal oxide nanoclusters on silicon substrates  

Microsoft Academic Search

Self-assembled monolayers (SAMs) of nanoclusters (<10 nm) on silicon wafer have been prepared via simply immersing the substrate into a FeCl3\\/HCl solution. The clusters are hydrous iron oxide resulting from the hydrolysis of FeCl3, and their deposition is driven by the electrostatic attraction between the clusters and the substrate, the surface groups of which are oppositely charged at suitable pH

Linyou Cao; Yang Chai; Pingjian Li; Zhiyong Shen; Jinlei Wu

2005-01-01

193

Electron magnetic resonance study of transition-metal magnetic nanoclusters embedded in metal oxides  

Microsoft Academic Search

Here, we report on the results of an electron magnetic resonance (EMR) study of a series of Ni\\/ZnO and Ni\\/gamma-Fe2O3 nanocomposites (NCs) to probe the resonance features of ferromagnetic (FM) Ni nanoclusters embedded in metal oxides. Interest in these NCs stems from the fact that they are promising for implementing the nonreciprocal functionality employed in many microwave devices, e.g., circulators.

Vincent Castel; Christian Brosseau

2008-01-01

194

Transition metal sulfide loaded catalyst  

DOEpatents

A zeolite based catalyst for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C.sub.2 + hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.

Maroni, Victor A. (Naperville, IL); Iton, Lennox E. (Downers Grove, IL); Pasterczyk, James W. (Westmont, IL); Winterer, Markus (Westmont, IL); Krause, Theodore R. (Lisle, IL)

1994-01-01

195

Fabrication of functional transition metal oxide and hydroxide used as catalysts and battery materials  

NASA Astrophysics Data System (ADS)

My research is focused on developing metal oxide and hydroxide nanomaterials which can be used as battery materials, organic transformation catalysts, and photocatalysts. This research involves studying ZnO with different morphologies as photocatalysts for phenol degradation, producing CuO as olefin epoxidation catalysts, developing V and Cu incorporated manganese oxides as cathode materials for Li-ion batteries, and fabricating alpha-nickel hydroxide for Li-air battery materials. The first part includes producing ZnO as a photocatalyst for phenol degradation. The goal of this study is the synthesis of ZnO with different morphologies using the solvothermal method. The influence of solvents has been studied in detail. Their properties and photocatalytic performances have been explored as well. The second part of the research is concerned with developing novel urchin-like CuO as an olefin epoxidation catalyst. The purpose of this study is to develop a new catalyst, CuO, for olefin epoxidation. The copper source and precipitators were optimized, and the possible self-assembly mechanism of the urchin-like morphology was proposed. The catalytic activity of CuO for olefin epoxidation was studied. The third part of this work includes developing V, Cu incorporated manganese oxide (V-Cu-OMS-2) as cathode materials for Li-ion batteries. The purpose of this project is to develop a new material with enhanced battery performance. V and Cu incorporated manganese oxide were developed using hydrothermal methods. Octahedral molecular sieve (OMS) materials show mixed valences of Mn 3+ and Mn4+, which produces novel properties in battery applications. Inexpensive starting materials make OMS materials more promising for commercial applications. How the incorporation of V and Cu affected OMS-2 materials was investigated in terms of their crystal structure, morphologies, and surface areas. The battery performance of the incorporated OMS-2 materials with different loading amounts of V and Cu was also studied. In the fourth part of this research, 3D flower-like alpha-nickel hydroxide with enhanced electrochemical activity was fabricated using a microwave-assisted hydrothermal method. The focus of this study is the synthesis of alpha-nickel hydroxide and its application for O2 reduction. The synthetic work focused on the preparation of flower-like alpha-nickel hydroxide using the microwave-assisted hydrothermal method. The alpha-nickel hydroxide shows superior electrochemical properties compared to those of the beta-form. However, it is difficult to make the alpha-form, since the structure of alpha-nickel hydroxide is unstable, and it prefers to transfer to the beta-form under basic conditions. In this study, flower-like alpha-nickel hydroxide was prepared using urea as the precipitating agent. The factors, which affected the formation of flower-like morphologies, have been investigated. The electrochemical activity of as-synthesized alpha-nickel hydroxide for oxygen reduction in an alkaline media was studied.

Xu, Linping

196

High Performance Ceramic Interconnect Material for Solid Oxide Fuel Cells (SOFCs): Ca- and Transition Metal-doped Yttrium Chromite  

SciTech Connect

The effect of transition metal substitution on thermal and electrical properties of Ca-doped yttrium chromite was investigated in relation to use as a ceramic interconnect in high temperature solid oxide fuel cells (SOFCs). 10 at% Co, 4 at% Ni, and 1 at% Cu substitution on B-site of 20 at% Ca-doped yttrium chromite led to a close match of thermal expansion coefficient (TEC) with that of 8 mol% yttria-stabilized zirconia (YSZ), and a single phase Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 remained stable between 25 and 1100 degree C over a wide oxygen partial pressure range. Doping with Cu significantly facilitated densification of yttrium chromite. Ni dopant improved both electrical conductivity and dimensional stability in reducing environments, likely through diminishing the oxygen vacancy formation. Substitution with Co substantially enhanced electrical conductivity in oxidizing atmosphere, which was attributed to an increase in charge carrier density and hopping mobility. Electrical conductivity of Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 at 900 degree C is 57 S/cm in air and 11 S/cm in fuel (pO2=510^-17 atm) environments. Chemical compatibility of doped yttrium chromite with other cell components was verified at the processing temperatures. Based on the chemical and dimensional stability, sinterability, and thermal and electrical properties, Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 is suggested as a promising SOFC ceramic interconnect to potentially overcome technical limitations of conventional acceptor-doped lanthanum chromites.

Yoon, Kyung J.; Stevenson, Jeffry W.; Marina, Olga A.

2011-10-15

197

Study of the L edges in the 3d transition metals and their oxides by electron-energy-loss spectroscopy with comparisons to theory  

Microsoft Academic Search

Excitations of the 2p subshell in the 3d transition metals and some of their oxides have been studied by inelastic scattering of 75-keV electrons. The L ''white lines'' which arise from dipole transitions to unoccupied d states have been investigated in terms of their threshold energies, widths, and intensity ratios. Shifts in the L threshold energy between the metal and

R. D. Leapman; L. A. Grunes; P. L. Fejes

1982-01-01

198

Carboxylate-substituted alumoxanes as processable precursors to transition metal-aluminum and lanthanide-aluminum mixed-metal oxides: Atomic scale mixing via a new transmetalation reaction  

Microsoft Academic Search

The preparation of carboxylate-alumoxanes of the formula, [Al(O)(OH){sub y}(OCR){sub z}]{sub n}, doped with transition metals or rare earths (M = Mn, La, Er, V, Ce) is described. Thermolysis of these compounds yields molecularly mixed homogeneous metal oxides. The precursors were characterized by IR and NMR spectroscopies and TG\\/DTA. The ceramic products were analyzed by SEM, ESR, microprobe analysis, and XRD.

Aivaras Kareiva; C. Jeff Harlan; D. Brent MacQueen; Ronald L. Cook; Andrew R. Barron

1996-01-01

199

Hydrolysis on transition metal oxide clusters and the stabilities of M-O-M bridges.  

PubMed

Water addition to molecular single, double and triple M-O-M bridges (M = Sc, Ti, V, Cr, and Mn) were considered, and the stabilities toward stepwise hydrolysis of the oxygen bridges were studied by means of quantum chemistry. The M-O bond distances for the studied systems were compared to experiment for demonstration of the applicability of the B3LYP functional to the investigated systems. While substantial exothermicities were found for the hydrolysis of double and triple M-O-M bridges, addition of water to a single bridge was generally found to be slightly endothermic. The lack of enthalpy drive for the (OH)yOxM-O-MOx(OH)y + H2O-->2MOx-1(OH)y+2 reaction was taken to suggest that entropy increase and the formation of mononuclear water complexe, would be decisive factors for the dissociation. A mechanism was proposed for the observed erosion of the protective chromium oxide scale on high-temperature alloys at elevated temperatures and high humidities, based on the formation of CrO2(OH)2(g). PMID:11196854

Johnson, J R; Panas, I

2000-07-24

200

Electron magnetic resonance study of transition-metal magnetic nanoclusters embedded in metal oxides  

NASA Astrophysics Data System (ADS)

Here, we report on the results of an electron magnetic resonance (EMR) study of a series of Ni/ZnO and Ni/?-Fe2O3 nanocomposites (NCs) to probe the resonance features of ferromagnetic (FM) Ni nanoclusters embedded in metal oxides. Interest in these NCs stems from the fact that they are promising for implementing the nonreciprocal functionality employed in many microwave devices, e.g., circulators. We observe that the EMR spectrum is strongly affected by the metallic FM content and its environment in the NC sample. We report the existence of broad and asymmetric features in the EMR spectra of these NCs. Our temperature dependent EMR data revealed larger linewidth and effective g factor, in the range of 2.1-3.7 (larger than the free electron value of ?2 ), for all samples as temperature is decreased from room temperature to 150K . The line broadening and asymmetry of the EMR features are not intrinsic properties of the metallic nanophase but reflect the local (nonmagnetic or magnetic) environment in which they are embedded. Furthermore, the results of a systematic dependence of the room temperature EMR linewidth and resonant field on the Ni content and the corresponding effective microwave losses measured in previous works show a remarkable correlation. This correlation has been attributed to the dipolar coupling between magnetic nanoparticles in the NCs.

Castel, Vincent; Brosseau, Christian

2008-04-01

201

Magnetic properties of 3d transition metal-phthalocyanine molecules on oxided Cu(110) surface  

NASA Astrophysics Data System (ADS)

After being extensively interesting topics in both fundamental researches and application practices for a decade, spintronics is now on its way of diversity. Molecular spintronics has attracted much attention in recent years, due to the accelerating miniaturization of electronic devices. In this work, based on first-principles calculations, we studied the electronic and magnetic properties of both isolated metal-phthalocyanines (commonly referred to as MPc) molecules (M=Mn, Fe and Co) and a single MPc molecule adsorbed on oxided Cu(110) [O-Cu(110)] surface. We find that the easy axis of FePc molecule switches from in-plane direction to perpendicular direction when it is adsorbed on O-Cu(110) surface. However, such a switch of direction of magnetization could not be observed for MnPc and CoPc molecules which are two neighbors of FePc with one electron less and more, respectively. Furthermore, we find that the magnetization of these MPc molecules on O-Cu(110) surface are rather stable, so they could not be altered by moderate hole/electron doping. The eg orbitals (dxz and dyz) of MPc molecules are found to be crucial for their magnetization on O-Cu(110) surface. Acknowledgement. This work was supported by DOE Grant DE-FG02-05ER46237.

Hu, Jun; Wu, Ruqian

2012-02-01

202

Designed synthesis of transition metal/oxide hierarchical peapods array with the superior lithium storage performance.  

PubMed

In this report, a novel hierarchical peapoded array with Co3O4 nanoparticles encapsulated in graphitized carbon fiber is introduced for the first time. The unique peapoded structure is suitable for the excellent anode in LIBs and demonstrates enhanced rate capability, cyclability and prolonged lifespan, e.g. the specific capacity can reach up to 1150?mAh/g. All the enhanced electrochemical performance is reasonably derived from the peapod-like and aligned conformation. Furthermore, due to the specialty of the structure and the versatility of Co3O4, the composite will find more applications in specific catalysis, biomedicine, electronics, optoelectronic engineering and gas sensing. The fabrication strategy developed here is also a rational and universal approach towards peapod-like architecture and has significantly widened the specific functional material domain we created before. In our design, more peapod-like aligned samples with various nanoparticles, e.g. oxides, phosphides, even nitrides, encapsulated in graphitized carbon fibers, have been lifted on the research agenda and the results will be presented soon. PMID:24056414

Zhang, Huijuan; Bai, Yuanjuan; Zhang, Yan; Li, Xiao; Feng, Yangyang; Liu, Qing; Wu, Kai; Wang, Yu

2013-09-23

203

Designed Synthesis of Transition Metal/Oxide Hierarchical Peapods Array with the Superior Lithium Storage Performance  

PubMed Central

In this report, a novel hierarchical peapoded array with Co3O4 nanoparticles encapsulated in graphitized carbon fiber is introduced for the first time. The unique peapoded structure is suitable for the excellent anode in LIBs and demonstrates enhanced rate capability, cyclability and prolonged lifespan, e.g. the specific capacity can reach up to 1150?mAh/g. All the enhanced electrochemical performance is reasonably derived from the peapod-like and aligned conformation. Furthermore, due to the specialty of the structure and the versatility of Co3O4, the composite will find more applications in specific catalysis, biomedicine, electronics, optoelectronic engineering and gas sensing. The fabrication strategy developed here is also a rational and universal approach towards peapod-like architecture and has significantly widened the specific functional material domain we created before. In our design, more peapod-like aligned samples with various nanoparticles, e.g. oxides, phosphides, even nitrides, encapsulated in graphitized carbon fibers, have been lifted on the research agenda and the results will be presented soon.

Zhang, Huijuan; Bai, Yuanjuan; Zhang, Yan; Li, Xiao; Feng, Yangyang; Liu, Qing; Wu, Kai; Wang, Yu

2013-01-01

204

Quantifying octahedral rotations in strained perovskite oxide films  

NASA Astrophysics Data System (ADS)

We have measured the oxygen positions in LaNiO3 films to elucidate the coupling between epitaxial strain and oxygen octahedral rotations. The oxygen positions are determined by comparing the measured and calculated intensities of half-order Bragg peaks, arising from the octahedral rotations. Combining ab initio density-functional calculations with these experimental results, we show how strain systematically modifies both bond angles and lengths in this functional perovskite oxide.

May, S. J.; Kim, J.-W.; Rondinelli, J. M.; Karapetrova, E.; Spaldin, N. A.; Bhattacharya, A.; Ryan, P. J.

2010-07-01

205

Giant magnetoresistance of manganese oxides with a layered perovskite structure  

Microsoft Academic Search

MANGANESE oxides with the cubic perovskite structure (typified by LaMnO3) have stimulated considerable interest because of their magnetoresistive properties1-9 they exhibit extremely large changes in electrical resistance in response to applied magnetic fields, a property that is of technological relevance for the development of magnetic memory and switching devices. But for such applications to be viable, great improvements will be

Y. Moritomo; A. Asamitsu; H. Kuwahara; Y. Tokura

1996-01-01

206

Giant magnetoresistance of manganese oxides with a layered perovskite structure  

NASA Astrophysics Data System (ADS)

MANGANESE oxides with the cubic perovskite structure (typified by LaMnO3) have stimulated considerable interest because of their magnetoresistive properties1-9 they exhibit extremely large changes in electrical resistance in response to applied magnetic fields, a property that is of technological relevance for the development of magnetic memory and switching devices. But for such applications to be viable, great improvements will be needed in both the sensitivity and temperature dependence of the magnetoresistive response. One approach under consideration for optimizing these properties is chemical substitution10. Here we demonstrate an alternative strategy, in which we synthesize layered variants of the cubic perovskite parent compounds that have a controlled number of MnO2 sheets per unit cell. This strategy is structurally analogous to that employed for the systematic exploration of the high-transition-temperature copper oxide superconductors11. We find that the magneto-resistive properties of these materials depend sensitively on the dimensionality of the manganese oxide lattice. Although the properties of our materials are still far from optimal, further exploration of this series of layered perovskites may prove fruitful.

Moritomo, Y.; Asamitsu, A.; Kuwahara, H.; Tokura, Y.

1996-03-01

207

Method for dry etching of transition metals  

DOEpatents

A method for dry etching of transition metals. The method for dry etching of a transition metal (or a transition metal alloy such as a silicide) on a substrate comprises providing at least one nitrogen- or phosphorous-containing .pi.-acceptor ligand in proximity to the transition metal, and etching the transition metal to form a volatile transition metal/.pi.-acceptor ligand complex. The dry etching may be performed in a plasma etching system such as a reactive ion etching (RIE) system, a downstream plasma etching system (i.e. a plasma afterglow), a chemically-assisted ion beam etching (CAIBE) system or the like. The dry etching may also be performed by generating the .pi.-acceptor ligands directly from a ligand source gas (e.g. nitrosyl ligands generated from nitric oxide), or from contact with energized particles such as photons, electrons, ions, atoms, or molecules. In some preferred embodiments of the present invention, an intermediary reactant species such as carbonyl or a halide ligand is used for an initial chemical reaction with the transition metal, with the intermediary reactant species being replaced at least in part by the .pi.-acceptor ligand for forming the volatile transition metal/.pi.-acceptor ligand complex.

Ashby, Carol I. H. (Edgewood, NM); Baca, Albert G. (Albuquerque, NM); Esherick, Peter (Albuquerque, NM); Parmeter, John E. (Albuquerque, NM); Rieger, Dennis J. (Tijeras, NM); Shul, Randy J. (Albuquerque, NM)

1998-01-01

208

Method for dry etching of transition metals  

DOEpatents

A method for dry etching of transition metals is disclosed. The method for dry etching of a transition metal (or a transition metal alloy such as a silicide) on a substrate comprises providing at least one nitrogen- or phosphorus-containing {pi}-acceptor ligand in proximity to the transition metal, and etching the transition metal to form a volatile transition metal/{pi}-acceptor ligand complex. The dry etching may be performed in a plasma etching system such as a reactive ion etching (RIE) system, a downstream plasma etching system (i.e. a plasma afterglow), a chemically-assisted ion beam etching (CAIBE) system or the like. The dry etching may also be performed by generating the {pi}-acceptor ligands directly from a ligand source gas (e.g. nitrosyl ligands generated from nitric oxide), or from contact with energized particles such as photons, electrons, ions, atoms, or molecules. In some preferred embodiments of the present invention, an intermediary reactant species such as carbonyl or a halide ligand is used for an initial chemical reaction with the transition metal, with the intermediary reactant species being replaced at least in part by the {pi}-acceptor ligand for forming the volatile transition metal/{pi}-acceptor ligand complex.

Ashby, C.I.H.; Baca, A.G.; Esherick, P.; Parmeter, J.E.; Rieger, D.J.; Shul, R.J.

1998-09-29

209

Autothermal reforming catalyst having perovskite structure  

DOEpatents

The invention addressed two critical issues in fuel processing for fuel cell application, i.e. catalyst cost and operating stability. The existing state-of-the-art fuel reforming catalyst uses Rh and platinum supported over refractory oxide which add significant cost to the fuel cell system. Supported metals agglomerate under elevated temperature during reforming and decrease the catalyst activity. The catalyst is a perovskite oxide or a Ruddlesden-Popper type oxide containing rare-earth elements, catalytically active firs row transition metal elements, and stabilizing elements, such that the catalyst is a single phase in high temperature oxidizing conditions and maintains a primarily perovskite or Ruddlesden-Popper structure under high temperature reducing conditions. The catalyst can also contain alkaline earth dopants, which enhance the catalytic activity of the catalyst, but do not compromise the stability of the perovskite structure.

Krumpel, Michael (Naperville, IL); Liu, Di-Jia (Naperville, IL)

2009-03-24

210

Synthesis and characterization of three-dimensional transition metal ions doped zinc oxide based dilute magnetic semiconductor thin films  

Microsoft Academic Search

Dilute magnetic semiconductors (DMS), especially 3d-transition metal (TM) doped ZnO based DMS materials are the most promising candidates for optoelectronics and spintronics applications; e.g. in spin light emitting diode (SLED), spin transistors, and spin field effect transistors (SFET), etc. In the present dissertation, thin films of Zn1-xTMxO (TM = Co2+, Cu2+, and Mn2+) were grown on (0001) oriented Al2O3 substrates

Kousik Samanta

2009-01-01

211

Molten salt synthesis of complex perovskite-related dielectric oxides  

Microsoft Academic Search

Perovskite related dielectric oxides of the formula Ba(Zn1\\/3Nb2\\/3)O3, Ba(Mg1\\/3Nb2\\/3)O3 and Ba(Zn1\\/3Ta2\\/3)O3 have been synthesized by the NaClKCl molten salt (flux) method at low temperatures of 900C for 3 or 6h. Sintering at 1000C retained the cubic structure. Sintering at 1300C generated minor amount (?10%) of Ba5Nb4O15 in addition to the main cubic phase for the niobium compounds. The tantalum compounds,

M. Thirumal; P. Jain; A. K. Ganguli

2001-01-01

212

Oligocyclopentadienyl transition metal complexes  

SciTech Connect

Synthesis, characterization, and reactivity studies of oligocyclopentadienyl transition metal complexes, namely those of fulvalene, tercyclopentadienyl, quatercyclopentadienyl, and pentacyclopentadienyl(cyclopentadienyl) are the subject of this account. Thermal-, photo-, and redox chemistries of homo- and heteropolynuclear complexes are described.

de Azevedo, Cristina G.; Vollhardt, K. Peter C.

2002-01-18

213

Transition-metal-free oxidative coupling reactions for the formation of C-C and C-N bonds mediated by TEMPO and its derivatives.  

PubMed

The application of nitroxides for the development of new synthetic methods and their implementation in polymer chemistry, material science and beyond is one of the major research topics in our laboratory in the institute of organic chemistry at the WWU Mnster. This short review focuses on our recent progress towards nitroxide-based transition-metal-free oxidative coupling reactions. The demand for organic surrogates for transition metals in such transformations is in our eyes unquestionable, since environmental and economic issues have become progressively more important in recent years. For this purpose, the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) is shown to be a highly efficient oxidant for the homo- and cross-coupling of Grignard reagents. This powerful C-C bond forming strategy allows the generation of conjugated polymers from bifunctional Grignard reagents. Moreover, cross-coupling of alkynyl Grignard compounds and nitrones can be accomplished under aerobic atmosphere with catalytic amounts of TEMPO. It is also shown that TEMPO-derived N-oxoammonium salts can act as suitable oxidants for formation of C-N bonds between non-preactivated benzoxazoles and secondary amines under metal-free conditions. PMID:22871285

Murarka, Sandip; Wertz, Sebastian; Studer, Armido

2012-01-01

214

Oxide-Ion Conduction in Anion-Deficient Perovskites.  

NASA Astrophysics Data System (ADS)

Strategies for the design of an improved oxide -ion electrolyte for medium-temperature fuel cells, sensors, and oxygen pumps are outlined. Anion-deficient oxides with the perovskite structure were chosen for study. Stoichiometric oxides contain a high fraction of vacancies, but vacancy ordering suppresses ionic conductivity. Nevertheless, the brownmillerite rm Ba_2In_2O _5 undergoes an order-disorder transition at 930^circC that allows an estimate of the motional enthalpy of the disordered vacancy. Substitution of both aliovalent and isovalent cations were investigated as a means of suppressing the long-range ordering. Substitution of Nb^{5+}, Ti ^{4+}, Sn^{4+ } could be made for In^{3+ } at the B sites of the ABO_ {3-delta} perovskite structure, and this strategy did suppress long-range ordering of the oxygen vacancies for more than 10 mole % dopant. However, it was found that these dopants have a sufficient preference for octahedral anion coordination that water is introduced into the vacancies below 400^circ C; the protons introduced with the water became dominant conductive species below 400^circ C. At high temperature, the oxide-ion conduction is dominant and, below 930^circC, is enhanced by two orders of magnitude for about 20% dopant whether quadravalent or pentavalent dopants are used. Substitution of La^{3+} for Ba ^{2+} suppressed the introduction of water at low temperature, and the best oxide-ion conduction was achieved in rm Ba_{0.6}La _{0.4}InO_{2.7} isovalent doping of Y for In in rm Ba_2In _2O_5<=ad to the discovery of an extremely high ionic conduction in rm Ba _3Y_4O_9 stabilized at room temperature by quenching from above 1200^ circC. On heating in air, it first decomposes into rm Y_2O_3 and a Ba(OH)_2-BaO_2 mix that becomes an immobilized melt and a fast ionic conductor above 370^circC. With time in air, the melt is transformed to BaCO_3 , which is not conductive. Superior oxide-ion conduction was discovered in the Sr and Mg doped perovskite LaGaO _3. Short-range order below 600 ^circC was evident in the temperature dependence of the conductivity, which had a value of 2.8 times 10^{-2 } S/cm at 650^circC in rm La_{0.9}Sr_ {0.1}Ga_{0.8}Mg_{0.2 }O_{2.85}.

Feng, Man

1995-01-01

215

Encapsulation of Transition Metals into Carbon Nanoclusters  

Microsoft Academic Search

\\u000a Fourteen transition metals were co-deposited with carbon in the conventional arc discharge. A composite anode that contained\\u000a the transition metal or its oxide stuffed into central bores of the graphite rods was used. Both soot sample at the reactor\\u000a walls and slag at the cathode were characterized using scanning and transmission electron microscopy, and x-ray diffraction.\\u000a Encapsulation occurs mainly in

Supapan Seraphin

216

Powder Diffraction Studies of Phase Transitions in Manganese Perovskites  

Microsoft Academic Search

The results of recent structural studies of some Manganese perovskites are presented, in particular oxides in the system Ca1-xSrxMnO3 and SrRu0.5Mn0.5O3. In the first series we firstly show the power of synchrotron X-ray powder diffraction to refine accurate and precise structures for oxides containing first row transition metals and then show the presence of a direct orthorhombic Pbnm to tetragonal

B. J. Kennedy

2008-01-01

217

Large-scale, hot-filament-assisted synthesis of tungsten oxide and related transition metal oxide nanowires.  

PubMed

A scalable and versatile method for the large-scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates, including amorphous quartz and fluorinated tin oxide, is reported. The synthesis involves the chemical-vapor transport of metal oxide vapor-phase species using air or oxygen flow over hot filaments onto substrates kept at a distance. The results show that the density of the nanowires can be varied from 10(6)-10(10) cm(-2) by varying the substrate temperature. The diameter of the nanowires ranges from 100-20 nm. The results also show that variations in oxygen flow and substrate temperature affect the nanowire morphology from straight to bundled to branched nanowires. A thermodynamic model is proposed to show that the condensation of WO(2) species primarily accounts for the nucleation and subsequent growth of the nanowires, which supports the hypothesis that the nucleation of nanowires occurs through condensation of suboxide WO(2) vapor-phase species. This is in contrast to the expected WO(3) vapor-phase species condensation into WO(3) solid phase for nanoparticle formation. The as-synthesized nanowires are shown to form stable dispersions compared to nanoparticles in various organic and inorganic solvents. PMID:17415736

Thangala, Jyothish; Vaddiraju, Sreeram; Bogale, Rahel; Thurman, Ryan; Powers, Trevor; Deb, Biswapriya; Sunkara, Mahendra K

2007-05-01

218

Catalytic hydrogen peroxide decomposition on La1-xSrxCo03-d perovskite oxides  

Microsoft Academic Search

Lanthanide perovskite oxides are mentioned as material for hydrogen peroxide sensor because they can catalytically decompose hydrogen peroxide in an aqueous medium. The catalytic properties of these perovskite oxides to hydrogen peroxide are suggested due to their oxygen vacancies influenced by the oxide non-stoichiometry. In this paper, we investigate the catalytic hydrogen peroxide decomposition of a La\\/sub 1-x\\/Sr\\/sub x\\/CoO\\/sub 3-\\/spl

Van-Ahn. T. Dam; W. Olthuis; P. Bergveld; Berg van den A

2005-01-01

219

Catalytic hydrogen peroxide decomposition on La1-xSrxCoO3-? perovskite oxide  

Microsoft Academic Search

Lanthanide perovskite oxides are mentioned as material for hydrogen peroxide sensor because they can catalytically decompose hydrogen peroxide in an aqueous medium. The catalytic properties of these perovskite oxides to hydrogen peroxide are suggested due to their oxygen vacancies influenced by the oxide non-stoichiometry. In this paper, we investigate the catalytic hydrogen peroxide decomposition of a La1-xSrxCoO3-? thin film with

V.-A. T. Dam; W. Olthuis; P. Bergveld; A. van den Berg

2005-01-01

220

Consistent LDA' + DMFT approach to the electronic structure of transition metal oxides: Charge transfer insulators and correlated metals  

NASA Astrophysics Data System (ADS)

We discuss the recently proposed LDA' + DMFT approach providing a consistent parameter-free treatment of the so-called double counting problem arising within the LDA + DMFT hybrid computational method for realistic strongly correlated materials. In this approach, the local exchange-correlation portion of the electron-electron interaction is excluded from self-consistent LDA calculations for strongly correlated electronic shells, e.g., d-states of transition metal compounds. Then, the corresponding double-counting term in the LDA' + DMFT Hamiltonian is consistently set in the local Hartree (fully localized limit, FLL) form of the Hubbard model interaction term. We present the results of extensive LDA' + DMFT calculations of densities of states, spectral densities, and optical conductivity for most typical representatives of two wide classes of strongly correlated systems in the paramagnetic phase: charge transfer insulators (MnO, CoO, and NiO) and strongly correlated metals (SrVO3 and Sr2RuO4). It is shown that for NiO and CoO systems, the LDA' + DMFT approach qualitatively improves the conventional LDA + DMFT results with the FLL type of double counting, where CoO and NiO were obtained to be metals. Our calculations also include transition-metal 4 s-states located near the Fermi level, missed in previous LDA + DMFT studies of these monoxides. General agreement with optical and the X-ray experiments is obtained. For strongly correlated metals, the LDA' + DMFT results agree well with the earlier LDA + DMFT calculations and existing experiments. However, in general, LDA' + DMFT results give better quantitative agreement with experimental data for band gap sizes and oxygen-state positions compared to the conventional LDA + DMFT method.

Nekrasov, I. A.; Pavlov, N. S.; Sadovskii, M. V.

2013-04-01

221

Direct decomposition of nitric oxide over perovskite-type catalysts  

Microsoft Academic Search

Direct decomposition of nitric oxide over three selected perovskites, both in the absence and the presence of oxygen added to the reaction gas mixture, was investigated at a steady state under a wide range of experimental conditions in a plug-flow reactor with 1g catalyst. Temperatures between 723 and 923K were covered. The catalysts (La0.87Sr0.13Mn0.2Ni0.8O3??, La0.66Sr0.34Ni0.3Co0.7O3??, and La0.8Sr0.2Cu0.15Fe0.85O3??) were prepared with

C Tofan; D Klvana; J Kirchnerova

2002-01-01

222

Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution.  

PubMed

The electronic structure of transition metal oxides governs the catalysis of many central reactions for energy storage applications such as oxygen electrocatalysis. Here we exploit the versatility of the perovskite structure to search for oxide catalysts that are both active and stable. We report double perovskites (Ln0.5Ba0.5)CoO3-? (Ln=Pr, Sm, Gd and Ho) as a family of highly active catalysts for the oxygen evolution reaction upon water oxidation in alkaline solution. These double perovskites are stable unlike pseudocubic perovskites with comparable activities such as Ba0.5Sr0.5Co0.8Fe0.2O3-? which readily amorphize during the oxygen evolution reaction. The high activity and stability of these double perovskites can be explained by having the O p-band centre neither too close nor too far from the Fermi level, which is computed from ab initio studies. PMID:24042731

Grimaud, Alexis; May, Kevin J; Carlton, Christopher E; Lee, Yueh-Lin; Risch, Marcel; Hong, Wesley T; Zhou, Jigang; Shao-Horn, Yang

2013-09-17

223

Oxidation Reactions of Ethane over Ba-Ce-O Based Perovskites  

SciTech Connect

Ethane oxidation reactions were studied over pure and Ca-, Mg-, Sr-, La-, Nd-, and Y-substituted BaCeO{sub 3} perovskites under oxygen limited conditions. Several of the materials, notably the Ca- and Y-substituted materials, show activity for complete oxidation of the hydrocarbon to CO{sub 2} at temperatures below 650 C. At higher temperatures, the oxidative dehydrogenation (ODH) to ethylene becomes significant. Conversions and ethylene yields are enhanced by the perovskites above the thermal reaction in our system in some cases. The perovskite structure is not retained in the high temperature reaction environment. Rather, a mixture of carbonates and oxides is formed. Loss of the perovskite structure correlates with a loss of activity and selectivity to ethylene.

Miller, James E.; Sault, Allen G.; Trudell, Daniel E.; Nenoff, Tina M.; Thoma, Steven G.; Jackson, Nancy B.

1999-08-18

224

Copper catalysts for soot oxidation: alumina versus perovskite supports.  

PubMed

Copper catalysts prepared using four supports (Mg- and Sr-modified Al2O3 and MgTiO3 and SrTiO3 perovskites) have been tested for soot oxidation by 02 and NOx/O2. Among the catalysts studied, Cu/SrTiO3 is the most active for soot oxidation by NOx/O2 and the support affects positively copper activity. With this catalyst, and under the experimental conditions used, the soot combustion by NOx/O2 presents a considerable rate from 500 degrees C (100 degrees C below the uncatalysed reaction). The Cu/ SrTiO3 catalyst is also the most effective for NOx chemisorption around 425 degrees C. The best activity of Cu/SrTiO3 can be attributed to the improved redox properties of copper originated by Cu-support interactions. This seems to be related to the presence of weakly bound oxygen on this sample. The copper species present in the catalyst Cu/SrTiO3 can be reduced more easily than those in other supports, and for this reason, this catalyst seems to be the most effective to convert NO into NO2, which explains its highest activity for soot oxidation. PMID:18983091

Lpez-Surez, F E; Bueno-Lpez, A; Illn-Gmez, M J; Adamski, A; Ura, B; Trawczynski, J

2008-10-15

225

Microstructure of perovskite oxide thin films grown on miscut\\/small lattice-mismatched substrates  

Microsoft Academic Search

Perovskite oxides exhibit a wide range of physical properties, such as high dielectric, piezoelectric, pyroelectric, ferroelectric\\/multiferroic, non-linear optical and high temperature superconducting properties. The unique combination of these properties makes possible the use of perovskite oxide thin films as core elements in the next generation devices for sensors and electric\\/electronic\\/optical circuits. Recent studies demonstrate that it is possible to tailor

Yanbin Chen

2008-01-01

226

Precipitation selectivity of perovskite phase from Ti-bearing blast furnace slag under dynamic oxidation conditions  

Microsoft Academic Search

The effects of dynamic oxidations on the crystal morphology and the precipitation behavior of the perovskite phase from Ti-bearing blast furnace slag were investigated. Air was blown into the molten slag as an oxygen source through a lance during the dynamic oxidation process. It was found that the dispersed Ti components were selectively taken into the perovskite phase (CaTiO3), and

L. Zhang; M. Y. Wang; G. Q. Li; Z. T. Sui

2007-01-01

227

Electronic structure of lanthanum perovskites with 3d transition elements  

Microsoft Academic Search

The systematics of the electronic structure of LaBO perovskite oxides, where the B element scans the 3d transition-metal series from Ti to Co, are examined. X-ray photoelectron spectra of valence bands and shallow core states are presented and compared with theoretical molecular cluster and free-ion multiplet models. Self-consistent ionic configurations are obtained from the embedded cluster calculations, which differ from

D. J. Lam; B. W. Veal; D. E. Ellis

1980-01-01

228

Modeling the global distribution of the oxygen isotopic composition of sulfate aerosols: Importance of transition metal catalyzed S(IV) oxidation chemistry  

NASA Astrophysics Data System (ADS)

The oxygen isotopic composition of sulfate aerosols (?17O ~ ?&&17O 0.5*?18O) reflects the relative importance of different photochemical oxidation pathways in the atmosphere. Simulated isotopic variability in a global chemical transport model (GEOS-Chem) shows good agreement with observations in oceanic [Alexander et al., 2005] and some continental sites. However, a large discrepancy exists between modeled and measured isotopic composition in the high northern latitudes, reflecting an incomplete understanding of the sulfur budget in this region. Recent oxygen isotope measurements of sulfate aerosols collected at Alert, Canada suggest that transition metal catalyzed oxidation of SO2 by O2 in the aqueous-phase is significant during winter [Mc Cabe et al.,2006]. Global chemistry models ignore this oxidation pathway because it is believed to be important only regionally, and because of the large uncertainties in atmospheric metal concentrations and oxidation states. We have incorporated Fe(III) and Mn(II) catalyzed oxidation of S(IV) (S(IV) = SO2H2O + HSO3- + SO32-) by O2 into the GEOS-Chem model using the McCabe et al. [2006] isotope measurements as a constraint. We will examine the importance of this oxidation pathway for the sulfur budget in the Arctic, and on the global scale. Preliminary results suggest that, during winter, up to 75% of aerosol sulfate at Alert forms via the metal catalysis pathway. The addition of this chemical pathway decreases the SO2 burden in the Arctic (north of 60N) by 40% due to an increase in the oxidation rate. The comparison of large-scale sulfate aerosol models study (COSAM) showed that on average, models over-predict SO2 mixing ratios by factors of 2 or more [Barrie et al., 2001]. This "missing" S(IV) oxidation pathway can partially explain this discrepancy.

Alexander, B.; Park, R. J.

2006-12-01

229

Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides  

Microsoft Academic Search

Oxygen-ion conduction in transition-metal oxides is exploited in, for example, electrolytes in solid-oxide fuel cells and oxygen-separation membranes, which currently work at high temperatures. Conduction at low temperature is a key to developing further utilization, and an understanding of the structures that enable conduction is also important to gain insight into oxygen-diffusion pathways. Here we report the structural changes observed

Satoru Inoue; Masanori Kawai; Noriya Ichikawa; Hiroshi Kageyama; Werner Paulus; Yuichi Shimakawa

2010-01-01

230

Dynamical mean-field theory for perovskites  

SciTech Connect

Using the Hubbard Hamiltonian for transition-metal 3{ital d} and oxygen 2{ital p} states with perovskite geometry, we present a dynamical mean-field theory that becomes exact in the limit of large coordination numbers or equivalently large spatial dimensions {ital D}. The theory is based on a description of these systems for large {ital D} using a selective treatment of different hopping processes, which cannot be generated by a unique scaling of the hopping element. The model is solved using a perturbational approach and an extended noncrossing approximation. We discuss the origin of the various 3{ital d} and 2{ital p} bands, the doping dependence of its spectral weight, and the evolution of quasiparticles at the Fermi level upon doping, leading to interesting insight into the dynamical character of the charge carriers near the metal insulator instability of transition-metal oxide systems, three-dimensional perovskites, and other strongly correlated transition-metal oxides. {copyright} {ital 1996 The American Physical Society.}

Lombardo, P. [Laboratoire d`Etudes des Proprietes Electroniques des Solides--CNRS, Boite Postale 166, 38042 Grenoble Cedex 9 (France); Schmalian, J. [Institut fuer Theoretische Physik, Freie Universitaet Berlin, Arnimallee 14, 14195 Berlin (Germany); Avignon, M. [Laboratoire d`Etudes des Proprietes Electroniques des Solides--CNRS, Boite Postale 166, 38042 Grenoble Cedex 9 (France); Bennemann, K. [Institut fuer Theoretische Physik, Freie Universitaet Berlin, Arnimallee 14, 14195 Berlin (Germany)

1996-08-01

231

Structural and dynamic properties of oxygen vacancies in perovskite oxides--analysis of defect chemistry by modern multi-frequency and pulsed EPR techniques.  

PubMed

Multi-frequency and pulsed electron paramagnetic resonance (EPR) provides a sensitive spectroscopic tool to elucidate the defect structure of transition-metal doped perovskite oxides, as well as to monitor dynamic processes of oxygen vacancies in these materials. In this regard, high-frequency EPR spectrometers and pulsed EPR techniques such as the hyperfine sublevel correlation experiment (HYSCORE) may now routinely be used for dedicated investigations, providing considerably more insight than the application of standard continuous-wave EPR. Recent results include the formation of defect complexes between acceptor-type transition-metal centers with either one or two oxygen vacancies for the reason of charge compensation. Furthermore, such defect complexes follow the domain switching upon poling ferroelectric compounds with correspondingly high electric fields. On the other hand, multi-valent manganese functional centers provide trapping centers for electronic and ionic charge carriers (e', ) such that valency altered acceptor states or defect complexes are formed. Additionally, the trapping of charge carriers at the intrinsic 'reduced' B-site ions, and , can be observed by means of EPR spectroscopy. PMID:21082136

Eichel, Rdiger-A

2010-11-17

232

Intermediate-Valence Behavior in the Transition-Metal Oxide CaCu3Ru4O12  

NASA Astrophysics Data System (ADS)

A detailed study of the electronic properties of strongly correlated CCRO will be presented. Along with transport, \\chi(T) and specific heat, we focus on NMR/NQR. CCRO is a metallic system with perovskite structure showing non-Fermi-liquid behavior below 2&,slash;K which is inferred from Cp/&,slash;T? -ln(T) and a deviation of 1/&,slash;T1 (T) from the Korringa law at the copper site [PRB {78}, 165126 (2008)]. On closer inspection, a volume change could be detected by neutron diffraction which comes along with a corresponding anomaly in the specific heat around 150&,slash;K. Furthermore, the 99Ru Knight shift shows a cross-over between paramagnetic behavior of localized moments at high T and itinerant band states at low T, respectively. Complementary density-functional calculations (LDA+DFT) relate these phenomena to the ruthenium d-electron number. We conclude that dynamic charge fluctuations originating from the strong electronic correlations are present in CCRO and give rise to the intermediate valence of the ruthenium ions [PRB {80}, 121101(R) (2009)].

Kraetschmer, Wolfgang; Buettgen, Norbert; Loidl, Alois; Scheidt, Ernst-Wilhelm

2010-03-01

233

Plasma assisted oxidation of perovskites for forming high temperature superconductors using inductively coupled discharges  

Microsoft Academic Search

This patent describes a method for plasma assisted oxidation of a perovskite material. It comprises placing the perovskite material in a toroidally shaped inductively coupled plasma reactor; operating the plasma reactor; operating the plasma reactor with an alternating current power supply at a frequency below 5 MHz; operating the plasma reactor with a power density between 0.1 and 10 W\\/cm³;

M. Borden; K. Daniell; M. Magida; C. Zarowin

1992-01-01

234

Dense perovskite membrane reactors for partial oxidation of methane to syngas  

Microsoft Academic Search

The partial oxidation of methane to synthesis gas (syngas, CO + H) was performed in a mixed-conducting perovskite dense membrane reactor at 850 C, in which oxygen was separated from air and simultaneously fed into the methane stream. Steady-state oxygen permeation rates for La{sub 1-x}AFe{sub 0.8} CO{sub 0.2}O{sub 3-δ} perovskite membranes in nonreacting air\\/helium experiments were in the order of

Chung-Yi Tsai; Anthony G. Dixon; William R. Moser; Yi Hua Ma

1997-01-01

235

Oxide Spintronics  

Microsoft Academic Search

Concomitant with the development of metal-based spintronics in the late 1980s and 1990s, important advances were made on the growth of high-quality oxide thin films and heterostructures. While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition-metal oxides and, notably, mixed-valence manganites. The discovery of

Manuel Bibes; Agns Barthelemy

2007-01-01

236

Chemical control of the properties of perovskite oxides  

NASA Astrophysics Data System (ADS)

Perovskite oxides show a variety of interesting properties that can be tuned by chemical control. In this talk, I will present three examples of how such approach can be used to study the nature of functional properties in perovskites: (1) RMnO3 (R=rare earth) show a variety of unusual states, including the spiral spin ordering and ferroelectricity in R=Tb and Dy. In [1], R=Ho-Lu have been obtained under high pressure, and their magnetic and structural properties have been studied. Combined with the data on larger R, the results show the importance of competing magnetic interactions on the complex phase diagram of RMnO3. (2) RCoO3 show a spin-state transition and an insulator-metal transition as a function of temperature. The nature of the excited states has been studied since the 1950's, but remains elusive. Here [2], I provide the complete electronic phase diagram of RCoO3 that has been obtained from high-pressure synthesis and heat capacity measurements. The results support a picture involving a high-spin state above the spin-state transition and an intermediate-spin state above the insulator-metal transition. (3) Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) is a relaxor ferroelectric system with extraordinary dielectric and piezoelectric properties. The average structure of the system changes from cubic to rhombohedral, monoclinic, and tetragonal with x. However, this system is also characterized by nanoscale phase inhomogeneities, and the role of polar nanoregions on the enhanced properties is not clear. Here [3], I will show that thermal conductivity and heat capacity of PMN-xPT show a systematic evolution from glasslike to crystalline behavior as a function of x. The results provide interesting perspectives on how polar nanoregions are transformed into macroscopic polarizations with increasing x. [4pt] [1] M. Tachibana et al., Phys. Rev. B 75, 144425 (2007). [0pt] [2] M. Tachibana et al., Phys. Rev. B 77, 094402 (2008). [0pt] [3] M. Tachibana et al., Phys. Rev. B 79, 100104(R) (2009).

Tachibana, Makoto

2010-03-01

237

Oxidations of Organic and Inorganic Substrates by Superoxo-, hydroperoxo-, and oxo-compounds of the transition metals.  

SciTech Connect

Chapters 1 and 2 dealt with the chemistry of superoxo-, hydroperoxo-, and oxo- complexes of chromium, rhodium and cobalt. Chapter 3 dealt with the mechanism of oxygen-atom transfer catalyzed by an oxo-complex of rhenium. In Chapter 1, it was shown that hydroperoxometal complexes of cobalt and rhodium react with superoxochromium and chromyl ions, generating reduced chromium species while oxidizing the hydroperoxometal ions to their corresponding superoxometal ions. It was shown that the chromyl and superoxochromium ions are the more powerful oxidants. Evidence supports hydrogen atom transfer from the hydroperoxometal ion to the oxidizing superoxochromium or chromyl ion as the reaction mechanism. There is a significant H/D kinetic isotope effect. Comparisons to the rate constants of other known hydrogen atom transfer reactions show the expected correlation with bond dissociation energies. In Chapter 2, it was found that the superoxometal complexes Cr{sub aq}OO{sup 2+} and Rh(NH{sub 3}){sub 4}(H{sub 2}O)OO{sup 2+} oxidize stable nitroxyl radicals of the TEMPO series with rate constants that correlate with the redox potentials of both the oxidant and reductant. These reactions fit the Marcus equation for electron transfer near the theoretical value. Acid catalysis is important to the reaction, especially the thermodynamically limited cases involving Rh(NH{sub 3}){sub 4}(H{sub 2}O)OO{sup 2+} as the oxidant. The rate constants are notably less than those measured in the reaction between the same nitroxyl radicals and other strong free-radical oxidants, an illustration of the delocalized and stabilized nature of the superoxometal ions. Chapter 3 showed that oxo-rhenium catalysts needed a nucleophile to complete the catalytic oxygen-atom transfer from substituted pyridine-N-oxides to triphenylphosphine. The reaction was studied by introducing various pyridine-derived nucleophiles and monitoring their effect on the rate, then fitting the observed rate constants to the Hammett correlation. It was found that the values of the Hammett reaction constant PN were -1.0(1) for 4-nitro-2-methylpyridine-N-oxide and -2.6(4) for 4-methylpyridine-N-oxide as substrates. The negative value confirms pyridine is acting as a nucleophile. Nucleophiles other than pyridine derivatives were also tested. In the end, it was found that the most effective nucleophiles were the pyridine-N-oxides themselves, meaning that a second equivalent of substrate serves as the most efficient promoter of this oxygen-atom transfer reaction. This relative nucleophilicity of pyridines and pyridine-N-oxides is similar to what is observed in other OAT reactions generating high-valent metal-oxo species.

Michael John Vasbinder

2006-12-12

238

Hydrocracking and hydroisomerization of long-chain alkanes and polyolefins over metal-promoted anion-modified transition metal oxides  

DOEpatents

A method of cracking a feedstock by contacting the feedstock with a metal-promoted anion-modified metal oxide catalyst in the presence of hydrogen gas. The metal oxide of the catalyst is one or more of ZrO.sub.2, HfO.sub.2, TiO.sub.2 and SnO.sub.2, and the feedstock is principally chains of at least 20 carbon atoms. The metal-promoted anion-modified metal oxide catalyst contains one or more of Pt, Ni, Pd, Rh, Ir, Ru, (Mn & Fe) or mixtures of them present between about 0.2% to about 15% by weight of the catalyst. The metal-promoted anion-modified metal oxide catalyst contains one or more of SO.sub.4, WO.sub.3, or mixtures of them present between about 0.5% to about 20% by weight of the catalyst.

Venkatesh, Koppampatti R. (Pittsburgh, PA); Hu, Jianli (Cranbury, NJ); Tierney, John W. (Pittsburgh, PA); Wender, Irving (Pittsburgh, PA)

2001-01-01

239

Analysis of charge-injection characteristics at electrode-organic interfaces: Case study of transition-metal oxides  

Microsoft Academic Search

The formation of resistance-free or Ohmic contacts at metal\\/organic interfaces remains a significant challenge for achieving high-performance organic electronic devices such as organic light-emitting diodes. Several oxides have recently been reported to yield extremely low-voltage devices and thus have excited a renewed interest in developing the next generation of contacting electrodes. In this paper, major metal oxides, CuO, Cu2O ,

Z. B. Wang; M. G. Helander; M. T. Greiner; J. Qiu; Z. H. Lu

2009-01-01

240

High temperature air separation by perovskite-type oxide sorbentsHeat effect minimization  

Microsoft Academic Search

Oxygen sorption on perovskite-type oxides can be advantageously used for air separation at high temperature. The large heat of oxygen sorption on these oxide sorbents presents a major challenge for the heat management of the high sorption separation process in practical applications. This paper reports a method to minimize the heat effects by taking advantage of an endothermic process of

Qinghua Yin; Jay Kniep; Y. S. Lin

2008-01-01

241

Reactions of Cp2M (M = Ni, V) with dilithium diamido-aryl reagents; retention and oxidation of the transition metal ions.  

PubMed

The reactions of dilithium 1,2-diamidobenzene, [1,2-(HN)2C6H4]Li2 (L(1)H2)Li2, and dilithium 1,8-diamidonaphthalene, [1,8-(NH)2C10H6]Li2 (L(2)H2)Li2, with Cp2Ni and Cp2V have been used to obtain the new complexes (L(2)H2)2Ni{Li(THF)2}2 (3), (L(2)H2)3V{Li(THF)2}3 (4) and (L(1)H2)6Ni6{[(L(1)H2)3(L(1)H)3Ni6Li(THF)](2-)2[Li(THF)4](+)} (5), in which retention or oxidation of the initial metal(ii) centre is observed. Whereas 3 and 4 contain one transition metal ion within ion-paired structures, 5 has a complicated co-crystalline composition which contains octahedral Ni6-cages constructed from six square-planar (16e) Ni(II) centres. PMID:23925263

Stokes, Francesca A; Vincent, Mark A; Hillier, Ian H; Ronson, Tanya K; Steiner, Alexander; Wheatley, Andrew E H; Wood, Paul T; Wright, Dominic S

2013-08-08

242

Oxidatively induced abstraction reactions. A synthetic approach to low-coordinate and reactive early transition metal complexes containing metal-ligand multiple bonds.  

PubMed

A library of low-coordinate titanium and vanadium complexes containing terminal metal-ligand multiply bonded functionalities such as alkylidenes, alkylidynes, and imides have been prepared by one-electron oxidatively induced alpha-hydrogen abstraction reactions. In the case of the alkylidene motif, the nucleophilic nature of the M-C multiple bond permits subsequent reactions such as alpha-hydrogen migration to generate other rare functionalities such as phosphinidene-alkyl and imide-alkyls. Identifying and fine-tuning of the supporting ancillary ligand on the metal has allowed the isolation of kinetically stable titanium alkylidene and phosphinidene systems. The former is a key functionality to generate transient titanium alkylidynes, which readily engage in intermolecular C-H activation reactions of arenes and alkanes, and the ring-opening metathesis of aromatic substrates such as pyridines. In this Account, we describe several synthetic strategies to achieve reactive functionalities, functionalities that were previously portrayed as "incompatible" or "too kinetically reactive" with 3d early transition metals. PMID:17115721

Mindiola, Daniel J

2006-11-01

243

Synthesis, characterization and catalytic oxidation of cyclohexane using a novel host (zeolite-Y)\\/guest (binuclear transition metal complexes) nanocomposite materials  

Microsoft Academic Search

Transition metal (M=Mn(II), Co(II), Ni(II) and Cu(II)) complexes with octahydro-Schiff base (H4-N4O4)=2,7,13,18-tetramethyl-3,6,14,17-tetraazatricyclo-[17.3.1.1]-tetracosa-1(23),2,6,8(24),9,11,13,17,19,21-decaene-9,11,20,22-tetraol; H4([H]8-N4O4)=2,7,13,,18-tetramethyl-3,6,14,17-tetraazatricyclo-[17.13.1.1.]-tetracosa-1(23),8(24),9,11,19,21-hexane-9,11,20,22-tetraol) have been encapsulated in nanopores of zeolite-Y; [M([H]8-N4O4)]@NaY; with Flexible Ligand Method (FLM) for the first time. The new Host-Guest Nanocomposite Materials (HGNM) was characterized by several techniques: chemical analysis, spectroscopic methods (DRS, FT-IR and UV\\/Vis), BET technique, conductometric and magnetic measurements. The catalytic activities for oxidation

Masoud Salavati-Niasari

2009-01-01

244

A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles.  

PubMed

The efficiency of many energy storage technologies, such as rechargeable metal-air batteries and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen evolution reaction (OER). We found that Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-?) (BSCF) catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than that of the state-of-the-art iridium oxide catalyst in alkaline media. The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an e(g) symmetry of surface transition metal cations in an oxide. The peak OER activity was predicted to be at an e(g) occupancy close to unity, with high covalency of transition metal-oxygen bonds. PMID:22033519

Suntivich, Jin; May, Kevin J; Gasteiger, Hubert A; Goodenough, John B; Shao-Horn, Yang

2011-10-27

245

Effect of Li ion in transition metal sites on electrochemical behavior of layered lithium manganese oxides solid solutions  

Microsoft Academic Search

Two solid solutions of Li[Li1\\/5Ni1\\/10Co1\\/5Mn1\\/2]O2 and Li[Ni1\\/4Co1\\/2Mn1\\/4]O2 compositions were synthesized using a solgel method. The solid solutions with layered manganese oxides were both formed with typical hexagonal structure with maintaining the manganese oxidation state of 4+. The electrochemical characterization of the synthesized materials showed a monotonous discharge curve shape without structural transformation to spinel during chargedischarge processes. The Li[Li1\\/5Ni1\\/10Co1\\/5Mn1\\/2]O2 and

K. S Park; M. H Cho; S. J Jin; K. S Nahm; Y. S Hong

2004-01-01

246

Solid electrolyte CO 2 sensor using NASICON and perovskite-type oxide electrode  

Microsoft Academic Search

Solid-state electrochemical sensor devices combined with sodium super ionic conductor (NASICON: Na1+xZr2SixP3?xO12) discs and perovskite-type oxide electrodes have been developed for the detection of CO2 in the range of 1002000 ppm. Among the various sensor devices tested, Co-based perovskite-type oxide electrodes were found to show excellent sensing properties to CO2 at 200300C. Especially, NdCoO3- and La0.8Ba0.2CoO3-based elements showed the best

Youichi Shimizu; Nami Yamashita

2000-01-01

247

The mechanism of the maximum photovoltage in perovskite oxide heterostructures with the critical thickness  

NASA Astrophysics Data System (ADS)

A calculation based on the one-dimensional time-dependent drift diffusion model was carried out in order to study the mechanism of the maximum photovoltage obtained in perovskite oxide heterostructures. We found that the critical thickness of the thin films with which the maximum photovoltage was obtained depends on the width of the depletion region, the diffusion length of the minority carriers, and the light absorption coefficient in the films. The results reveal some physical insights in the maximum photovoltage and should be useful in designing the perovskite oxide heterostructures to improve the photovoltaic effect.

He, Xu; Jin, Kui-juan; Ge, Chen; Wang, Cong; Lu, Hui-bin; Yang, Guo-zhen

2013-05-01

248

Preparation and characterization of A-site doped Perovskite-type manganese oxides  

NASA Astrophysics Data System (ADS)

Perovskite type manganese oxide is a kind of in solid state chemistry and the field of condensed matter physics are used extensively in the study of materials. Application of sol-gel method on perovskite type manganese oxide doping, preparation out series of samples La0.6BaxSr(0.4-x)MnO3, including powder and flake. After annealing treatment, the X ray diffraction tests of different annealing temperature on the influence of the structure of the samples was studied.

Huang, Yingqun; Li, Xiong; Che, Xinglai; Ju, Hailang

2013-08-01

249

Analysis of charge-injection characteristics at electrode-organic interfaces: Case study of transition-metal oxides  

NASA Astrophysics Data System (ADS)

The formation of resistance-free or Ohmic contacts at metal/organic interfaces remains a significant challenge for achieving high-performance organic electronic devices such as organic light-emitting diodes. Several oxides have recently been reported to yield extremely low-voltage devices and thus have excited a renewed interest in developing the next generation of contacting electrodes. In this paper, major metal oxides, CuO, Cu2O , Ni2O3 , Co3O4 , WO3 , MoO3 , V2O5 , and indium tin oxide, have been systematically studied to compare their relative performance as hole injection anodes, as well as to provide an experimental database for theoretical analysis of current-voltage (IV) characteristics with a diverse range of injection barrier heights. Contrary to previous reports in the literature, none of the oxides studied in this work were found to form a true Ohmic contact with commonly used hole transport layers, such as N,N-diphenyl-N, N-bis-1-naphthyl-1-1-biphenyl-4,4-diamine ( ? -NPD). This discrepancy is attributed to incorrect IV data analysis of the quasi-Ohmic injection regimethe region in between space-charge limited current (SCLC) and injection limited current (ILC)in previous studies. It is found that the quasi-Ohmic regime is much larger (i.e., covers a greater range of injection barrier height) than has previously been expected. A criterion that defines Ohmic, quasi-Ohmic, and injection limited contacts has been quantified based on a time-domain simulation of charge transport across ? -NPD single-carrier devices. This criterion includes the effects of the electric field dependent mobility, organic layer thickness, and charge-injection barrier height. The effects of the built-in potential on the IV characteristics are also evaluated. A barrier-thickness-voltage phase diagram that defines the regions of SCLC, quasi-Ohmic, and ILC for ? -NPD is presented.

Wang, Z. B.; Helander, M. G.; Greiner, M. T.; Qiu, J.; Lu, Z. H.

2009-12-01

250

Partial oxidation of methane over Ni catalysts based on hexaaliminate- or perovskite-type oxides  

Microsoft Academic Search

Ni\\/Al2O3, hexaaluminate-type BaNixAl12?xO19?? and perovskite-type LaAl5\\/6Ni1\\/6O3 were investigated for partial oxidation of methane (POM). The hexaaluminate-type BaNiAl11O19?? calcined at 1400C demonstrated more stable and higher activity than Ni\\/Al2O3 calcined at 1000C. The BaNiAl11O19?? catalyst reduced at 1000C was more active than the catalyst reduced at 800C. The results from XPS analysis revealed that the hexaaluminate-type catalysts calcined and reduced at

Toshimasa Utaka; Saud Abdulaziz Al-Drees; Jun Ueda; Yasuyuki Iwasa; Tatsuya Takeguchi; Ruji Kikuchi; Koichi Eguchi

2003-01-01

251

Exploring the color of transition metal ions in irregular coordination geometries: new colored inorganic oxides based on the spiroffite structure, Zn(2-x)M(x)Te3O8 (M = Co, Ni, Cu).  

PubMed

We describe the synthesis, crystal structures, and optical absorption spectra of transition metal-substituted spiroffite derivatives, Zn(2-x)M(x)Te3O8 (M(II) = Co, Ni, Cu; 0 < x ? 1.0). The oxides are readily synthesized by solid state reaction of stoichiometric mixtures of the constituent binaries at 620 C. Reitveld refinement of the crystal structures from powder X-ray diffraction (XRD) data shows that the Zn/MO6 octahedra are strongly distorted, as in the parent Zn2Te3O8 structure, consisting of five relatively short Zn/M(II)-O bonds (1.898-2.236 ) and one longer Zn/M(II)-O bond (2.356-2.519 ). We have interpreted the unique colors and the optical absorption/diffuse reflectance spectra of Zn(2-x)M(x)Te3O8 in the visible, in terms of the observed/irregular coordination geometry of the Zn/M(II)-O chromophores. We could not however prepare the fully substituted M2Te3O8 (M(II) = Co, Ni, Cu) by the direct solid state reaction method. Density Functional Theory (DFT) modeling of the electronic structure of both the parent and the transition metal substituted derivatives provides new insights into the bonding and the role of transition metals toward the origin of color in these materials. We believe that transition metal substituted spiroffites Zn(2-x)M(x)Te3O8 reported here suggest new directions for the development of colored inorganic materials/pigments featuring irregular/distorted oxygen coordination polyhedra around transition metal ions. PMID:23659639

Tamilarasan, S; Sarma, Debajit; Bhattacharjee, S; Waghmare, U V; Natarajan, S; Gopalakrishnan, J

2013-05-09

252

Co-free, iron perovskites as cathode materials for intermediate-temperature solid oxide fuel cells  

Microsoft Academic Search

We have developed a Co-free solid oxide fuel cell (SOFC) based upon Fe mixed oxides that gives an extraordinary performance in test-cells with H2 as fuel. As cathode material, the perovskite Sr0.9K0.1FeO3?? (SKFO) has been selected since it has an excellent ionic and electronic conductivity and long-term stability under oxidizing conditions; the characterization of this material included X-ray diffraction (XRD),

Shu-en Hou; Jos Antonio Alonso; John B. Goodenough

2010-01-01

253

Noble metal-doped perovskites for the oxidation of organic air pollutants  

Microsoft Academic Search

Ag-Pt- and Pd-doped LaMnO3-based perovskite catalysts were prepared and their activity in the oxidation of toluene, n-heptane and ethanol was investigated. The activity of LaMnO3-based catalyst was very high in the oxidation of each compound tested. The Ag-doped catalyst was the most active in the oxidation of each compound and it displayed the highest BET specific surface area (SSA) also.

Anna Musialik-Piotrowska; Holger Landmesser

2008-01-01

254

Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal-air batteries  

NASA Astrophysics Data System (ADS)

The prohibitive cost and scarcity of the noble-metal catalysts needed for catalysing the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries limit the commercialization of these clean-energy technologies. Identifying a catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant transition-metal-oxide catalysts to replace platinum. Here, we demonstrate that the ORR activity for oxide catalysts primarily correlates to ?*-orbital (eg) occupation and the extent of B-site transition-metal-oxygen covalency, which serves as a secondary activity descriptor. Our findings reflect the critical influences of the ?* orbital and metal-oxygen covalency on the competition between O22-/OH- displacement and OH- regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.

Suntivich, Jin; Gasteiger, Hubert A.; Yabuuchi, Naoaki; Nakanishi, Haruyuki; Goodenough, John B.; Shao-Horn, Yang

2011-07-01

255

K-edge x-ray absorption spectra in transition-metal oxides beyond the single-particle approximation: Shake-up many-body effects  

NASA Astrophysics Data System (ADS)

Near-edge structures in K-edge x-ray absorption spectra (XAS) are widely investigated to understand the electronic and local structure in materials. The precise interpretation of these spectra with the help of calculations is hence of prime importance, especially for the study of correlated materials which have a complicated electronic structure per se. The single-particle approach, for example, has generally limited itself to the dominant dipolar cross section. It has long been known, however, that effects beyond this approach should be taken into account, due to both the inadequacy of such calculations when compared to experiment and the presence of shakeup many-body satellites in core-level photoemission spectra of correlated materials. This effect should manifest itself in XANES spectra, and the question is first how to account for it theoretically and second how to verify it experimentally. By using state-of-the-art first-principles electronic structure calculations and 1s photoemission measurements, we demonstrate that shakeup many-body effects are present in K-edge XAS dipolar spectra of NiO, CoO, and CuO at all energy scales. We show that shakeup effects can be included in K-edge XAS spectra in a simple way by convoluting the single-particle first-principles calculations including core-hole effects with the 1s photoemission spectra. We thus describe all features appearing in the XAS dipolar cross section of NiO and CoO and obtain a dramatic improvement with respect to the single-particle calculation in CuO. These materials being prototype correlated magnetic oxides, our work points to the presence of shakeup effects in K-edge XANES of most correlated transition-metal compounds and shows how to account for them, paving the way to a precise understanding of their electronic structure.

Calandra, M.; Rueff, J. P.; Gougoussis, C.; Colin, D.; Gorgoi, M.; Benedetti, S.; Torelli, P.; Shukla, A.; Chandesris, D.; Brouder, Ch.

2012-10-01

256

Structural and Electronic Properties of Reduced Transition Metal Oxide Clusters, M?O? and M?O?? (M = Cr, W), from Photoelectron Spectroscopy and Quantum Chemical Calculations  

SciTech Connect

We report a comparative study of reduced transition metal oxide clusters, M?O?? (M = Cr, W) anions and their neutrals, via anion photoelectron spectroscopy (PES) and density functional theory (DFT) and molecular orbital theory (CCSD(T)) calculations. Well-resolved PES spectra are obtained for M?O?? (M = Cr, W) at 193 and 157 nm photon energies. Different PES spectra are observed for M = Cr versus M = W. ExtensiveDFT and CCSD(T) calculations are performed to locate the ground and low-lying excited states for the neutrals and anions. The ground states of Cr?O? and Cr?O?? are predicted to be the B? and ?B? states of a C?v structure, respectively, revealing ferromagnetic spin coupling for Cr 3d electrons. In contrast, the ground states of W?O? and W?O?? are predicted to be the A? state (Cs symmetry) and the A? state (C?v symmetry), respectively, showing metal-metal d-d bonding in the anion. The current cluster geometries are in qualitative agreement with prior DFT studies at the PBE level for M = Cr and the B3LYP level for M = W. The BP86 and PW91 functionals significantly outperform the B3LYP functional for the Cr species, in terms of relative energies, electron detachment energies, and electronic excitation energies, whereas the B3LYP functional is better for the W species. Accurate heats of formation for the ground states of M?O? are calculated from the clustering energies and the heats of formation of MO? and MO?. The energetics have been used to predict redox reaction thermochemistry.

Li, Shenggang; Zhai, Hua Jin; Wang, Lai S.; Dixon, David A.

2009-09-28

257

Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure  

Microsoft Academic Search

Colossal magnetoresistance-a huge decrease in resistance in response to a magnetic field-has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the

K.-I. Kobayashi; T. Kimura; H. Sawada; K. Terakura; Y. Tokura

1998-01-01

258

Kinetics of non-isothermal precipitation process of perovskite phase in oxidized Ti-bearing blast furnace slag  

Microsoft Academic Search

Kinetics of non-isothermal precipitation process and crystal growth of perovskite phase in oxidized Ti-bearing slag were investigated. The oxidized slag was obtained by blowing the air into the molten Ti-bearing blast furnace slag through a lance. The experimental results show that the cooling rate has important effect on precipitation and growth of perovskite phase in oxidized slag; the lower cooling

WANG Ming-yu; WANG Xue-wen; LOU Tai-ping

259

New pathways for organic synthesis. Practical applications of transition metals  

SciTech Connect

This book contains a considerable number of transition-metal-based procedures that have genuine applications in synthesis, and which are arranged according to the nature of the organic product or synthetic transformation being carried out. The objective is to provide those engaged in the preparation of pharmaceuticals, natural products, herbicides, dyestuffs, and other organic chemicals with a practical guide to the application of transition metals in organic synthesis. Topics considered include the formation of carbon-carbon bonds, the formation of carbocyclic compounds, the formation of heterocyclic compounds, the isomerization of alkenes, the direct introduction and removal of carbonyl groups, reduction, oxidation, and preparing and handling transition metal catalysts.

Colquhoun, H.M.; Holton, J.; Thompson, D.J.; Twigg, M.V.

1984-01-01

260

Crystal and Magnetic Structure of Conducting Double Perovskite Sr 2FeMoO 6  

Microsoft Academic Search

We have investigated, crystal and magnetic structure of conducting double perovskite transition metal oxide, Sr2FeMoO6, which has been grown by the floating-zone method.The resisitvity shows T2-dependence in a wide temperature range (50 350 K), perhaps due to magnon scattering.We further have determined the crystal and magnetic structure in the ferrimagnetic state by means of synchrotron radiation X-ray powder measurements as

Yutaka Moritomo; Sheng Xu; Akihiko Machida; Takumi Akimoto; Eiji Nishibori; Masaki Takata; Makoto Sakata; Kenji Ohoyama

2000-01-01

261

Reducibility of Co3+ in perovskite-type LaCoO3 and promotion of copper on the reduction of Co3+ in perovskite-type oxides  

NASA Astrophysics Data System (ADS)

The reducibility of Co3+ in LaCoO3 and the promotion of copper on the reduction of Co3+ in perovskite-type oxides have been studied by temperature programmed reduction (TPR), temperature programmed oxidation (TPO), X-ray diffraction (XRD) and infrared spectroscopy (IR). Quantitative TPR and TPO analyses propose that the reduction of Co3+ in LaCoO3 to Co0 belongs to a one-step process and that every TPR peak represents the reduction of every Co3+ species to Co0 in the crystallite structure. Accordingly, the produced Co0 is assumed to be atomically located in the perovskite lattice provided the perovskite structure is retained after reduction. Quantitative TPR analyses also indicates that copper located in LaCo0.85Cu0.15O3 promotes regularly the reduction of various Co3+ species whereas copper doped on LaCoO3 does irregularly. The action of the latter leads to more efficient reduction of Co3+ to atomically dispersed Co0 over a useful temperature range for catalytic purposes. TPR, XRD and IR studies show that the perovskite structure of LaCo0.85Cu0.15O3 is somewhat less stable than that of LaCoO3 and that the perovskite structural stability of LaCoO3 is not weakened by the doping of copper.

Huang, Lin; Bassir, Mahbod; Kaliaguine, Serge

2005-04-01

262

Spin-orbital superstructure in strained ferrimagnetic perovskite cobalt oxide.  

PubMed

We have investigated the Co-3d spin-orbital state in a thin film of perovskite LaCoO3 to clarify the origin of strain induced spontaneous magnetization (T(C)=94 K) by means of x-ray diffraction, optical spectroscopy, and magnetization measurements. A lattice distortion with the propagation vector (1/4 -1/4 1/4) and an anomalous activation of optical phonons coupled to Co-3d orbital are observed below 126 K. Combined with the azimuthal angle analysis of superlattice reflection, we propose that the ordering of Co-3d orbital promoted by an epitaxial strain produces a unique ferrimagnetic structure. PMID:23889438

Fujioka, J; Yamasaki, Y; Nakao, H; Kumai, R; Murakami, Y; Nakamura, M; Kawasaki, M; Tokura, Y

2013-07-09

263

Intergranular giant magnetoresistance in a spontaneously phase separated perovskite oxide.  

PubMed

We present small-angle neutron scattering data proving that, on the insulating side of the metal-insulator transition, the doped perovskite cobaltite La(1-x)Sr(x)CoO(3) phase separates into ferromagnetic metallic clusters embedded in a nonferromagnetic matrix. This induces a hysteretic magnetoresistance, with temperature and field dependence characteristic of intergranular giant magnetoresistance (GMR). We argue that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces. PMID:15698314

Wu, J; Lynn, J W; Glinka, C J; Burley, J; Zheng, H; Mitchell, J F; Leighton, C

2005-01-25

264

Resonant x-ray reflectivity study of perovskite oxide superlattices  

NASA Astrophysics Data System (ADS)

Resonant x-ray reflectivity (XRR) is utilized to characterize the structural properties of La0.7Sr0.3MnO3/La0.7Sr0.3FeO3 superlattices. XRR spectra at the Mn and Fe absorption edges provide additional structural information compared to spectra obtained with a conventional Cu x-ray source. We demonstrate that this technique provides individual layer thicknesses and intermixing behavior of perovskite superlattices with small density contrast in a non-destructive manner. These results are consistent with scanning transmission electron microscopy and electron energy loss spectroscopy.

Kemik, N.; Gu, M.; Yang, F.; Chang, C.-Y.; Song, Y.; Bibee, M.; Mehta, A.; Biegalski, M. D.; Christen, H. M.; Browning, N. D.; Takamura, Y.

2011-11-01

265

Structural and magnetic properties of SrMn1-xRuxO3 perovskites  

Microsoft Academic Search

Ferromagnetism of SrRuO3 is unique among 4d transition metal based perovskite oxides. On substitution of Mn its TC decreases from 163 K to 0 for x0.5-0.6 followed by a formation of an antiferromagnetic insulating state at a quantum critical point. The other end member of the SrMn1-xRuxO3 family, a cubic perovskite SrMnO3 is a G-type antiferromagnet with TN=233 K. We

B. Dabrowski; S. Kolesnik; O. Chmaissem; T. Maxwell

2007-01-01

266

Homogeneous catalysis by transition metal oxygen anion clusters  

Microsoft Academic Search

This review provides a thorough discussion of homogeneous catalysis by transition metal oxygen anion clusters (polyoxometalates), with a focus on mechanism. The primary catalysts examined are mixed addenda and d-electron-transition-metal-substituted polyoxometalate (TMSP) complexes. The unusual versatility and tunability of these catalysts and their compatibility with environmentally and economically attractive conditions (solvents, oxidants, etc.) are outlined. A general compilation of the

Craig L. Hill; Christina M. Prosser-McCartha

1995-01-01

267

Powder Diffraction Studies of Phase Transitions in Manganese Perovskites  

NASA Astrophysics Data System (ADS)

The results of recent structural studies of some Manganese perovskites are presented, in particular oxides in the system Ca1-xSrxMnO3 and SrRu0.5Mn0.5O3. In the first series we firstly show the power of synchrotron X-ray powder diffraction to refine accurate and precise structures for oxides containing first row transition metals and then show the presence of a direct orthorhombic Pbnm to tetragonal I4/mcm transition associated with the tilting of the MnO6 octahedra. The inclusion of a heavier second row transition metal reduces the precision of the structure, however the details of the tetragonal to cubic phase transition in SrRu0.5Mn0.5O3 are still established.

Kennedy, B. J.

2008-03-01

268

Investigation of composite Ni-doped perovskite anode catalyst for electrooxidation of hydrogen in solid oxide fuel cell  

Microsoft Academic Search

Ni-doped perovskite anodes have been investigated for oxidation of hydrogen in IT-SOFC. Perovskite materials are characterized by mixed electronicionic conductivity that combined to the high intrinsic catalytic activity of Ni allows to achieve high reaction rates. Suitable power densities of about 420mW\\/cm2 for the direct oxidation of hydrogen were achieved at 800C. Temperature programmed reduction analysis carried out in hydrogen

Giovanni Brunaccini; Massimiliano Lo Faro; Daniela La Rosa; Vincenzo Antonucci; Antonino Salvatore Aric

2008-01-01

269

Self-propagating high-temperature synthesis of Sr-doped LaMnO 3 perovskite as oxidation catalyst  

Microsoft Academic Search

Sr-doped LaMnO3 perovskite oxide has been focused on as one of the alternative catalysts to precious metals such as platinum that are used for cleaning automotive emission gas. The conventional Solid-state reaction method is a popular productive process for perovskite oxide, however, it is time and energy consuming process because it requires repeated prolonged heat treatment at high temperatures. Therefore,

T. Hirano; H. Purwanto; T. Watanabe; T. Akiyama

2007-01-01

270

High Mobility in a Stable Transparent Perovskite Oxide  

NASA Astrophysics Data System (ADS)

We discovered that La-doped BaSnO3 with the perovskite structure has an unprecedentedly high mobility at room temperature while retaining its optical transparency. In single crystals, the mobility reached 320 cm2 V-1 s-1 at a doping level of 8 1019 cm-3, constituting the highest value among wide-band-gap semiconductors. In epitaxial films, the maximum mobility was 70 cm2 V-1 s-1 at a doping level of 4.4 1020 cm-3. We also show that resistance of (Ba,La)SnO3 changes little even after a thermal cycle to 530 C in air, pointing to an unusual stability of oxygen atoms and great potential for realizing transparent high-frequency, high-power functional devices.

Kim, Hyung Joon; Kim, Useong; Kim, Hoon Min; Kim, Tai Hoon; Mun, Hyo Sik; Jeon, Byung-Gu; Hong, Kwang Taek; Lee, Woong-Jhae; Ju, Chanjong; Kim, Kee Hoon; Char, Kookrin

2012-06-01

271

Optical Response of High-Dielectric-Constant Perovskite-Related Oxide  

Microsoft Academic Search

Optical conductivity measurements on the perovskite-related oxide CaCu3Ti4O12 provide a hint of the physics underlying the observed giant dielectric effect in this material. A low-frequency vibration displays anomalous behavior, implying that there is a redistribution of charge within the unit cell at low temperature. At infrared frequencies (terahertz), the value for the dielectric constant is ~80 at room temperature, which

C. C. Homes; T. Vogt; S. M. Shapiro; S. Wakimoto; A. P. Ramirez

2001-01-01

272

A SIMS study of hydrogen in acceptor-doped perovskite oxides  

Microsoft Academic Search

The incorporation of protons into two perovskite oxides has been investigated using Secondary Ion Mass Spectrometry (SIMS). The SIMS analysis was carried out on an Atomika 6500 instrument using Cs+ primary bombardment and negative secondary ions. Samples of SrCe0.95Yb0.05O3 ? ? were subjected to anneals in either wet or dry oxygen at 600 C prior to analysis. Proton incorporation into

R. A. De Souza; J. A. Kilner; B. C. H. Steele

1995-01-01

273

Atomic level observation of octahedral distortions at the perovskite oxide heterointerface  

PubMed Central

For perovskite oxides, ABO3, slight octahedral distortions have close links to functional properties. While perovskite oxide heterostructures offer a good platform for controlling functionalities, atomistic understanding of octahedral distortion at the interface has been a challenge as it requires precise measurements of the oxygen atomic positions. Here we demonstrate an approach to clarify distortions at an atomic level using annular bright-field imaging in aberration-corrected scanning transmission electron microscopy, which provides precise mappings of cation and oxygen atomic positions from distortion-minimized images. This technique revealed significant distortions of RuO6 and ScO6 octahedra at the heterointerface between a SrRuO3 film and a GdScO3 substrate. We also found that structural mismatch was relieved within only four unit cells near the interface by shifting the oxygen atomic positions to accommodate octahedral tilt angle mismatch. The present results underscore the critical role of the oxygen atom in the octahedral connectivity at the perovskite oxide heterointerface.

Aso, Ryotaro; Kan, Daisuke; Shimakawa, Yuichi; Kurata, Hiroki

2013-01-01

274

Thermal diffusivity of oxide perovskite compounds at elevated temperature  

NASA Astrophysics Data System (ADS)

The phonon component of thermal diffusivity (D) for eleven compounds (synthetic SrTiO3, SrTiO3:Fe3+, BaTiO3, KTaO3, KNbO3, NdGaO3, YAlO3, YAlO3:Tm, LaAlO3, La0.29Sr0.66Al0.65Ta0.35O3, and natural Ca1.01Mn0.001Fe0.007Ti0.99O3) with various perovskite structures was measured from ambient temperature (T) up to ~2000 K using contact-free, laser-flash analysis, from which effects of ballistic radiative transfer were removed. Structural transitions (e.g., orthorhombic to tetragonal) below 800 K were manifest as sharp steps in 1/D. Above 800 K, structural transitions occur over intervals of ~150 K. Similarly broad peaks accompany changes from colorless to black, attributable to partial reduction in Ti, Nb, or Ta from contact with graphite coatings. Otherwise, D decreases with increasing T and, if substitutional disorder exists, approaches a constant (Dsat) near 1600 K. Our data are best described as D-1 following a low order polynomial in T. Ordered, cubic perovskites occupy a single trend for D(T)-1, defining the contribution of the ideal lattice. Distortion, disorder, and polymorphism affect D-1 in a manner that is consistent with the damped harmonic oscillator-phonon gas model which relates phonon lifetimes to infrared peak widths. Calculated D-values at ambient and high T agree with measurements. The behavior of D is simple compared to that of thermal conductivity, k=?CPD, where ? is density and CP is heat capacity. Combining our data with cryogenic measurements of YAlO3 and LaAlO3 shows that D-1 depends on T similarly to CP, consistent with phonon lifetime depending on the density of states but, the best description for D-1(T) is a proportionality to ?T from ~0 K up to the limit of measurements, where ? is thermal expansivity, a strongly anharmonic property. At low T, D-1 due to phonon scattering follows that of CP, generally~T3, so klat=k0+k1T. Defects being present preclude scattering at sample walls, adding a small constant D0-1 ~0.0001 mm-2 s as T-->0, and an additional contribution of kdfctT3. Forms previously inferred for thermal insulators include systematic errors stemming from ballistic radiative transfer and/or interface resistance. Our results show that optical phonons largely govern heat transport of complex insulators.

Hofmeister, Anne M.

2010-05-01

275

First insertion of NO into a transition-metal cluster-carbon bond: regioselective formation, structure, and reactions of the first alkanenitrile oxide complexes  

SciTech Connect

The chemistry of NO in the presence of transition metals is receiving considerable current attention because of its role in air pollution, its potential in organic synthesis by carbon-nitrogen bond formation, and an increasing interest in its basic features. The nitrosyl cation has been reacted with many mono and polynuclear metal systems, leading mainly to substitution and reduction. Insertion into alkyl and aryl metal bonds in mono-metallic complexes is documented. The unprecedented title reaction and some preliminary chemistry of the products are reported here. 27 references, 1 figure.

Goldhaber, A.; Vollhardt, K.P.C.; Walborsky, E.C.; Wolfgruber, M.

1986-02-05

276

Nonaqueous synthesis of metal oxide nanoparticles: Short review and doped titanium dioxide as case study for the preparation of transition metal-doped oxide nanoparticles  

Microsoft Academic Search

The liquid-phase synthesis of metal oxide nanoparticles in organic solvents under exclusion of water is nowadays a well-established alternative to aqueous solgel chemistry. In this article, we highlight some of the advantages of these routes based on selected examples. The first part reviews some recent developments in the synthesis of ternary metal oxide nanoparticles by surfactant-free nonaqueous solgel routes, followed

Igor Djerdj; Denis Arcon; Zvonko Jaglicic; Markus Niederberger

2008-01-01

277

Transparent and conductive oxide films with the perovskite structure: La and Sb-doped BaSnO3  

Microsoft Academic Search

We report that (La0.07Ba0.93)SnO3 films grown epitaxially on SrTiO3(001) substrates by laser ablation at 760 C show a cubic perovskite structure of lattice constant 4.121 A?, excellent optical transmittance in the visible range, and a weak metallic behavior with low resistivity of about 4 m? cm within 10300 K. The transparent perovskite oxide films are n-type conductors, with carrier concentration

H. F. Wang; Q. Z. Liu; F. Chen; G. Y. Gao; Wenbin Wu; X. H. Chen

2007-01-01

278

Thermal-expansion behaviors and mechanisms for Ca or Sr-doped lanthanum manganite perovskites under oxidizing atmospheres  

Microsoft Academic Search

The thermal expansion behavior and mechanism of A-site-deficient lanthanum manganite perovskites, La{sub 1-x}MnO (0 < x 0.1), and alkaline earth metal (AE)-doped lanthanum manganite perovskites, La{sub 1-x}AEMnO (AE = Ca and Sr, 0 x 0.4), under oxidizing atmospheres have been investigated. The average linear thermal expansion coefficients of the AE-doped lanthanum manganites decreased with increasing Ca content

Masashi Mori; Yoshiko Hiei; Nigel M. Sammes; Geoff A. Tompsett

2000-01-01

279

Protons and lattice defects in perovskite-related oxides  

SciTech Connect

The recent work on this project has focused on two perovskite- structured materials: single crystalline KTaO{sub 3} doped with iron-group ions, and ceramic BaCeO{sub 3} doped with Nd, Gd, Sc and Ca. In the case of KTaO{sub 3}, we have shown that protons are introduced into the crystal from H{sub 2}O vapor so that it becomes primarily a protonic conductor. The defect reactions involved have been considered in detail with the aid of EPR and IR measurements. In the study of BaCeO{sub 3}, it was found that the Nd-doped material is an excellent protonic conductor with activation energy for proton hopping of only 0.54 eV. In addition, weight-change measurements were used to determine the amount of water uptake, while Nernst-cell measurements gave values for the transport numbers of oxygen and of protons. In contrast to the case of Nd doping, Gd or Yb-doped material are not protonic conductors. Other studies include those of high-{Tc} superconductors, the first showing a large elastic constant and internal friction anomaly in La{sub 2x}Sr{sub x}CuO{sub 4}, and the second showing under what conditions degradation occurs due to H{sub 2}O and CO{sub 2} annealing treatments. Finally, a study of dielectric loss in Li-borate glasses shows that the dielectric loss peak in glasses widely believed to be due to a bulk phenomenon, is actually an electrode blocking effect.

Nowick, A.S.

1990-01-01

280

Protons and lattice defects in perovskite-related oxides  

NASA Astrophysics Data System (ADS)

The recent work on this project has focused on two perovskite-structured materials: single crystalline KTaO3 doped with iron-group ions, and ceramic BaCeO3 doped with Nd, Gd, Sc and Ca. In the case of KTaO3, it is shown that protons are introduced into the crystal from H2O vapor so that it becomes primarily a protonic conductor. The defect reactions involved have been considered in detail with the aid of EPR and IR measurements. In the study of BaCeO3, it was found that the Nd-doped material is an excellent protonic conductor with activation energy for proton hopping of only 0.54 eV. In addition, weight-change measurements were used to determine the amount of water uptake, while Nernst-cell measurements gave values for the transport numbers of oxygen and of protons. In contrast to the case of Nd doping, Gd or Yb-doped material are not protonic conductors. Other studies include those of high-(Tc) superconductors, the first showing a large elastic constant and internal friction anomaly in La(2x)Sr(x)CuO4, and the second showing under what conditions degradation occurs due to H2O and CO2 annealing treatments. Finally, a study of dielectric loss in Li-borate glasses shows that the dielectric loss peak in glasses widely believed to be due to a bulk phenomenon, is actually an electrode blocking effect.

Nowick, A. S.

281

Ferrite-based perovskites as cathode materials for anode-supported solid oxide fuel cells  

Microsoft Academic Search

The properties and the applicability of iron- and cobalt-containing perovskites were evaluated as cathodes for solid oxide fuel cells (SOFCs) in comparison to state-of-the-art manganite-based perovskites. The materials examined were La1?x?ySrxCo0.2Fe0.8O3?? (x=0.2 and 0.4; y=0?0.05), La0.8Sr0.2FeO3??, La0.7Ba0.3Co0.2Fe0.8O3?? and Ce0.05Sr0.95Co0.2Fe0.8O3??. The main emphasis was placed on the electrochemical properties of the materials, which were investigated on planar anode-supported SOFCs with 8

Andreas Mai; Vincent A. C. Haanappel; Sven Uhlenbruck; Frank Tietz; Detlev Stver

2005-01-01

282

Influence of magnetic ordering on structural instabilities in insulating perovskites  

NASA Astrophysics Data System (ADS)

We propose a general model, based on Anderson superexchange and the pseudo Jahn-Teller effect, to explain why ferroelectric and octahedral tilting distortions are favored by ferromagnetic ordering when compared to antiferromagnetic tilting distortions in ABX3 (A = alkali, alkaline-earth ion, f-filling rare earth; B = magnetic transition-metal ion; X = O2-, F-) perovskite crystals. Ab initio calculations are used to check the model under a wide range of pressures for oxides and fluorides, with the latter displaying an enhancement of the ferroelectric distortion when pressure is increased.

Garcia-Fernandez, Pablo; Aramburu, Jose Antonio; Moreno, Miguel

2011-05-01

283

Fe doped LaGaO 3 perovskite oxide as an oxygen separating membrane for CH 4 partial oxidation  

Microsoft Academic Search

Fe doped LaGaO3 based perovskite oxide was investigated in this study as an oxygen permeating membrane for CH4 partial oxidation. La0.7Sr0.3Ga0.6Fe0.4O3 exhibits a high oxygen permeation rate from air to He as high as 102 ?mol\\/min cm2 (2.5 cm3-std\\/mincm2) at 1273 K and 0.3 mm thickness. Application of LSGF membrane for CH4 partial oxidation was further studied. Since the PO2

Tatsumi Ishihara; Yuko Tsuruta; Toshitsune Todaka; Hiroyasu Nishiguchi; Yusaku Takita

2002-01-01

284

Correlation between thermal expansion and oxide ion transport in mixed conducting perovskite-type oxides for SOFC cathodes  

Microsoft Academic Search

Oxygen deficiency, electrical conductivity, ionic transport and thermal expansion of alternative cathode materials of the A1?aA?aBO3 perovskite-type oxides (A=La, Pr, Ce; A?=Sr; B=Mn, Fe, Co, Ni, Ga, Mg) were measured as a function of temperature and oxygen partial pressure. Additionally, lanthanum strontium gallates were investigated as electrolyte materials. On the basis of the results a general picture of the A-

H. Ullmann; N. Trofimenko; F. Tietz; D. Stver; A. Ahmad-Khanlou

2000-01-01

285

A new preparation method of LaMnO3 perovskite using electrochemical oxidation  

NASA Astrophysics Data System (ADS)

LaMnO3 perovskite was prepared by heat treatment of an oxide film which was electrochemically deposited on a Pt electrode in solutions containing both La3+ and Mn2+ ions. The La/Mn ratio in the deposited oxide films increased with an increase in the [La3+]/[Mn2+] ratio in solution and with a positive shift of electrode potential. The maximum value in this ratio was about one, and was obtained in solution with [La3+]/[Mn2+] > 500 and/or in a potential region more positive than about 1.9 V vs. Ag/AgCl. It was presumed from IR, TG, XRD, and XPS measurements that the incorporation of La3+ ion into the oxide film during electrochemical deposition is based on a specific adsorption of La3+ ions onto the surface of amorphous and hydrous ?-MnO2, which has some oxygen deficiencies, and that the La3+ ion exists as a hydroxide in the oxide. A single phase of LaMnO3 perovskite was obtained by heat-treatment of the deposited oxide film with La/Mn ? 1.0 at a temperature higher than about 750C, which was always lower than the preparation temperature in the ceramic technique (about 1200C). The crystallization process in the heat treatment was also discussed from TG and XRD analysis.

Sasaki, Takeshi; Matsumoto, Yasumichi; Hombo, Jukichi; Ogawa, Yoriyuki

1991-03-01

286

Nonaqueous synthesis of metal oxide nanoparticles: Short review and doped titanium dioxide as case study for the preparation of transition metal-doped oxide nanoparticles  

NASA Astrophysics Data System (ADS)

The liquid-phase synthesis of metal oxide nanoparticles in organic solvents under exclusion of water is nowadays a well-established alternative to aqueous sol gel chemistry. In this article, we highlight some of the advantages of these routes based on selected examples. The first part reviews some recent developments in the synthesis of ternary metal oxide nanoparticles by surfactant-free nonaqueous sol gel routes, followed by the discussion of the morphology-controlled synthesis of lanthanum hydroxide nanoparticles, and the presentation of structural peculiarities of manganese oxide nanoparticles with an ordered Mn vacancy superstructure. These examples show that nonaqueous systems, on the one hand, allow the preparation of compositionally complex oxides, and, on the other hand, make use of the organic components (initially present or formed in situ) in the reaction mixture to tailor the morphology. Furthermore, obviously even the crystal structure can differ from the corresponding bulk material like in the case of MnO nanoparticles. In the second part of the paper we present original results regarding the synthesis of dilute magnetic semiconductor TiO2 nanoparticles doped with cobalt and iron. The structural characterization as well as the magnetic properties with special attention to the doping efficiency is discussed.

Djerdj, Igor; Ar?on, Denis; Jagli?i?, Zvonko; Niederberger, Markus

2008-07-01

287

Iron Partitioning And Oxidation State In Mg-Perovskite And Ferropericlase At Lower Mantle Conditions  

NASA Astrophysics Data System (ADS)

Many processes that occur within the Earth are determined by the physical properties of the lower mantle. Albeit their thermodynamic stability in the lower mantle, the physical and chemical properties of its two main phases, namely (Mg,Fe)SiO3 magnesium silicate perovskite and (Mg,Fe)O ferropericlase, are affected by variations in the chemistry of iron. We report results concerning the behaviour of iron under lower mantle conditions. Iron partitioning experiments between Mg-perovskite and ferropericlase were conducted on samples synthesized from a San Carlos olivine compressed in a laser-heated diamond anvil cell. Recovered run products were thinned to electron transparency by focused ion beam and studied by analytical transmission electron microscopy (ATEM). Energy-dispersive x-ray measurements were used to calculate the partitioning coefficients of the iron between the phases at relevant lower mantle conditions. As the oxidation state of iron is known to influence the distribution of iron in the mantle phases, there is a strong motivation to achieve a quantitative determination of Fe3+ content in the lower mantle assemblage Mg-perovskite and ferropericlase. The Fe3+ /Fe concentrations were determined by electron energy loss spectroscopy. We also heated a (Mg0.83 Fe0.17)O at 92 GPa and investigated the recovered samples by ATEM. We see no evidence of decomposition into magnesium oxide and ferrous rich oxide components, which is at odds with a recent report. Our work provides important data on Fe/Mg partitioning and the oxidation state in the two main phases of lower mantle.

Auzende, A.; Badro, J.; Gloter, A.

2005-12-01

288

Interplay of octahedral rotations and breathing distortions in charge-ordering perovskite oxides  

NASA Astrophysics Data System (ADS)

We investigate the structure-property relationships in ABO3 perovskites exhibiting octahedral rotations and cooperative octahedral breathing distortions (CBD) using group theoretical methods. Rotations of octahedra are ubiquitous in the perovskite family, while the appearance of breathing distortionsoxygen displacement patterns that lead to approximately uniform dilation and contraction of the BO6 octahedraare rarer in compositions with a single, chemically unique B site. The presence of a CBD relies on electronic instabilities of the B-site cations, either orbital degeneracies or valence-state fluctuations, and often appear concomitant with charge order metal-insulator transitions or B-site cation ordering. We enumerate the structural variants obtained from rotational and breathing lattice modes and formulate a general Landau functional describing their interaction. We use this information and combine it with statistical correlation techniques to evaluate the role of atomic scale distortions on the critical temperatures in representative charge ordering nickelate and bismuthate perovskites. Our results provide microscopic insights into the underlying structure-property interactions across electronic and magnetic phase boundaries, suggesting plausible routes to tailor the behavior of functional oxides by design.

Balachandran, Prasanna V.; Rondinelli, James M.

2013-08-01

289

Optical and energy-loss spectra of the antiferromagnetic transition metal oxides MnO, FeO, CoO, and NiO including quasiparticle and excitonic effects  

NASA Astrophysics Data System (ADS)

We calculate the frequency-dependent dielectric function for the series of antiferromagnetic transition metal oxides (TMOs) from MnO to NiO using many-body perturbation theory. Quasiparticle, excitonic, and local-field effects are taken into account by solving the Bethe-Salpeter equation in the framework of collinear spin polarization. The optical spectra are based on electronic structures which have been obtained using density-functional theory with a hybrid functional containing screened exchange (HSE03) and a subsequent quasiparticle calculation in the GW approximation to describe exchange and correlation effects adequately. These sophisticated quasiparticle band structures are mapped to electronic structures resulting from the computationally less expensive GGA+U+? scheme that includes an on-site interaction U and a scissors shift ? and allows us to calculate the large number of electronic states that is necessary to construct the Bethe-Salpeter Hamiltonian. For an accurate description of the optical spectra, an appropriate treatment of the strong electron-hole attraction is mandatory to obtain agreement with the experimentally observed absorption-peak positions. The itinerant s and p states as well as the localized transition metal 3d states have to be considered on an equal footing. We find that a purely atomic picture is not suitable to understand the optical absorption spectra of the TMOs. Reflectivity spectra, absorption coefficients, and loss functions at vanishing momentum transfer are computed in a wide spectral range and discussed in light of the available experimental data.

Rdl, Claudia; Bechstedt, Friedhelm

2012-12-01

290

Intrasphere and extrasphere effects of the influence of ammine complexes of transition metals on their catalytic activity in reactions of oxidation of manganese sulfate to the dioxide by atmospheric oxygen  

SciTech Connect

Manganese(II) sulfate is a waste product from the large-tonnage redox process for production of anthraquinone from aniline. Recovery of the original manganese dioxide and removal of manganese(II) sulfate from the wastewaters are economically and ecologically necessary. Liquid-phase low-temperature conversion of manganese(II) sulfate by the action of atmospheric oxygen blown through an aqueous solution of the salt containing ammine complexes of transition metals was studied earlier. It was shown that it does not depend only on intrasphere cations or extrasphere anions; it may be influenced by the effects of their interactions. The purpose of the present work was to study this influence in a complete factorial experiment of the CFE-3T type. It was found that homogeneous catalysis of oxidation of manganese(II) sulfate by atmospheric oxygen in presence of transition-metal ammines is determined to a considerable extent by the effects of pair interactions between complex-forming cations and extrasphere anions.

Eremeev, A.P.; Veselovskii, P.F.

1986-12-10

291

Electrospun Zn(1-x)Mn(x)Fe2O4 nanofibers as anodes for lithium-ion batteries and the impact of mixed transition metallic oxides on battery performance.  

PubMed

The structural and electrochemical properties of the mixed transition metallic oxides Zn1-xMnxFe2O4 nanofibers, which crystallize in a cubic spinel AFe2O4 structure, are investigated systematically with a gradual substitution of Zn by Mn. The crystal structural information studied by X-ray diffraction (XRD) depicts the formation of single phase spinel structure, while electron-dispersive X-ray spectroscopy (EDS) reveals the stoichiometric ratio between Zn and Mn. ZnFe2O4 exhibits a good capacity of ~532 mAh g(-1) at 50th cycle through the interbeneficial conversion reaction and alloy-dealloy mechanism, with a first discharge working voltage of ~0.83 V. Subsequently, the characteristic redox potential of each spinel is gradually reduced with the replacement of Mn. Furthermore, Zn0.3Mn0.7Fe2O4 demonstrates the highest capacity of ~612 mA h g(-1) at 50th cycle among the solid solution series. Ex situ characterization by high-resolution transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) is conducted to study the participation of Mn in the battery performance. This report represents an example of how the electrochemical performance could be flexibly adjusted by tuning the ratio of transition metals within the spinel. PMID:23688028

Teh, Pei Fen; Pramana, Stevin Snellius; Sharma, Yogesh; Ko, Yah Wen; Madhavi, Srinivasan

2013-06-05

292

Transition metal surface electronic structure  

NASA Astrophysics Data System (ADS)

In the automotive industry, solid surface effects are encountered in areas such as exhaust gas catalysis and sensing, corrosion, adhesion, friction and wear. Transition metals are involved in all of these types of interfaces. Our surface and interface physics programs are designed to characterize relevant physical phenomena in the atomistic sense. Transition metals are characterized in large part by their d-electron behavior. The localized nature of the d-orbitals has necessitated the formulation of a self-consistent local orbital method in order to compute surface electronic structure. Results for the (100) surfaces on work functions agree well with experiment. We found a surprisingly large density of surface states, i.e., localized to the surface region, on palladium, copper, and silver. One-fifth to one-fourth of the electrons in the surface layer are in surface states on these metals. A prominent surface state band found theoretically on copper has subsequently have been observed by Heimann et al., and Kevan and Shirley via angular photoemission. It is clear that henceforth surface states will have to be considered in the analysis of physical and chemical processes on transition metal surfaces.

Smith, J. R.; Arlinghaus, F. J.; Gay, J. G.

1981-01-01

293

Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal-air batteries  

Microsoft Academic Search

The prohibitive cost and scarcity of the noble-metal catalysts needed for catalysing the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries limit the commercialization of these clean-energy technologies. Identifying a catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant transition-metal-oxide catalysts to replace platinum. Here, we demonstrate

Jin Suntivich; Hubert A. Gasteiger; Naoaki Yabuuchi; Haruyuki Nakanishi; John B. Goodenough; Yang Shao-Horn

2011-01-01

294

2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy.  

PubMed

Using a combination of high-angle annular dark-field scanning transmission electron microscopy and atomically resolved electron energy-loss spectroscopy in an aberration-corrected transmission electron microscope we demonstrate the possibility of 2D atom by atom valence mapping in the mixed valence compound Mn3O4. The Mn L(2,3) energy-loss near-edge structures from Mn2+ and Mn3+ cation sites are similar to those of MnO and Mn2O3 references. Comparison with simulations shows that even though a local interpretation is valid here, intermixing of the inelastic signal plays a significant role. This type of experiment should be applicable to challenging topics in materials science, such as the investigation of charge ordering or single atom column oxidation states in, e.g., dislocations. PMID:21981530

Tan, Haiyan; Turner, Stuart; Ycelen, Emrah; Verbeeck, Jo; Van Tendeloo, Gustaaf

2011-09-01

295

Brazing perovskite ceramics with silver\\/copper oxide braze alloys  

Microsoft Academic Search

In this study the feasibility of eutectic braze alloys based on the silver\\/copper oxide (Ag\\/CuO) system were investigated for use in joining lead magnesium niobate (PMN) ceramics. PMN was successfully brazed in air at 1050 and 1100C using Ag\\/CuO alloys. The brazed samples had an average four-point-bend fracture strength of approximately 40 percent of the average monolithic PMN strength (with

K. M. Erskine; A. M. Meier; S. M. Pilgrim

2002-01-01

296

Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor  

Microsoft Academic Search

Ionic-electronic mixed-conducting perovskite-type oxide La0.6Sr0.4Co0.8Fe0.2O3 was applied as a dense membrane for oxygen supply in a reactor for methane coupling. The oxygen permeation properties were studied in the pO2-range of 10?3?1 bar at 10731273 K, using helium as a sweeping gas at the permeate side of the membrane. The oxygen semi-permeability has a value close to 1 mmol m?2 s?1

J. E. ten Elshof; H. J. M. Bouwmeester; H. Verweij

1995-01-01

297

Influence of order-disorder transitions on oxygen permeability through selected nonstoichiometric perovskite-type oxides  

Microsoft Academic Search

New results on the oxygen permeability of perovskite-type oxides SrCo0.8B'0.2O3 (with B'=Cr, Fe, Co and Cu) and La0.6Sr0.4CoO3 are presented. The occurrence of order-disorder transitions at elevated temperatures (790940C) in these phases has been confirmed by DSC measurements and, in some cases, by X-ray powder diffraction of samples either slowly cooled or quenched from high temperature after annealing in different

H. Kruidhof; H. J. M. Bouwmeester; Doorn van R. H. E; A. J. Burggraaf

1993-01-01

298

Quantum confinement in perovskite oxide heterostructures: Tight binding instead of a nearly free electron picture  

NASA Astrophysics Data System (ADS)

Most recently, orbital-selective quantum well states of d electrons have been observed in SrVO3 ultrathin films and SrTiO3 surfaces. We construct from first-principles simple tight-binding models for such perovskite oxide heterostructures and surfaces. We show that this model provides a simple intuitive physical picture and yields, already with only two parameters, quantitatively reliable results, consistent with experiment. For quantum wells confined to only a few atomic layers or a higher quantum number, a nearly free electron description, on the other hand, does not work.

Zhong, Zhicheng; Zhang, Qinfang; Held, Karsten

2013-09-01

299

Nano-structured perovskite oxide electrodes for planar electrochemical sensors using tape casted YSZ layers  

Microsoft Academic Search

Solid-state NO2 sensors based on yttria stabilized zirconia (YSZ), an O2- conductor, combined either with an n- (WO3), or a p-type semiconducting oxide (LaFeO3), or a mixed electronic and ionic conductor (La0.8Sr0.2FeO3) were investigated. Platinum parallel finger electrodes were applied on the surface of tape-casted YSZ layers and attached with gold wires for current collection. Nanocrystalline perovskite powders were prepared

Elisabetta Di Bartolomeo; Narin Kaabbuathong; Alessandra D'Epifanio; Maria Luisa Grilli; Enrico Traversa; Hiromichi Aono; Yoshihiko Sadaoka

2004-01-01

300

Design Principles for Oxygen-Reduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal-air Batteries  

SciTech Connect

The prohibitive cost and scarcity of the noble-metal catalysts needed for catalysing the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries limit the commercialization of these clean-energy technologies. Identifying a catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant transition-metal-oxide catalysts to replace platinum. Here, we demonstrate that the ORR activity for oxide catalysts primarily correlates to {sigma}*-orbital (e{sub g}) occupation and the extent of B-site transition-metal-oxygen covalency, which serves as a secondary activity descriptor. Our findings reflect the critical influences of the {sigma}* orbital and metal-oxygen covalency on the competition between O{sub 2}{sup 2-}/OH{sup -} displacement and OH{sup -} regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.

J Suntivich; H Gasteiger; N Yabuuchi; H Nakanishi; J Goodenough; Y Shao-Horn

2011-12-31

301

Structural Transformations in Ceramics: Perovskite-like Oxides and Group III, IV, and V Nitrides  

SciTech Connect

1 Overview of Results and their Significance Ceramic perovskite-like oxides with the general formula (A. A0. ...)(B. B0. ...)O3and titanium-based oxides are of great technological interest because of their large piezoelectric and dielectric response characteristics.[1] In doped and nanoengineered forms, titantium dioxide finds increasing application as an organic and hydrolytic photocatalyst. The binary main-group-metal nitride compounds have undergone recent advancements of in-situ heating technology in diamond anvil cells leading to a burst of experimental and theoretical interest. In our DOE proposal, we discussed our unique theoretical approach which applies ab initio electronic calculations in conjunction with systematic group-theoretical analysis of lattice distortions to study two representative phase transitions in ceramic materials: (1) displacive phase transitions in primarily titanium-based perovskite-like oxide ceramics, and (2) reconstructive phase transitions in main-group nitride ceramics. A sub area which we have explored in depth is doped titanium dioxide electrical/optical properties.

James P. Lewis (PI, former Co-PI), Dorian M. Hatch (Co-PI, former PI), and Harold T. Stokes (Co-PI)

2006-12-31

302

Nanostructured magnetoelectric thin films of ferroelectric perovskite and magnetic oxides  

Microsoft Academic Search

Nanostructured thin films of ferroelectric (e.g., BaTiO3, PZT, and BiFeO3) and magnetic oxides (e.g., cobalt-ferrites and nickel-ferrites) prepared by physical deposition techniques (e.g., pulsed laser deposition) and chemical solution processing (e.g., sol-gel spin-coating method) have recently become new routes to multiferroic magnetoelectric (ME) composites. In comparison to bulk ME ceramic composites, the nanostructured thin films provide more degrees of freedom,

Ce-Wen Nan

2008-01-01

303

Resistive hystersis effects in perovskite oxide-based heterostructure junctions  

Microsoft Academic Search

The electrical and structural properties of the oxide-based metal\\/ferroelectric\\/metal (MFM) junctions are reported. The heterostructures are composed of ultrathin layers of La0.7Ca0.3MnO3 (LCMO) as a metallic layer and BaTiO3 (BTO) as a ferroelectric layer. Junction based devices, having the dimensions of 400200 mum2, have been fabricated upon LCMO\\/BTO\\/LCMO heterostructures by photolithography and Ar-ion milling technique. The dc current-voltage (I-V) characteristics

M. P. Singh; L. Mchin; W. Prellier; M. Maglione

2006-01-01

304

Resistive hystersis effects in perovskite oxide-based heterostructure junctions  

Microsoft Academic Search

The electrical and structural properties of the oxide-based metal\\/ferroelectric\\/metal (MFM) junctions are reported. The heterostructures are composed of ultrathin layers of La0.7Ca0.3MnO3 (LCMO) as a metallic layer and BaTiO3 (BTO) as a ferroelectric layer. Junction based devices, having the dimensions of 400200 ?m2, have been fabricated upon LCMO\\/BTO\\/LCMO heterostructures by photolithography and Ar-ion milling technique. The dc current-voltage (I-V) characteristics

M. P. Singh; L. Mechin; W. Prellier; M. Maglione

2006-01-01

305

Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications.  

PubMed

Transition metals of copper, zinc, chromium and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental composition were characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Phase analysis of transition metal-substituted cobalt ferrite nanoparticles was performed via X-ray diffraction. Surface wettability was measured using the water contact angle technique. The surface roughness of all nanoparticles was measured using profilometry. Moreover, thermogravimetric analysis and differential scanning calorimetry were performed to determine the temperature at which the decomposition and oxidation of the chelating agents took place. Results indicated that the substitution of transition metals influences strongly the microstructure, crystal structure and antibacterial property of the cobalt ferrite nanoparticles. PMID:23137676

Sanpo, Noppakun; Berndt, Christopher C; Wen, Cuie; Wang, James

2012-11-05

306

The Optical Properties of R Titanium Oxides Perovskites  

NASA Astrophysics Data System (ADS)

Reflectance spectroscopy (50-40000 cm ^{-1}) has been used to study the rare earth titanium (III) oxides (RTiO_3 , R = La, Ce, Pr, Nd, Sm, Gd). A mid-infrared absorption has been observed in all members of the series. With decreasing rare earth radius (and decreasing Ti-O-Ti bond angle) the band shifts to higher frequencies. A tight-binding calculation of the effect of the Ti-O-Ti bond angle on the conduction band width supports the picture in which the mid-infrared band is due to transition from the lower Hubbard to the upper Hubbard band. Because this intrinsic absorption is at lower energy than transitions corresponding to O(2p) -Ti(3d) charge transfer (~4 eV) the rare earth titanites are classified as Mott-Hubbard insulators. Systematic variations in the infrared active phonon spectra were observed and related to a tetragonal distortion of the TiO_6 octahedron in NdTiO_3, SmTiO_3 and GdTiO_3. In a second study, the reflectance of four well characterized (x-ray, neutron activation, thermogravimetric, dc resistivity, magnetization) doped LaTiO_3 samples was measured. Although the starting materials contained equimolar amounts of La and Ti, the neutron activation results and structural considerations indicate the presence of La vacancies. There is a transfer of oscillator strength from the mid-infrared band to lower frequencies in highly doped samples but transitions placing two electrons on one site (from the lower to the upper Hubbard band) retain considerable oscillator strength even in a sample where approximately one-fifth of the sites are empty.

Crandles, David Anthony

307

Electronic and ionic transport properties and other physical aspects of perovskites  

NASA Astrophysics Data System (ADS)

The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn-Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum-critical-point behaviour at lowest temperatures. Crossover of sgr-bonding electrons in the presence of localized spins associated with pgr-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

Goodenough, J. B.

2004-11-01

308

Electrical conductivity and performance of doped LaCrO 3 perovskite oxides for solid oxide fuel cells  

Microsoft Academic Search

The crystalline structure, redox stability and electrical conductivity of LaCrO3, (La1?xMx)CrO3 (M=Mg, Ca, Ba for x=0.3 and M=Sr for x=0.25), and (La0.75Sr0.25)(Cr0.5Mn0.5)O3 (LSCM) perovskites are studied from 500 to 800C in both oxidizing and reducing atmospheres. Dopability, redox stability and electrical conductivity are compared and examined. A-site doping with alkaline elements is found to improve significantly the electrical conductivity, particularly

San Ping Jiang; Li Liu; Khuong P. Ong; Ping Wu; Jian Li; Jian Pu

2008-01-01

309

Catabolism of hyaluronan: involvement of transition metals  

PubMed Central

One of the very complex structures in the vertebrates is the joint. The main component of the joint is the synovial fluid with its high-molar-mass glycosaminoglycan hyaluronan, which turnover is approximately twelve hours. Since the synovial fluid does not contain any hyaluronidases, the fast hyaluronan catabolism is caused primarily by reductive-oxidative processes. Eight transition metals V23, Mn25, Fe26, Co27, Ni28, Cu29, Zn30, and Mo42 naturally occurring in living organism are essential for the control of various metabolic and signaling pathways. They are also the key elements in catabolism of hyaluronan in the joint. In this overview, the role of these metals in physiological and pathophysiological catabolism of hyaluronan is described. The participation of these metals in the initiation and propagation of the radical degradation hyaluronan is critically reviewed.

Soltes, Ladislav; Kogan, Grigorij

2009-01-01

310

Catabolism of hyaluronan: involvement of transition metals.  

PubMed

One of the very complex structures in the vertebrates is the joint. The main component of the joint is the synovial fluid with its high-molar-mass glycosaminoglycan hyaluronan, which turnover is approximately twelve hours. Since the synovial fluid does not contain any hyaluronidases, the fast hyaluronan catabolism is caused primarily by reductive-oxidative processes.Eight transition metals - V(23), Mn(25), Fe(26), Co(27), Ni(28), Cu(29), Zn(30), and Mo(42) - naturally occurring in living organism are essential for the control of various metabolic and signaling pathways. They are also the key elements in catabolism of hyaluronan in the joint.In this overview, the role of these metals in physiological and pathophysiological catabolism of hyaluronan is described. The participation of these metals in the initiation and propagation of the radical degradation hyaluronan is critically reviewed. PMID:21217859

Solts, Ladislav; Kogan, Grigorij

2009-12-28

311

The effect of octahedral tilting on proton binding sites and transition states in pseudo-cubic perovskite oxides  

Microsoft Academic Search

Proton conducting oxide ceramics have shown potential for use in fuel cell technologies. Understanding the energy pathways for proton conduction could help us design more efficient fuel cell materials. This paper describes how octahedral tilting affects the relative energies of proton binding sites, transition states, and conduction pathways in cubic and pseudo-cubic perovskites. First, the structure for cubic and pseudo-cubic

Maria A. Gomez; Mary A. Griffin; Saryu Jindal; Kristin D. Rule; Valentino R. Cooper

2005-01-01

312

Transparent and conductive oxide films with the perovskite structure: La and Sb-doped BaSnO3  

Microsoft Academic Search

We report that (La0.07Ba0.93)SnO3 films grown epitaxially on SrTiO3(001) substrates by laser ablation at 760 C show a cubic perovskite structure of lattice constant 4.121 A˚, excellent optical transmittance in the visible range, and a weak metallic behavior with low resistivity of about 4 mOmega cm within 10-300 K. The transparent perovskite oxide films are n-type conductors, with carrier concentration

H. F. Wang; Q. Z. Liu; F. Chen; G. Y. Gao; Wenbin Wu; X. H. Chen

2007-01-01

313

Thermophysical properties of perovskite type alkaline-earth metals and plutonium complex oxides  

NASA Astrophysics Data System (ADS)

Polycrystalline specimens of strontium plutonate, SrPuO3, have been prepared by mixing the appropriate amounts of PuO2 and SrCO3 powders followed by reacting and sintering at 1600 K under the flowing gas atmosphere of dry-air. The sintered specimens had a single phase of orthorhombic perovskite structure and were crack-free. The elastic moduli of SrPuO3 were determined from the longitudinal and shear sound velocities. The Debye temperature was also determined from the sound velocities and lattice parameter measurements. The thermal conductivity of SrPuO3 was calculated from the measured density at room temperature, literature values of heat capacity, and thermal diffusivity measured by laser flash method in vacuum. Although the thermal conductivity of SrPuO3 slightly decreased with increasing temperature to 800 K, the range of change was extremely narrow and the temperature dependence did not completely follow the 1/T law. The thermal conductivity of SrPuO3 was lower than those of other perovskite type oxides.

Tanaka, Kosuke; Sato, Isamu; Hirosawa, Takashi; Kurosaki, Ken; Muta, Hiroaki; Yamanaka, Shinsuke

2012-03-01

314

The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte-cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation.  

PubMed

The stability of the valence state of the 3d transition metal ions and the stoichiometry of LiMO(2) (M = Co, Ni, Mn) layered oxides at the surface-electrolyte interface plays a crucial role in energy storage applications. The surface oxidation/reduction of the cations caused by the contact of the solids to air or to the electrolyte results in the blocking of the Li-transport through the interface that leads to the fast batteries deterioration. The influence of the end-of-charge voltage on the chemical composition and the oxidation state of 3d transition metal ions, as well as the stability of the solid-electrolyte interface formed during the electrochemical Li-deintercalation/intercalation of the LiCoO(2) and Li(Ni,Mn,Co)O(2), have been investigated by X-ray photoelectron spectroscopy. While the chemical composition of the solid-electrolyte interface is similar for both layered oxide surfaces, the electrochemical cycling to some critical voltage values leads to the disappearance of the interface. By the analysis of the shape of the 2p and 3s photoelectron emissions we show that the formation of the solid-electrolyte interface layer correlates with the partial reduction of the trivalent Co ions at the electrolyte-LiCoO(2) interface and the amount of the Co(2+) ions is increased as the solid-electrolyte interface vanishes. In contrast, the Mn(4+), Co(3+) and Ni(2+) ions of the Li(Ni,Mn,Co)O(2) are stable at the interface under the electrochemical cycling to higher end-of-charge voltage. A correlation between deterioration of the LiCoO(2) and Li(Ni,Mn,Co)O(2) batteries and the change of electronic structure at the surface/interface after the electrochemical cycling has been found. The dissolution of the solid-electrolyte interface layer might be the reason for the fast deterioration of the Li-ion batteries. PMID:22858824

Cherkashinin, Gennady; Nikolowski, Kristian; Ehrenberg, Helmut; Jacke, Susanne; Dimesso, Lucangelo; Jaegermann, Wolfram

2012-08-03

315

Synthesis of transition metal carbonitrides  

DOEpatents

Transition metal carbonitrides (in particular, titanium carbonitride, TiC.sub.0.5 N.sub.0.5) are synthesized by a self-propagating reaction between the metal (e.g., titanium) and carbon in a nitrogen atmosphere. Complete conversion to the carbonitride phase is achieved with the addition of TiN as diluent and with a nitrogen pressure .gtoreq.0.6 MPa. Thermodynamic phase-stability calculations and experimental characterizations of quenched samples provided revealed that the mechanism of formation of the carbonitride is a two-step process. The first step involves the formation of the nonstoichiometric carbide, TiC.sub.0.5, and is followed by the formation of the product by the incorporation of nitrogen in the defect-structure carbide.

Munir, Zuhair A. R. (Davis, CA); Eslamloo-Grami, Maryam (Davis, CA)

1994-01-01

316

Thermogravimetric Study on Oxygen Adsorption\\/Desorption Properties of Double Perovskite Structure Oxides REBaCo 2 O 5+? (RE = Pr, Gd, Y)  

Microsoft Academic Search

The oxygen adsorption\\/desorption properties of double perovskite structure oxides PrBaCo2O5+?, GdBa-Co2O5+? and YBaCo2O5+? were investigated by the thermogravimetry (TG) method in the temperature range of 400 ? 900 C. The calculated oxygen adsorption\\/desorption surface reaction rate constants ka and kd of these double perovskite structure oxides were larger than the commonly used cubic perovskite oxides, such as Ba0.95Ca0.05Co0.8Fe0.2O3-? and Ba0.5

Hao Haoshan; Zheng Lu; Wang Yingfang; Liu Shijiang; Hu Xing

2007-01-01

317

Mechanistic differences between electrochemical and gas-phase thermal oxidation of platinum-group transition metals as discerned by surface-enhanced Raman spectroscopy  

SciTech Connect

The oxidation of five Pt-group metals--platinum, palladium, iridium, rhodium, and ruthenium--is examined by means of surface-enhanced Raman spectroscopy (SERS) in aqueous electrochemical and gaseous dioxygen environments as a function of electrode potential and temperature, respectively, with the objective of intercomparing systematically the conditions required for surface oxide formation and discerning the reaction mechanisms involved. The SERS strategy, utilizing ultrathin Pt-group metal films electrodeposited on a gold substrate, enables monolayer-level metal oxide vibrational spectra to readily be obtained in both the electrochemical and gaseous environments. The SER spectra obtained during positive- and then negative-going potential excursions in aqueous 0.1 M HCLO{sub 4} display metal-oxygen vibrational bands signaling anodic oxide formation and subsequent removal at potentials consistent with corresponding voltammetric data. The nature of the amorphous oxides (or hydroxides) formed is deduced by comparison with bulk-phase metal oxide Raman spectra. The onset potentials for surface oxide formation are comparable to the thermodynamic potentials for the bulk-phase metal oxides. In contrast, the onset of surface oxidation even in ambient-pressure dioxygen uniformly requires elevated temperatures, {gt}200 C for each metal except for iridium, where oxide formation occurs at ca. 100 C. While spectral differences are evident, especially on palladium and ruthenium, the nature of the oxides formed in the electrochemical and gaseous systems is largely similar. The highly activated nature of the gaseous O{sub 2} oxidations is consistent with literature reports for Pt-group surfaces in ultrahigh vacuum as well as higher-pressure conditions. Likely reasons for the markedly more efficacious metal electrooxidations are discussed. Thermodynamic factors are not responsible, since the free-energy driving forces for the gaseous O{sub 2} oxidations are larger than for the electrochemical reactions at the applied potentials where surface oxidation for the latter processes proceeds at room temperature. The electrostatic driving forces for oxygen incorporation into the metal lattice (via high-field ion transport) are also typically more favorable for the gaseous systems, as evidenced by a comparison of the metal-solution and metal-gas surface potentials. The intrinsically more facile electrochemical processes thereby deduced are attributed to the occurrence of direct oxide production via a metal-oxygen place-exchange mechanism, expedited by interfacial solvation and therefore being energetically unfavorable in the anhydrous gas-phase environment. Other factors, such as the formation of precursor chemisorbed oxygen, are also considered.

Chan, H.Y.H.; Zou, S.; Weaver, M.J.

1999-12-16

318

Optical response of high-dielectric-constant perovskite-related oxide.  

PubMed

Optical conductivity measurements on the perovskite-related oxide CaCu3Ti4O12 provide a hint of the physics underlying the observed giant dielectric effect in this material. A low-frequency vibration displays anomalous behavior, implying that there is a redistribution of charge within the unit cell at low temperature. At infrared frequencies (terahertz), the value for the dielectric constant is approximately 80 at room temperature, which is far smaller than the value of approximately 10(5) obtained at lower radio frequencies (kilohertz). This discrepancy implies the presence of a strong absorption at very low frequencies due to dipole relaxation. At room temperature, the characteristic relaxation times are fast (less than or approximately 500 nanoseconds) but increase dramatically at low temperature, suggesting that the large change in dielectric constant may be due to a relaxor-like dynamical slowing down of dipolar fluctuations in nanosize domains. PMID:11474105

Homes, C C; Vogt, T; Shapiro, S M; Wakimoto, S; Ramirez, A P

2001-07-27

319

Autothermal reforming of propane on Ni-supported perovskite, hydrotalcite, and metal oxide catalysts.  

PubMed

This study evaluated the autotherrmal reforming of propane over Ni-supported perovskite (LaAlO3), hydrotalcite as a layered structure (MgAl), and metal oxide (alpha-Al2O3, ZrO2) catalysts. The catalysts were also characterized via physicochemical property analysis, XRD, SEM, TEM, and TPR. The order of catalytic activity was ZrO2 > or = LaAlO3 > Al2O3 > MgAl, and coincided with the reducibility of these catalysts to reduce the Ni2+ metallic ion of the NiO, as measured via TPR. Finally the reducility of Ni in the form of NiO was important to the reaction in the ATR process. The diffraction peak of Ni was observed after a 600 degrees C reduction in ZrO2, LaAlO3, and alpha-Al2O3. PMID:21456265

Park, Namcook; Kim, WooRi; Moon, DongJu; Seo, Gon; Kim, YoungChul

2011-02-01

320

Determination of the surface and interface phase shifts in metallic quantum well structures of perovskite oxides  

NASA Astrophysics Data System (ADS)

We propose an experimental approach to extract separately the surface and interface phase shift of standing waves in metallic quantum well (QW) structures composed of isostructural perovskite oxides. The asymmetric vacuum/SrVO3/SrTiO3 and symmetric SrTiO3/SrVO3/SrTiO3 QW structures are fabricated in an epitaxial multilayer form. Using these metallic QW structures, the phase shifts at the surface (vacuum/SrVO3) and interface (SrTiO3/SrVO3) are successfully obtained by analyzing a thickness series of angle-resolved photoemission spectra. The difference of the phase shift between the two boundaries reveals that nearly ideal quantum confinement is achieved at the interface, indicating that a SrTiO3 layer acts as a useful potential barrier.

Yoshimatsu, K.; Sakai, E.; Kobayashi, M.; Horiba, K.; Yoshida, T.; Fujimori, A.; Oshima, M.; Kumigashira, H.

2013-09-01

321

Rectifying characteristic in all-perovskite oxide film p-n junction with room temperature ferromagnetism  

NASA Astrophysics Data System (ADS)

We fabricated an all-perovskite oxide p-n junction comprised of hole-doped (p-) manganite La0.9Ba0.1MnO3 and electron-doped (n-) titanate Sr0.99La0.01TiO3 films. The junction showed good rectifying properties at both room temperature and low temperature in a simple structure without inserting an insulating layer. By optimizing junction fabrication conditions, a thin La0.9Ba0.1MnO3 layer in the junction exhibited room temperature ferromagnetism and metallic conduction, which may be modulated by carrier injection from the n-type layer under an electric field. These results indicate that this p-n junction may be developed into functional, strongly correlated electronic devices able to work at room temperature.

Zhang, Jun; Tanaka, Hidekazu; Kawai, Tomoji

2002-06-01

322

Transition-metal-free one-pot synthesis of biaryls from Grignard reagents and substituted cyclohexanones.  

PubMed

A new strategy for the construction of biaryls by a transition-metal-free process is presented. A sequence of a Grignard reaction, dehydration, and oxidative aromatization affords the desired products in a one-pot fashion. PMID:23559468

Zhou, Feng; Simon, Marc-Oliver; Li, Chao-Jun

2013-04-04

323

[Photoreduction of Se (VI) by marine algae-transitional metals-light system].  

PubMed

Seven marine phytoplankton, including five green algae (Tetraselmis levis, Chlorella autotrophica, Dunaliella salina, Nannochloropsis sp. and Tetraselmis subcordiformis), one diatom (Phaeodactylum tricornutum), one red alga (Porphyridium purpureum), and three usual transitional metals (Fe(III), Cu(II), Mn(II)) were used to make up marine phytoplankton-light or transitional metals-light or marine phytoplankton-transitional metals-light system. In such system, Se(VI) could be transformed into Se(IV) by photoreduction. The species transformation of selenium could be photo-induced by redox reaction of transitional metals. The photochemical activity of marine phytoplankton was confirmed for the first time, because marine phytoplankton could adsorb and concentrated of selenium, transitional metals and organic substances (including the exudation of algae, as reducing agent) which redox potentials were changed. The ratios of Se(VI) to Se(IV) were dominated by the species, the concentration of marine phytoplankton and transitional metals, and it could be enhanced through increasing the concentration of marine algae or the combined effect from marine algae and transitional metals. After photoreduction by ternary system, the ratio of Se(VI) to Se(IV) ranges from 1.17 to 2.85, which is close to the actual value in euphotic layer of seawater. The photochemical process that is induced by marine algae and transitional metals dominative the leading effects on the distribution of oxidation states of selenium. PMID:16212166

Li, Shun-Xing; Zheng, Feng-Ying; Deng, Nan-Sheng; Hong, Hua-Sheng; Zhu, Guo-Hui

2005-07-01

324

Chemical Pathways of Peptide Degradation. VIII. Oxidation of Methionine in Small Model Peptides by Prooxidant\\/Transition Metal Ion Systems: Influence of Selective Scavengers for Reactive Oxygen Intermediates  

Microsoft Academic Search

In the presence of oxygen, Fe(III), and an appropriate electron donor (e.g. ascorbic acid, dithiothreitol), the oxidation of methionine residues to methionine sulfoxides in small model peptides can be induced. It is shown in this study that these oxidations can be retarded by catalase in a pH-dependent manner, by some hydroxyl radical scavengers, and by azide. In contrast, superoxide dismutase

Shihong Li; Christian Schneich; Ronald T. Borchardt

1995-01-01

325

Thermal hysteresis in the dielectric properties of composites based on transition metal oxide and sulfide nanoparticles stabilized in a low-density polyethylene matrix  

NASA Astrophysics Data System (ADS)

We have studied the temperature dependence of the complex dielectric permittivity of composites based on a low-density polyethylene matrix containing dispersed nanoparticles of zinc oxide and manganese oxide and an analogous composite with nanoparticles of cadmium sulfide. Laws governing the electric conduction and dielectric polarization as functions of the temperature and the size of nanoparticles are established and a possible mechanism responsible for the appearance of thermal hysteresis of the dielectric properties in the materials studied is proposed.

Ul'Zutuev, A. N.; Ushakov, N. M.; Yurkov, G. Yu.; Kosobudskii, I. D.

2009-05-01

326

Mechanistic differences between electrochemical and gas-phase thermal oxidation of platinum-group transition metals as discerned by surface-enhanced Raman spectroscopy  

Microsoft Academic Search

The oxidation of five Pt-group metals--platinum, palladium, iridium, rhodium, and ruthenium--is examined by means of surface-enhanced Raman spectroscopy (SERS) in aqueous electrochemical and gaseous dioxygen environments as a function of electrode potential and temperature, respectively, with the objective of intercomparing systematically the conditions required for surface oxide formation and discerning the reaction mechanisms involved. The SERS strategy, utilizing ultrathin Pt-group

Ho Yeung H. Chan; Shouzhong Zou; Michael J. Weaver

1999-01-01

327

Tilts, dopants, vacancies and non-stoichiometry: Understanding and designing the properties of complex solid oxide perovskites from first principles  

NASA Astrophysics Data System (ADS)

Perovskite oxides of formula ABO3 have a wide range of structural, electrical and mechanical properties, making them vital materials for many applications, such as catalysis, ultrasound machines and communication devices. Perovskite solid solutions with high piezoelectric response, such as ferroelectrics, are of particular interest as they can be employed as sensors in SONAR devices. Ferroelectric materials are unique in that their chemical and electrical properties can be non-invasively and reversibly changed, by switching the bulk polarization. This makes ferroelectrics useful for applications in non-volatile random access memory (NVRAM) devices. Perovskite solid solutions with a lower piezoelectric response than ferroelectrics are important for communication technology, as they function well as electroceramic capacitors. Also of interest is how these materials act as a component in a solid oxide fuel cell, as they can function as an efficient source of energy. Altering the chemical composition of these solid oxide materials offers an opportunity to change the desired properties of the final ceramic, adding a degree of flexibility that is advantageous for a variety of applications. These solid oxides are complex, sometimes disordered systems that are a challenge to study experimentally. However, as it is their complexity which produces favorable properties, highly accurate modeling which captures the essential features of the disordered structure is necessary to explain the behavior of current materials and predict favorable compositions for new materials. Methodological improvements and faster computer speeds have made first-principles and atomistic calculations a viable tool for understanding these complex systems. Offering a combination of accuracy and computational speed, the density functional theory (DFT) approach can reveal details about the microscopic structure and interactions of complex systems. Using DFT and a combination of principles from both inorganic chemistry and materials science, I have been able to gain insights into solid oxide perovskite-based systems.

Bennett, Joseph W.

328

Structural and surface analysis of unsupported and alumina-supported La(Mn,Fe,Mo)O3 perovskite oxides.  

PubMed

A series of bulk and Al(2)O(3)-supported perovskite oxides of the type LaMn(1-x-y)Fe(x)Mo(y)O(3) (x = 0.00-0.90 and y = 0.00-0.09) were synthesized by the citric acid complexation-gelation method followed by annealing in air at 800 degrees C. For all samples, the local environment and the chemical state and concentration of surface species were determined. Mssbauer spectra revealed the only presence of octahedral Fe(3+) ions dispersed in the perovskite structure, however well-crystallized together with a poorly crystalline LaFeO(3) phases were detected for larger substitutions (x = 0.90). A similar picture was obtained for Mo-loaded (y = 0.02 and 0.05) samples but a new phase most likely related to Fe(3+) ions dispersed aside from the perovskite structure was found for larger substitutions (y = 0.09). Together with these structures, supported samples showed the presence of LaFeO(3) nanoparticles. Finally, photoelectron spectroscopy indicated that the chemical state and composition of the samples in the surface region (2-3 nm) approaches that of the bulk. For the unsupported substituted samples, iron (and molybdenum) enters into the perovskite structure while manganese tends to be slightly segregated. Moreover, in supported perovskites, a fraction of Mo and La atoms interact with the alumina surface. All these oxides were active in methane combustion and best performance was recorded for the Fe-rich composition (x = 0.9) in which both Mn(3+) and Mo(3+) ions were in the same proportion (y = 0.05). PMID:19956932

Rosmaninho, M G; Tristo, J C; Moura, F C C; Lago, R M; Arajo, M H; Fierro, J L G

2009-12-03

329

Electrochemical hydrogen property improved in nano-structured perovskite oxide LaFeO3 for Ni/MH battery  

NASA Astrophysics Data System (ADS)

Perovskite oxide LaFeO3, as a novel candidate for the electrode of Ni/MH battery, holds high specific capacity and good cyclical durability at elevated temperatures. However, the poor electrochemical kinetics is a bottleneck for the application of this type of material. By use of nano-structured materials, there are greatly enhanced values of exchange current density I0 and hydrogen diffusion coefficient D, which resulted in an improvement of electrochemical kinetics, a much higher specific capacity and excellent stability during cycling for nano-structured LaFeO3. In theory, there is a significant possibility of further advancing the hydrogen reaction kinetics of perovskite type oxides for Ni/MH battery.

Wang, Qiang; Deng, Gang; Chen, Zhiqian; Chen, Yungui; Cheng, Nanpu

2013-02-01

330

New class of single-source precursors for the synthesis of main group-transition metal oxides: heterobimetallic Pb-Mn beta-diketonates.  

PubMed

Heterometallic lead-manganese beta-diketonates have been isolated in pure form by several synthetic methods that include solid-state and solution techniques. Two compounds with different Pb/Mn ratios, PbMn(2)(hfac)(6) (1) and PbMn(hfac)(4) (2) (hfac = hexafluoroacetylacetonate), can be obtained in quantitative yield by using different starting materials. Single crystal X-ray investigation revealed that the solid-state structure of 1 contains trinuclear molecules in which lead metal center is sandwiched between two [Mn(hfac)(3)] units, while 2 consists of infinite chains of alternating [Pb(hfac)(2)] and [Mn(hfac)(2)] fragments. The heterometallic structures are held together by strong Lewis acid-base interactions between metal atoms and diketonate ligands acting in chelating-bridging fashion. Spectroscopic investigation confirmed the retention of heterometallic structures in solutions of non-coordinating solvents as well as upon sublimation-deposition procedure. Thermal decomposition of heterometallic diketonates has been systematically investigated in a wide range of temperatures and annealing times. For the first time, it has been shown that thermal decomposition of heterometallic diketonates results in mixed-metal oxides, while both the structure of precursors and the thermolysis conditions have a significant influence on the nature of the resulting oxides. Five different Pb-Mn oxides have been detected by X-ray powder diffraction when studying the decomposition of 1 and 2 in the temperature range 500-800 degrees C. The phase that has been previously reported as "Pb(0.43)MnO(2.18)" was synthesized in the pure form by decomposition of 1, and crystallographically characterized. The orthorhombic unit cell parameters of this oxide, obtained by electron diffraction technique, have been subsequently refined using X-ray powder diffraction data. Besides that, a previously unknown lead-manganese oxide has been obtained at low temperature decomposition and short annealing times. The parameters of its monoclinically distorted unit cell have been determined. The EDX analysis revealed that this compound has a Pb/Mn ratio close to 1:4 and contains no appreciable amount of fluorine. PMID:19653652

Zhang, Haitao; Yang, Jen-Hsien; Shpanchenko, Roman V; Abakumov, Artem M; Hadermann, Joke; Clrac, Rodolphe; Dikarev, Evgeny V

2009-09-01

331

High temperature electrical properties of the perovskite-type oxide La 1 ? x Sr xMnO 3 ? d  

Microsoft Academic Search

In order to elucidate the conduction mechanism of the perovskite-type oxide La1 ? xSrxMnO3 ? d(0$?= x $?= 0.4), the electrical conductivity, ?, and Seebeck coefficient, Q, were measured as a function of temperature, T, up to 1100 C in l atm O2 gas and as a function of the oxygen partial pressure, P(O2), at 8001100 dgC. At T ?

Hiroyuki Kamata; Yuki Yonemura; Junichiro Mizusaki; Hiroaki Tagawa; Kazunori Naraya; Ta-i Sasamoto

1995-01-01

332

Chemical Pressure Effects On Tc In Double Perovskite Oxides A2FeMoO6 (A = Ba, Sr)  

NASA Astrophysics Data System (ADS)

The effect of chemical pressure on the ferromagnetic transition temperature has been investigated in the double perovskite oxides Sr2FeMoO6 and Ba2FeMoO6 using first principle electronic structure calculations. While the Curie temperature is found to be larger by 50-100 K for Sr2FeMoO6 over that for Ba2FeMoO6 in experiments, the difference we find in our calculations is smaller.

Nandy, Ashis K.; Mahadevan, Priya; Sarma, D. D.

2008-04-01

333

Laves Phases of Uranium and Transition Metals.  

National Technical Information Service (NTIS)

Pauling's valence-bond theory of metals was used to calculate bond numbers of uranium-transition metal Laves phase compounds. Hybridization was determined from Pauling's relationships between single bond radii and d orbital participation in bonding. The a...

E. C. Beahm

1975-01-01

334

New Application of Transition Metal Carbides Detailed.  

National Technical Information Service (NTIS)

Many transition metal family metal carbides are compounds with high melting points of more than 3,000 C. Recently, growth of their monocrystals has become possible with the elucidation of their basic properties. The processes are detailed for manufacturin...

Y. Ishizawa

1987-01-01

335

Dissociation of methane on different transition metals  

Microsoft Academic Search

Dissociation of methane on different transition metals M (M=Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag, Au) has been investigated using a quasi-relativistic density-functional method. Reaction enthalpies for the steps involved are determined. The activation energies have been estimated using the analytic BOC-MP formula. The transition metals, Ru, Rh, , Pt are shown to exhibit high activity in the dissociation

Meng-Sheng Liao; Qian-Er Zhang

1998-01-01

336

Doped PrMnO3 perovskite oxide as a new cathode of solid oxide fuel cells for low temperature operation  

Microsoft Academic Search

Cathodic overpotentials of Ln(0.6)Sr(0.4)MnO3 Ln = La, Pr, Nd, Sm, Gd, Yb, and Y) were studied for a new cathode of solid oxide fuel cell (SOFC). Cathodic overpotentials as well as the electrical conductivity strongly depended on the rare earth cations used for the A sites of perovskite oxide. Strontium doped PrMnO3 exhibited the highest electrical conductivity among the examined

Tatsumi Ishihara; Takanari Kudo; Hideaki Matsuda; Yusaku Takita

1995-01-01

337

Perovskite-Type Metal Oxides Exhibiting Negligible Grain Boundary Resistance to Total Electrical Conductivity.  

PubMed

In this paper, we report the synthesis, structure and electrical properties of the perovskite-type AZn(0.33+x)Nb(0.67-x)O(3-?) (A = Sr or Ba; 0 ? x ? 0.08). The investigated compounds were prepared by employing the solid-state (ceramic) reaction using alkaline nitrates, zinc oxide, and niobium oxide at elevated temperatures in air. Powder X-ray diffraction (PXRD) showed the formation of disordered Zn and Nb at the B-sites of space group Pm3?m with cubic structure and a lattice constant comparable to that of the literature. The AC impedance study showed mainly bulk contribution to the total electrical conductivity over the investigated frequency range of 0.01 Hz to 1 MHz in all the investigated atmospheres even at low temperatures, which is significantly different from that of the well-known perovskite-type B-site ordered BaCa(0.33+x)Nb(0.67-x)O(3-?) and the disordered acceptor-doped BaCeO(3). The bulk dielectric constant determined at 500 and 700 C in air was found to be in the range of 35-100. In air, the isothermal bulk dielectric constant seems to increase with an increasing Zn content, and a similar trend was observed for total electrical conductivity. In dry and wet H(2), the electrical conductivity decreases with an increasing Zn content in AZn(0.33+x)Nb(0.67-x)O(3-?), and the x = 0 member of the Ba compound exhibits the highest total conductivity of 7.2 10(-3) S cm(-1) in dry H(2) at 800 C. Both Sr and Ba compounds were found to be stable against the reaction with pure CO(2) at 700 C and H(2)O at 100 C for a long period of time. SrZn(0.33+x)Nb(0.67-x)O(3-?) was found to be stable in 30 ppm H(2)S at 800 C, while the corresponding Ba compound formed reaction products such as BaS (JCPDS Card 01-0757), BaS(2) (JCPDS Card 21-0087), and BaS(3) (JCPDS Card 03-0824). PMID:21142004

Pannu, Tania; Pannu, Kanwar Gulsher Singh; Thangadurai, Venkataraman

2010-12-01

338

Additional effects of rare earth elements on formation and properties of some transition metal pyrophosphates  

Microsoft Academic Search

Transition metal phosphates have been used as inorganic pigments. By the addition of rare earth elements, new functional materials are expected to form. In this work, rare earth oxides (Y2O3, La2O3, CeO2, and Nd2O3) were added in the formation systems of copper, manganese, and cobalt pyrophosphates prepared from ammonium dihydrogenphosphate and the transition metal carbonate. The thermal behaviors of the

Hiroaki Onoda; Kazuo Kojima; Hiroyuki Nariai

2006-01-01

339

Dissolution of transition metals and oxides. Synthesis and structure of copper complexes in. cap alpha. ,. cap alpha. '-dipyridyl-amide systems. Role of the solvent  

SciTech Connect

The dissolution of metallic copper and its oxides in ..cap alpha..,..cap alpha..'-dipyridyl-amide systems (containing N,N'-dimethylacetamide, N-methylacetamide, N-methylformamide, and formamide) has been studied. The following complex compounds of copper have been isolated and characterized: Cu/sub 3/(OH)/sub 2/(CO/sub 3/)/sub 2/(dipy)/sub 3/ x 10H/sub 2/O (I), Cu/sub 2/CO/sub 5/(HCO/sub 3/)/sub 2/(dipy)/sub 2/ x 2(CH/sub 3/)/sub 2/NC(CH/sub 3/)O x 2H/sub 2/O (II), Cu/sub 2/(HCOO)/sub 4/(dipy)/sub 2/ x CH/sub 3/NHCHO (III), Cu/sub 2/(HCOO)/sub 2/(CN)/sub 2/(dipy)/sub 2/ x 5NH/sub 2/CHO (IV), and Cu(HCOO)/sub 2/(H/sub 2/NCHO)/sub 2/ (V). The molecular and crystal structures of complexes I and V have been determined. On the basis of an analysis of the anionic structure of the compounds, which is formed from the solvent molecules, and the chemistry of the processes of the dissolution of Cu/sup 0/, Cu/sub 2//sup I/O, and Cu/sup I/O, it has been concluded that there is electronic and coordinative amphoterism in the series of amides of acetic and formic acid investigated.

Chulkevich, A.K.; Lavrent'ev, I.P.; Moravskii, A.P.; Khidekel', M.L.; Ponomarev, V.I.; Filipenko, O.S.; Atovmyan, L.O.

1987-02-01

340

Solid-state transition metal chemistry with quinolin-4-yl-methyl-[N-(n-butyl)amino]-diphenylphosphine oxide (4-qmape): Crystal structure of the 4-qmape ligand  

NASA Astrophysics Data System (ADS)

A series of three perchlorate coordination compounds of bioactive diethyl quinolin-4-yl-methyl(N-butylamino)diphenylphosphine oxide (4-qmape) ligand, with the following stoichiometries [M(2-qmape)2](ClO4)2 MCu and Ni and [Co(2-qmape)2](ClO4)2, were obtained and studied. Stoichiometry and stereochemistry of the compounds was confirmed by spectroscopic and magnetic studies as well as by elemental analyses. In particular, the crystal structure of the free ligand was determined. The 4-qmape ligand has a potential capacity to coordinate to metal ions by following atoms: phosphoryl oxygen, amino nitrogen and quinolyl nitrogen. In studied compounds, 4-qmape adopts the didentate N,O-coordination mode, bonding metal centers through the phosphoryl oxygen and amino nitrogen. Quinoline nitrogen atom does not participate in coordination. The all complexes are monomeric with tetrahedral environment of metal ions. Magnetic studies (1.8-300 K) indicate existence of a very weak exchange coupling between metal centers in crystal lattice.

?urowska, Bogumi?a; Brzuszkiewicz, Anna; Boduszek, Bogdan

2012-11-01

341

Transition metal oxide pillared clay. 1: A comparative study of textural and acidic properties of Fe(III) pillared montmorillonite and pillared acid activated montmorillonite  

SciTech Connect

Fe(III) pillared montmorillonite samples have been prepared by intercalating trinuclear acetato hydroxy-iron (III) nitrate [Fe(COOCH{sub 3}){sub 7}OH 2H{sub 2}O]{sup +} NO{sub 3}{sup {minus}} between the layers of both Na-exchanged and acid-activated montmorillonite, followed by calcination and characterized by various techniques. The multistep ion exchange process gave better complex loading than the single step process. FTIR and Moessbauer spectral analysis showed the presence of the complex inside the silicate layers. Materials prepared from the two starting materials are thermally stable up to 500 C, having basal spacings of 18.0 and 17.6 {angstrom} and high surface areas of 284 and 276 m{sup 2}/g, respectively. The acid-activated pillared montmorillonite shows somewhat low complex intake, but has high acidity in comparison to the Na-exchanged material. Iron oxide pillared clay has gained considerable importance as it can be used in demetalization, reduction of NO by NH{sub 3}, and Fischer-Tropsch reactions.

Mishra, T.; Parida, K.M.; Rao, S.B. [Council of Scientific and Industrial Research, Bhubaneswar (India). Regional Research Lab.

1996-10-15

342

Correlation between resistance-change effect in transition-metal oxides and secondary-electron contrast of scanning electron microscope images  

NASA Astrophysics Data System (ADS)

Conductive atomic-force microscopy (C-AFM) writing is attracting attention as a technique for clarifying the switching mechanism of resistive random-access memory by providing a wide area filled with filaments, which can be regarded as one filament with large radius. The writing area on a nickel-oxide (NiO) film formed by conductive atomic-force microscopy was observed by scanning electron microscope, and a correlation between the contrast in a secondary-electron image (SEI) and the resistance written by C-AFM was revealed. In addition, the dependence of the SEI contrast on the beam accelerating voltage (Vaccel) suggests that the resistance-change effect occurs near the surface of the NiO film. As for the effects of electron irradiation and vacuum annealing on the C-AFM writing area, it was shown that the resistance-change effect is caused by exchange of oxygen with the atmosphere at the surface of the NiO film. This result suggests that the low-resistance and high-resistance areas are, respectively, p-type Ni1+?O (? < 0) and insulating (stoichiometric) or n-type Ni1+?O (? >= 0).

Kinoshita, K.; Yoda, T.; Kishida, S.

2011-09-01

343

Synthesis of ferromagnetic nanoparticles, formic acid oxidation catalyst nanocomposites, and late-transition metal-boride intermetallics by unique synthetic methods and single-source precursors  

NASA Astrophysics Data System (ADS)

The design, synthesis, and characterization of magnetic alloy nanoparticles, supported formic acid oxidation catalysts, and superhard intermetallic composites are presented. Ferromagnetic equatomic alloy nanoparticles of FePt, FePd, and CoPt were synthesized utilizing single-source heteronuclear organometallic precursors supported on an inert water-soluble matrix. Direct conversion of the precursor-support composite to supported ferromagnetic nanoparticles occurs under elevated temperatures and reducing conditions with metal-ion reduction and minimal nanoparticle coalescence. Nanoparticles were easily extracted from the support by addition of water and characterized in structure and magnetic properties. Palladium and platinum based nanoparticles were synthesized with microwave-based and chemical metal-ion reduction strategies, respectively, and tested for catalytic performance in a direct formic acid fuel cell (DFAFC). A study of palladium carbide nanocomposites with various carbonaceous supports was conducted and demonstrated strong activity comparable to commercially available palladium black, but poor catalytic longevity. Platinum-lead alloy nanocomposites synthesized with chemical reduction and supported on Vulcan carbon demonstrated strong activity, excellent catalytic longevity, and were subsequently incorporated into a prototype DFAFC. A new method for the synthesis of superhard ceramics on polymer substrates called Confined Plasma Chemical Deposition (CPCD) was developed. The CPCD method utilizes a tuned Free Electron Laser to selectively decompose the single-source precursor, Re(CO)4(B3H8), in a plasma-like state resulting in the superhard intermetallic ReB2 deposited on polymer substrates. Extension of this method to the synthesis of other hard of superhard ceramics; WB4, RuB2, and B4C was demonstrated. These three areas of research show new synthetic methods and novel materials of technological importance, resulting in a substantial advance in their respective fields.

Wellons, Matthew S.

344

Bandgap engineering in perovskite oxides: Al-doped SrTiO3  

NASA Astrophysics Data System (ADS)

The ability to modulate the bandgap of a material without altering its functional properties is crucial for fabricating heterojunctions for device applications. Here, we explore experimentally and theoretically the effect of the substitution of Ti with Al on the bandgap of perovskite oxide SrTiO3. We grow Al-doped SrTiO3 films directly on Si(100) and show from electron energy loss spectra that the bandgap is increased by approximately 0.3 eV over undoped SrTiO3. This bandgap increase is confirmed by spectroscopic ellipsometry measurements on identical films grown on LaAlO3 substrates, which show a 0.3 eV blue shift in the steep increase of the absorption edge. Current vs. voltage measurements show a reduction in leakage current by six orders of magnitude at a field of 1 MV/cm. We use density functional theory to explain how Al replacing Ti modifies the conduction band edge density of states resulting in the experimentally observed increase in the bandgap.

Posadas, Agham B.; Lin, Chungwei; Demkov, Alexander A.; Zollner, Stefan

2013-09-01

345

Process for producing a powder of perovskite-type double oxide  

SciTech Connect

A process is described for producing a powder of perovskite-type double oxide represented by the formula ABO[sub 3], wherein A represents a metal material which is at least one member selected from the group consisting of Pb and a combination of Pb and at least one element selected form among Ba, Sr, Ca, La, and Li, B represents a metal material selected from the group consisting of a combination of Nb and at least one additional element selected from among Zn, Mg, Zr, Ti, Ni, Fe, W, Co, and Mn, and 0 represents oxygen, comprising the steps of: forming a homogeneous solution containing metal material B, by adding a solution containing a metal ion of Nb to a solution containing a metal ion of at least on of said additional element thereby preparing a homogeneous containing metal material B; reacting a solution containing a metal material A, and an aqueous solution of alkaline metal in a reaction vessel, thereby precipitating the hydroxide of metal material A; adding said homogeneous solution containing metal material B to said hydroxide precipitate of metal material A in a reaction vessel, thereby precipitating the hydroxide of metal material B in the obtained mixture; reacting hydrothermally the obtained mixture in the reaction vessel; filtering the reacted mixture; cleaning and drying the filtered precipitate; and heat-treating and pulverizing the dried precipitate.

Watanabe, Munetoshi; Hata, Hiroyuki.

1993-07-20

346

Radiation enhanced diffusion of hydrogen in perovskite-type oxide ceramics under reactor irradiation  

NASA Astrophysics Data System (ADS)

Electrical conductivity of yttrium-doped perovskite-type barium-cerium oxide ceramics (BaCe0.9Y0.1O3-?), implanted with 10 keV H2+ ions, was measured in situ under fission reactor irradiation. An increment of the electrical conductivity, called radiation induced conductivity (RIC), was observed with increasing ionizing dose rate. The RIC for the specimen with implanted H at 1.1 kGy/s and irradiation temperatures 473 673 K was higher by about four orders of magnitude than the base conductivity without radiation at 0 Gy/s, and was about two orders of magnitude higher than that without H. The RIC is attributed to electronic excitation as well as hydrogen enhanced diffusion. The RIC greatly depended on the irradiation temperature, but was insensitive to the fast neutron fluence in the range 3.3 7.4 1023 n/m2. The results show that the radiation induced defects, produced by neutron collisions, and radiolysis have no influence on the electronic and protonic conduction.

Tsuchiya, B.; Shikama, T.; Nagata, S.; Toh, K.; Narui, M.; Yamazaki, M.

2007-08-01

347

Distinctive Charge Density Distributions of Perovskite-Type Antiferroelectric Oxides PbZrO3 and PbHfO3 in Cubic Phase  

Microsoft Academic Search

The electron charge density distributions of simple perovskite oxides, PbBO3 (B = Ti, Zr and Hf), in their cubic phase are investigated by analyzing high-energy synchrotron powder diffraction data by the maximum entropy method (MEM)\\/Rietveld method. Clear structural differences between the antiferroelectric and ferroelectric perovskites are revealed. In the cubic phase of PbZrO3 and PbHfO3 that undergo antiferroelectric phase transitions,

Yoshihiro Kuroiwa; Hiroshi Fujiwara; Akikatsu Sawada; Shinobu Aoyagi; Eiji Nishibori; Makoto Sakata; Masaki Takata; Hitoshi Kawaji; Tooru Atake

2004-01-01

348

Transparent and conductive oxide films with the perovskite structure: La- and Sb-doped BaSnO3  

NASA Astrophysics Data System (ADS)

We report that (La0.07Ba0.93)SnO3 films grown epitaxially on SrTiO3(001) substrates by laser ablation at 760 C show a cubic perovskite structure of lattice constant 4.121 A?, excellent optical transmittance in the visible range, and a weak metallic behavior with low resistivity of about 4 m? cm within 10-300 K. The transparent perovskite oxide films are n-type conductors, with carrier concentration and mobility at room temperature of about 21021 cm-3 and 0.69 cm2/V s, respectively, and a direct allowed band gap of 4.02 eV. The effect of deposition temperature on structural, optical, and electric properties of the Sb-doped Ba(Sn0.93Sb0.07)O3 films was also demonstrated.

Wang, H. F.; Liu, Q. Z.; Chen, F.; Gao, G. Y.; Wu, Wenbin; Chen, X. H.

2007-05-01

349

Determination of the surface spin-polarization of perovskite oxides using point-contact Andreev reflection spectroscopy  

NASA Astrophysics Data System (ADS)

Materials with surface spin-polarization are invaluable for incorporation into devices that utilize spin-polarized currents. Point-contact Andreev reflection spectroscopy is currently one of the few techniques capable of direct measurement of surface spin-polarization. Niobium wire was electrochemically etched in a potassium hydroxide solution to form sharp tips which were used to form point-contacts with perovskite oxides in single crystal and thin film forms. Surface spin-polarization values were determined at 4.2 K for several materials including La0.7Sr0.3MnO3, which is a material with purported 100% spin polarization. The results show that surface spin polarization of perovskites is smaller than theoretically predicted.

Grimley, Everett; Biswas, Amlan

2013-03-01

350

Resistive switching mechanisms in random access memory devices incorporating transition metal oxides: TiO2, NiO and Pr0.7Ca0.3MnO3.  

PubMed

Resistance change random access memory (RRAM) cells, typically built as MIM capacitor structures, consist of insulating layers I sandwiched between metal layers M, where the insulator performs the resistance switching operation. These devices can be electrically switched between two or more stable resistance states at a speed of nanoseconds, with long retention times, high switching endurance, low read voltage, and large switching windows. They are attractive candidates for next-generation non-volatile memory, particularly as a flash successor, as the material properties can be scaled to the nanometer regime. Several resistance switching models have been suggested so far for transition metal oxide based devices, such as charge trapping, conductive filament formation, Schottky barrier modulation, and electrochemical migration of point defects. The underlying fundamental principles of the switching mechanism still lack a detailed understanding, i.e. how to control and modulate the electrical characteristics of devices incorporating defects and impurities, such as oxygen vacancies, metal interstitials, hydrogen, and other metallic atoms acting as dopants. In this paper, state of the art ab initio theoretical methods are employed to understand the effects that filamentary types of stable oxygen vacancy configurations in TiO(2) and NiO have on the electronic conduction. It is shown that strong electronic interactions between metal ions adjacent to oxygen vacancy sites results in the formation of a conductive path and thus can explain the 'ON' site conduction in these materials. Implication of hydrogen doping on electroforming is discussed for Pr(0.7)Ca(0.3)MnO(3) devices based on electrical characterization and FTIR measurements. PMID:21572196

Magyari-Kpe, Blanka; Tendulkar, Mihir; Park, Seong-Geon; Lee, Hyung Dong; Nishi, Yoshio

2011-05-16

351

Physical, mechanical and electrochemical characterization of all-perovskite intermediate temperature solid oxide fuel cells  

NASA Astrophysics Data System (ADS)

Strontium- and magnesium-doped lanthanum gallate (LSGM) has been considered as a promising electrolyte for solid oxide fuel cell (SOFC) systems in recent years due to its high ionic conductivity and chemical stability over a wide range of oxygen partial pressures and temperatures. This research describes synthesis, physical and mechanical behavior, electrochemical properties, phase evolution, and microstructure of components of an all-perovskite anode-supported intermediate temperature solid oxide fuel cell (ITSOFC), based on porous La 0.75Sr0.25Cr0.5Mn0.5O3 (LSCM) anode, La0.8Sr0.2Ga0.8Mg0.2O 2.8 (LSGM) electrolyte, and porous La0.6Sr0.4Fe 0.8Co0.2O3 (LSCF) cathode. The phase evolution of synthesized LSGM and LSCM powders has been investigated, and it has been confirmed that there is no reaction between LSGM and LSCM at sintering temperature. Using different amounts of poreformers and binders as well as controlling firing temperature, porosity of the anode was optimized while still retaining good mechanical integrity. The effect of cell operation conditions under dry hydrogen fuel on the SOFC open circuit voltage (OCV) and cell performance were also investigated. Characterization study of the synthesized LSGM indicates that sintering at 1500C obtains higher electrical conductivity compared to the currently published results, while conductivity of pellets sintered at 1400C and 1450C would be slightly lower. The effect of sintering temperature on bulk and grain boundary resistivities was also discussed. The mechanical properties, such as hardness, Young's modulus, fracture toughness and modulus of rupture of the electrolyte were determined and correlated with scanning electron microscopy (SEM) morphological characterization. Linear thermal expansion and thermal expansion coefficient of LSGM were also measured.

Mohammadi, Alidad

352

Synthesis of intercalatable layered stable transition metal chalcogenides and alkali metal-transition metal chalcogenides  

SciTech Connect

Intercalatable layered or lamellar stable transition metal chalcogenides, such as CrS/sub 2/, usable as cathode active materials and intercalatable layered or lamellar stable alkali metal-transition metal chalcogenides, such as LiCrS/sub 2/, usable as cathode active materials as well as anode active materials are synthesized by ion implantation and annealing techniques.

Palmer, D. N.

1985-09-17

353

Study of Ba3M(II)M(IV)WO9 (M(II) = Ca, Zn; M(IV) = Ti, Zr) perovskite oxides: competition between 3C and 6H perovskite structures.  

PubMed

We describe an investigation of Ba3MIIMIVWO9 oxides for MII = Ca, Zn, and other divalent metals and MIV = Ti, Zr. In general, a 1:2-ordered 6H (hexagonal, P63/mmc) perovskite structure is stabilized at high temperatures (1300 degrees C) for all of the Ba3MIITiWO9 oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba2(Zn2/3Ti1/3)(W2/3Ti1/3)O6, is obtained at 1200 degrees C for Ba3ZnTiWO9. Sr substitution for Ba in the latter stabilizes the cubic 3C structure instead of the 6H structure. A metastable Ba3CaZrWO9 that adopts the 3C (cubic, Fmm) structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the present investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d0-induced distortion of metal-oxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the Ba3MIIMIVWO9 series. PMID:17630730

Mani, Rohini; Selvamani, P; Joy, Joby E; Gopalakrishnan, J; Mandal, Tapas Kumar

2007-07-14

354

Activation of methane by transition metal-substituted aluminophosphate molecular sieves  

DOEpatents

Aluminophosphate molecular sieves substituted with cobalt, manganese or iron and having the AlPO.sub.4 -34 or AlPO.sub.4 -5, or related AlPO.sub.4 structure activate methane starting at approximately 350.degree. C. Between 400.degree. and 500.degree. C. and at methane pressures .ltoreq.1 atmosphere the rate of methane conversion increases steadily with typical conversion efficiencies at 500.degree. C. approaching 50% and selectivity to the production of C.sub.2+ hydrocarbons approaching 100%. The activation mechanism is based on reduction of the transition metal(III) form of the molecular sieve to the transition metal(II) form with accompanying oxidative dehydrogenation of the methane. Reoxidation of the - transition metal(II) form to the transition metal(III) form can be done either chemically (e.g., using O.sub.2) or electrochemically.

Iton, Lennox E. (Downers Grove, IL); Maroni, Victor A. (Naperville, IL)

1991-01-01

355

Method of boronizing transition metal surfaces  

DOEpatents

A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB[sub 2], or CrB[sub 2]. A transition metal to be coated is immersed in the melt at a temperature of no more than 700 C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface. 4 figs.

Koyama, Koichiro; Shimotake, Hiroshi.

1983-08-16

356

Control of Copper and Iron Oxidation States in Some Triple- and Double-Perovskite Oxides.  

National Technical Information Service (NTIS)

This thesis consists of eight publications and a summary of the obtained experimental results, reviewed together with the most essential literature related to the topic. The work deals with ways of tuning and analyzing the translation metal oxidation meta...

K. Lehmus

2003-01-01

357

Oxide ion conductivity in doubly doped PrGaO perovskite-type oxide  

Microsoft Academic Search

In this study, oxide ion conduction characteristics of doubly doped PrGaO were investigated. It is found that PrGaO doped with Sr (or Ca) for the Pr site and Mg for the Ga site exhibits the high oxide ion conduction which was slightly less than that measured for doubly doped La(Sr)Ga(Mg)O (LSGM). Doping alkaline earth cations for the Pr site and

Tatsumi Ishihara; Haruyoshi Furutani; Hiroshi Arikawa; Miho Honda; Taner Akbay; Yusaku Takita

1999-01-01

358

Nanomagnetism in Transition Metal Doped Si Nanocrystals  

Microsoft Academic Search

\\u000a We study the influence of transition-metal doping on silicon nanocrystals by means of ab-initio first-principles calculations and a supercell approach. As transition-metals Manganese and Iron are investigated on either\\u000a substitutional or interstitial doping sites. For description of the localized d-levels we use the GGA+U approach with an Hubbard-like\\u000a Coulomb repulsion of about 3eV. We focus on the energetic stability of

Christian Panse; Roman Leitsmann; Friedhelm Bechstedt

359

Maximum-valence radii of transition metals  

PubMed Central

In many of their compounds the transition metals have covalence 9, forming nine bonds with use of nine hybrid spd bond orbitals. A set of maximum-valence single-bond radii is formulated for use in these compounds. These radii are in reasonably good agreement with observed bond lengths. Quadruple bonds between two transition metal atoms are about 50 pm (iron-group atoms) or 55 pm (palladium and platinum-group atoms) shorter than single bonds. This amount of shortening corresponds to four bent single bonds with the best set of bond angles, 79.24 and 128.8.

Pauling, Linus

1975-01-01

360

Antiferromagnetism in 4 d transition metals  

SciTech Connect

Total-energy band calculations employing a fixed-spin-moment procedure and the augmented-spherical-wave method are used to study the volume dependences and existence ranges of antiferromagnetic behavior in the 4{ital d} transition metals constrained to cubic lattices. At expanded volumes we find stable antiferromagnetic solutions of the Kohn-Sham equations for bcc niobium, molybdenum, and technetium, and for fcc technetium, and metastable antiferromagnetic solutions for fcc ruthenium. We find no stable antiferromagnetic solutions for fcc rhodium or palladium. Comparisons are made with the occurrence of antiferromagnetism in the 3{ital d} transition metals.

Moruzzi, V.L.; Marcus, P.M. (IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, NY (USA))

1990-12-01

361

Structural analysis and characterization of layer perovskite oxynitrides made from Dion-Jacobson oxide precursors  

SciTech Connect

A three-layer oxynitride Ruddlesden-Popper phase Rb{sub 1+x}Ca{sub 2}Nb{sub 3}O{sub 10-x}N{sub x}.yH{sub 2}O (x=0.7-0.8, y=0.4-0.6) was synthesized by ammonialysis at 800{sup o}C from the Dion-Jacobson phase RbCa{sub 2}Nb{sub 3}O{sub 10} in the presence of Rb{sub 2}CO{sub 3}. Incorporation of nitrogen into the layer perovskite structure was confirmed by XPS, combustion analysis, and MAS NMR. The water content was determined by thermal gravimetric analysis and the rubidium content by ICP-MS. A similar layered perovskite interconversion occurred in the two-layer Dion-Jacobson oxide RbLaNb{sub 2}O{sub 7} to yield Rb{sub 1+x}LaNb{sub 2}O{sub 7-x}N{sub x}.yH{sub 2}O (x=0.7-0.8, y=0.5-1.0). Both compounds were air- and moisture-sensitive, with rapid loss of nitrogen by oxidation and hydrolysis reactions. The structure of the three-layer oxynitride Rb{sub 1.7}Ca{sub 2}Nb{sub 3}O{sub 9.3}N{sub 0.7}.0.5H{sub 2}O was solved in space group P4/mmm with a=3.887(3) and c=18.65(1)A, by Rietveld refinement of X-ray powder diffraction data. The two-layer oxynitride structure Rb{sub 1.8}LaNb{sub 2}O{sub 6.3}N{sub 0.7}.1.0H{sub 2}O was also determined in space group P4/mmm with a=3.934(2) and c=14.697(2)A. GSAS refinement of synchrotron X-ray powder diffraction data showed that the water molecules were intercalated between a double layer of Rb+ ions in both the two- and three-layer Ruddlesden-Popper structures. Optical band gaps were measured by diffuse reflectance UV-vis for both materials. An indirect band gap of 2.51eV and a direct band gap of 2.99eV were found for the three-layer compound, while an indirect band gap of 2.29eV and a direct band gap of 2.84eV were measured for the two-layer compound. Photocatalytic activity tests of the three-layer compound under 380nm pass filtered light with AgNO{sub 3} as a sacrificial electron acceptor gave a quantum yield of 0.025% for oxygen evolution.

Schottenfeld, Joshua A. [Department of Chemistry, Pennsylvania State University, University Park, PA 16802 (United States); Benesi, Alan J. [Department of Chemistry, Pennsylvania State University, University Park, PA 16802 (United States); Stephens, Peter W. [Department of Physics and Astronomy, State University of New York, Stony Brook, New York 11794 (United States); Chen, Gugang [Departments of Physics, Pennsylvania State University, University Park, PA 16802 (United States); Eklund, Peter C. [Departments of Physics, Pennsylvania State University, University Park, PA 16802 (United States); Mallouk, Thomas E. [Department of Chemistry, Pennsylvania State University, University Park, PA 16802 (United States)]. E-mail: tom@chem.psu.edu

2005-07-15

362

Role of covalency in ``Charge Ordering'' perovskite ferrates  

NASA Astrophysics Data System (ADS)

Transition-metal oxides (TMO) with the perovskite crystal structure exhibit strong electron--electron correlation effects and complex structural distortions. The balance of those factors determines the stability of charge ordered states in chemistries susceptible to valence instabilities. We use first-principles density functional calculations to investigate the role of symmetry-unique structural distortions on covalent bonding in the ``charge-ordered'' insulator CaFeO3. We evaluate the electronic density distribution along the Fe--O bonds to assess the ground state stability by tracing the evolution in the oxygen environment, which appears as octahedral expansion/contractions and rotations. We show that nearly zero charge transfer occurs; the insulating phase results from a complex interplay of symmetry-lowering structural distortions and enhanced covalent interactions. Finally, we discuss possible routes to control the metal--insulator transition by fine-tuning the covalency.

Cammarata, Antonio; Rondinelli, James

2013-03-01

363

Investigation of a reference electrode based on perovskite oxide for second kind potentiometric gas sensor in open systems  

Microsoft Academic Search

Different oxygen electrodes are compared to be used in second kind gas sensors. Perovskite oxides (La1?xSrxMnO3), RuO2, platinum and gold deposited on NASICON electrolyte were characterized by impedance spectroscopy in different atmospheres (argon, synthetic pure air or with 5% CO2). They were also mixed with Na2Ti6O13TiO2 and Na2Ti3O7Na2Ti6O13 to improve the reversible exchange of oxygen and sodium at the interfaces.

J Ramirez; P Fabry

2001-01-01

364

Monte Carlo Methods in Transition Metal Alloys  

Microsoft Academic Search

Giant moments of several Bohr magnetons are formed in transition metal alloys where the matrix is palladium or platinum. The interaction between these giant moments produces a phase transition from paramagnetism to ferromagnetism, when the alloy is below the Curie temperature. These giant moments can be measured mainly by neutron diffraction, although several characteristics can be determined by magnetization measurements.

Rixio Parra

1997-01-01

365

Magnetism in embedded transition metal clusters  

Microsoft Academic Search

There is considerable current interest in the magnetic properties of transition metal nanoclusters embedded in a matrix of another material. Such materials generally show an enhancement in the magnetic moment as compared with the bulk. In a number of cases the orbital moment, which is quenched in the bulk, contributes significantly. Recently we introduced [Xie and Blackman, Phys Rev B

Yuannan Xie; John Blackman

2002-01-01

366

Intercalation studies of some transition metal dichalcogenides  

Microsoft Academic Search

Some of the Group IVA and VA transition metal dichaicogenides have been intercalated with aniline, pyridine, cyclopropylamine and n-octadecylamine Transmission spectra at 77 K have been recorded before and after intercalation, and electron diffraction patterns of the intercalation complexes have also been recorded. The spectra of the intercalation complexes, which show evidence for charge transfer from the organic molecules to

A. R. Beal; W. Y. Liang

1973-01-01

367

Solvation of isolated transition metal ions.  

National Technical Information Service (NTIS)

X-ray absorption spectroscopy is used to study the structure of transition metal ions as they encounter excess solvent and dissolve; more particularly, Cu in bentonite, with methanol solvent. The distance between the Cu ion and the oxygen atom increases b...

S. R. Wasserman K. A. Carrado

1994-01-01

368

Infrared Spectra of Transition Metal Alkoxides.  

National Technical Information Service (NTIS)

The infrared spectra of a number of transition metal isopropoxides and tetra tertiary butoxides were obtained from 5000 cm-1 to 285 cm-1 for the first time. The spectra were compared to existing infrared data on these classes of compounds. Characteristic ...

C. T. Lynch K. S. Mazdiyasni J. S. Smith W. J. Crawford

1964-01-01

369

High Pressure Synthesis of Transition Metal Carbonyls.  

ERIC Educational Resources Information Center

Presents an experiment which uses readily available starting materials and inexpensive equipment for synthesis of transition metal carbonyls at 1000 atm and which is intended to give students experience in techniques used in research and industry. Safety precautions are emphasized. (Author/SA)

Hagen, A. P.; And Others

1979-01-01

370

Studies of "reactive templated grain growth" in some complex-oxide perovskites  

NASA Astrophysics Data System (ADS)

This thesis details research on "reactive-templated grain growth" (RTGG) processing of complex-oxide perovskites. Four independent yet interwoven investigations are described. In the first, molten-salt synthesis yielded plate-like (0.2 x 5 x 5 mum) Ruddlesden-Popper (Sr3Ti 2O7) and Aurivillius (Bi4Ti3O 12 (BiT) and BaBi2Nb2O9) phases which served as "templates" in the subsequent studies. "Chemical preparation" routes were designed to produce intimately mixed, fine-grain matrix powders for the second and third investigations. A "citrate gel" method was developed for Bi1/2Na 1/2TiO3 (BNT)-based powders and a "sequential precipitation" method for PbNi1/3Nb2/3O3 - PbTiO3 (PNNT)-based powders. Both methods were original to the present investigation and resulted in dense, mum to sub-mum scale microstructures when applied to un-templated compositions. Microstructure studies of templated, chemically-prepared mixtures indicated that both Aurivillius phases behaved similarly with BNT-based matrices: An initially heterogeneous microstructure evolved towards a dense assemblage of anisometric (1 x 5 x 5 mum) grains. Sr3Ti 2O7 exhibited consistent behavior with both BNT and PNNT-based matrices: The microstructure evolved towards an aggregation of equi-axed, mum-scale grains with larger, isolated pores. A qualitative model was proposed to account for the behavior. It considered the excess matrix constituents after reaction to ABO3 perovskite and predicted an A-excess matrix in BNT-based mixtures templated with the Aurivillius phases and a B-excess matrix in BNT-based mixtures templated with Sr3Ti2O7. Predictions for PNNT-based matrices were less definite but not contradictory. In the final phase of the investigation, the solids formulation for BiT-templated, RTGG-processed Bi1/2Na17/40K3/40TiO 3 (BNKT) was systematically varied. This approach, original to this thesis, involved pre-reacting to well-defined alkali and bismuth titanate precursors. Use of these in different BNKT formulations indicated that the amount of desintering associated with reacting at 600--800C was a function of formulation. Formulation affected the microstructure and texture development in the RTGG-processed BNKT. Pre-reacting the alkali carbonates and TiO2 (to Na2Ti3O7 and K2Ti2O 5) inhibited texture development. No such effect was observed when Bi 2O3 and TiO2 were pre-reacted to Bi2Ti 4O11, instead the measured Lotgering factor F 001 increased from 0.5 to 0.7 for samples sintered at 1200C.

West, David Lawrence

371

La, Ca and Fe oxide perovskites: preparation, characterization and catalytic properties for methane combustion  

Microsoft Academic Search

La1?xCaxFeO3 (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K have been investigated as catalysts for methane combustion. The formation of the perovskite structure has been shown by X-ray diffraction (XRD) for all samples. The surface area (SA) values are in the range 36m2g?1 for the samples up to x=0.4, whereas SA is 0.7m2g?1

P Ciambelli; S Cimino; L Lisi; M Faticanti; G Minelli; I Pettiti; P Porta

2001-01-01

372

Unusual oxidation states give reversible room temperature magnetocaloric effect on perovskite-related oxides SrFe0.5Co0.5O3  

NASA Astrophysics Data System (ADS)

The magnetic properties and the magnetocaloric effect are presented for the perovskite-related oxide SrFe0.5Co0.5O3 prepared using electrochemical oxidation. SrFe0.5Co0.5O3 exhibits a second order paramagnetic-ferromagnetic transition close to room temperature (TC=330 K). The maximal magnetic entropy change ?SMMax , the maximal adiabatic temperature change ?Tad and the refrigerant capacity are found to be equal to respectively 4.0 J/kgK, 1.8 K and 258 J/kg while raising the B-field change from 0 to 5 T.

Yin, C.; Liu, Q.; Decourt, R.; Pollet, M.; Gaudin, E.; Toulemonde, O.

2011-12-01

373

A comparative study of the magnetic and electrical properties of perovskite oxides and the corresponding two-dimensional oxides of K2NiF4 structure  

Microsoft Academic Search

Electrical and magnetic properties of several oxide systems of K2NiF4 structure have been compared to those of the corresponding perovskites. Members of the La1-xSr1+xCoO4 system are all semiconductors with a high activation energy for conduction unlike La1-xSrxCoO3 (x >= 0.3) which is metallic; the latter oxides are ferromagnetic. La0.5Sr1.5CoO4 shows a magnetization of 0.5 muB at 0 K (compared to

C. N. R. Rao; P. Ganguly; K. K. Singh; R. A. Mohan Ram

1988-01-01

374

PARTICLE SIZE AND MORPHOLOGY CONTROL OF PEROVSKITE OXIDE NANOPOWDERS FOR NANOSTRUCTURED MATERIALS  

Microsoft Academic Search

Due to the high permittivity and low losses, ferroelectric ceramics with perovskite structure are largely used in the electronics industry for many applications. Presently, miniaturization of the components is one of the main goal in the field of microelectronics and communication. Accordingly, the current trend in high-tech ceramics is to realize sintered bodies with grain size below a few hundred

A. BASSANO; M. T. BUSCAGLIA; V. BUSCAGLIA; P. NANNI

2009-01-01

375

Effect of the rare earth in the perovskite-type mixed oxides AMnO3 (A=Y, La, Pr, Sm, Dy) as catalysts in methanol oxidation  

NASA Astrophysics Data System (ADS)

The effect of the rare earth in the perovskite-type mixed oxides AMnO3 (A=Y, La, Pr, Sm, Dy) on catalytic properties in methanol oxidation was investigated in this work. The perovskites were prepared by reactive grinding in order to enhance the specific surface area in comparison with other classical synthesis procedures. These catalysts were characterized by N2 adsorption, X-ray diffraction, H2 temperature-programmed reduction (TPR-H2), O2-, CH3OH- and CO2-temperature-programmed desorption (TPD). The activity of the five catalysts under study in the methanol oxidation reaction was evaluated. The behaviour of the ?-O2 from the surface of the perovskite was strongly related to the nature of the A-site cation and particularly to its electronegativity but also to its density. Concerning the ?-O2 from the bulk, the rare earth only induces an indirect effect notably due to structural modifications. As suggested in a previous study, the activity in methanol oxidation was directly linked with the surface oxygen density. Under an excess of ?-oxygen, the reaction intermediate was found to be a monodentate carbonate that decomposes into CO2. The stability of monodentate carbonates was also found to be related to the electronegativity of the rare earth during both CH3OH- and CO2-temperature-programmed desorption. However, as soon as a lack of ?-oxygen was observed in the structure, the dominant reaction intermediate was a bidentate carbonate that induces a consumption of anion vacancies in spite of the production of CO2. Nevertheless, the accumulation of these carbonates leads to a decrease in the oxidation rate since their desorption requires high temperatures.

Levasseur, B.; Kaliaguine, S.

2008-11-01

376

Perovskite ferroelectric nanomaterials.  

PubMed

In this review, the main concept of ferroelectricity of perovskite oxides and related materials at nanometer scale and existing difficulties in the synthesis of those nanocrystals are discussed. Important effects, such as depolarization field and size effect, on the existence of ferroelectricity in perovskite nanocrystals are deliberated. In the discussion of modeling works, different theoretical calculations are pinpointed focusing on their studies of lattice dynamics, phase transitions, new origin of ferroelectricity in nanostructures, etc. As the major part of this review, recent research progress in the facile synthesis, characterization and various applications of perovskite ferroelectric nanomaterials, such as BaTiO3, PbTiO3, PbZrO3, and BiFeO3, are also scrutinized. Perspectives concerning the future direction of ferroelectric nanomaterials research and its potential applications in renewable energy, etc., are presented. This review provides an overview in this area and guidance for further studies in perovskite ferroelectric nanomaterials and their applications. PMID:23912964

Nuraje, Nurxat; Su, Kai

2013-08-05

377

(Electronic structure and reactivities of transition metal clusters)  

SciTech Connect

The following are reported: theoretical calculations (configuration interaction, relativistic effective core potentials, polyatomics, CASSCF); proposed theoretical studies (clusters of Cu, Ag, Au, Ni, Pt, Pd, Rh, Ir, Os, Ru; transition metal cluster ions; transition metal carbide clusters; bimetallic mixed transition metal clusters); reactivity studies on transition metal clusters (reactivity with H{sub 2}, C{sub 2}H{sub 4}, hydrocarbons; NO and CO chemisorption on surfaces). Computer facilities and codes to be used, are described. 192 refs, 13 figs.

Not Available

1992-01-01

378

[Electronic structure and reactivities of transition metal clusters  

SciTech Connect

The following are reported: theoretical calculations (configuration interaction, relativistic effective core potentials, polyatomics, CASSCF); proposed theoretical studies (clusters of Cu, Ag, Au, Ni, Pt, Pd, Rh, Ir, Os, Ru; transition metal cluster ions; transition metal carbide clusters; bimetallic mixed transition metal clusters); reactivity studies on transition metal clusters (reactivity with H{sub 2}, C{sub 2}H{sub 4}, hydrocarbons; NO and CO chemisorption on surfaces). Computer facilities and codes to be used, are described. 192 refs, 13 figs.

Not Available

1992-08-01

379

Chemical vapor deposition and electric characterization of perovskite oxides LaMO{sub 3} (M=Co, Fe, Cr and Mn) thin films  

SciTech Connect

Oxides with a perovskite structure are important functional materials often used for the development of modern devices. In view of extending their applicability, it is necessary to efficiently control their growth as thin films using technologically relevant synthesis methods. Pulsed spray evaporation CVD was used to grow several perovskite-type oxides on planar silicon substrates at temperatures ranging from 500 to 700 deg. C. The optimization of the process control parameters allows the attainment of the perovskite structure as a single phase. The electrical characterization using the temperature-dependent conductivity and thermopower indicates the p-type conduction of the grown films and shows a decreasing concentration of the charge carrier, mobility and band gap energy in the sequence LaCoO{sub 3}>LaMnO{sub 3}>LaCrO{sub 3}>LaFeO{sub 3}. The investigation of the electric properties of the obtained perovskite thin films shows the versatility of CVD as a method for the development of innovative devices. - Graphical abstract: We report a single step deposition of perovskite thin films LaMO{sub 3} (M: Co, Mn, Cr, Fe) using pulsed spray evaporation chemical vapor deposition. Electrical and thermopower properties, similar to these of bulk materials, could promote the development of modern thermoelectric devices based on thin films technology.

Ngamou, Patrick Herve Tchoua [Department of Chemistry, Bielefeld University, Universitaetsstr. 25, 33615 Bielefeld (Germany); Bahlawane, Naoufal, E-mail: naoufal@pc1.uni-bielefeld.d [Department of Chemistry, Bielefeld University, Universitaetsstr. 25, 33615 Bielefeld (Germany)

2009-04-15

380

Perovskite ferroelectric nanomaterials  

NASA Astrophysics Data System (ADS)

In this review, the main concept of ferroelectricity of perovskite oxides and related materials at nanometer scale and existing difficulties in the synthesis of those nanocrystals are discussed. Important effects, such as depolarization field and size effect, on the existence of ferroelectricity in perovskite nanocrystals are deliberated. In the discussion of modeling works, different theoretical calculations are pinpointed focusing on their studies of lattice dynamics, phase transitions, new origin of ferroelectricity in nanostructures, etc. As the major part of this review, recent research progress in the facile synthesis, characterization and various applications of perovskite ferroelectric nanomaterials, such as BaTiO3, PbTiO3, PbZrO3, and BiFeO3, are also scrutinized. Perspectives concerning the future direction of ferroelectric nanomaterials research and its potential applications in renewable energy, etc., are presented. This review provides an overview in this area and guidance for further studies in perovskite ferroelectric nanomaterials and their applications.In this review, the main concept of ferroelectricity of perovskite oxides and related materials at nanometer scale and existing difficulties in the synthesis of those nanocrystals are discussed. Important effects, such as depolarization field and size effect, on the existence of ferroelectricity in perovskite nanocrystals are deliberated. In the discussion of modeling works, different theoretical calculations are pinpointed focusing on their studies of lattice dynamics, phase transitions, new origin of ferroelectricity in nanostructures, etc. As the major part of this review, recent research progress in the facile synthesis, characterization and various applications of perovskite ferroelectric nanomaterials, such as BaTiO3, PbTiO3, PbZrO3, and BiFeO3, are also scrutinized. Perspectives concerning the future direction of ferroelectric nanomaterials research and its potential applications in renewable energy, etc., are presented. This review provides an overview in this area and guidance for further studies in perovskite ferroelectric nanomaterials and their applications. The authors declare no competing financial interests.

Nuraje, Nurxat; Su, Kai

2013-09-01

381

Electronic properties of intercalation complexes of the transition metal dichalcogenides  

Microsoft Academic Search

Intercalation of the layer type transition metal dichalcogenides by a variety of organic molecules, alkali metals, or 3d transition metals, provides a powerful way to finely tune the electron occupation of the relatively narrow d bands met in these solids. These transition metal dichalcogenides are highly anisotropic solids, sometimes referred to as two-dimensional solids, and the intercalant molecules which are

R. H. Friend; A. D. Yoffe

1987-01-01

382

Phonon self energy in transition metals  

NASA Astrophysics Data System (ADS)

We present ab initio calculations of the phonon self energy of transition metals obtained using second order many body perturbation theory.ootnotetextS. Narasimhan and D. Vanderbilt, Phys. Rev. B, 43, 4541 (1991) The code we have implementedootnotetextL. Chaput, A. Togo, I. Tanaka and G. Hug, submitted to Phys. Rev. B use the symmetry properties of the phonon-phonon interactions to express the self energy as a sum over irreducible triplets. It is analogous to the reduction of integration to the irreducible part of the Brillouin zone for one particle properties. The self energy of transition metals is then calculated. We show that the Peierls approximationootnotetextR. E. Peierls, Quantum Theory of Solids, Oxford University Press,1964 is in fact reasonable for bcc and fcc metals, but fails for the hcp. The decays paths of phonons producing the self energy is finally analyzed using surfaces of reciprocal space defined by conservation law.

Chaput, Laurent; Togo, Atsushi; Tanaka, Isao; Hug, Gilles

2011-03-01

383

Transition metal dichalcogenide\\/polymer nanocomposites  

Microsoft Academic Search

Single molecular layers of transition metal dichalcogenides (TMDs) such as MoS2, MoSe2 and WS2, have been formed in suspension in water by lithium intercalation of crystalline powders and then exfoliation in water. The two-dimensionality of such systems is readily identified using powder X-ray diffraction, where the strong asymmetric shape of the (hk0) peaks and the absence of (00?) peaks and

D. Yang; P. Westreich; R. F. Frindt

1999-01-01

384

Early Transition Metal Alkyl and Tetrahydroborate Complexes  

Microsoft Academic Search

An investigation of early transition metal alkyl and tetrahydroborate complexes as catalytic models and ceramic precursors has been initiated. The compounds MX _2 (dmpe)_2, dmpe = 1,2-bis(dimethylphosphino)ethane, for M = Ti, V, Cr, and X = Br, I, BH_4, have been prepared. These complexes are paramagnetic and have been shown by X-ray crystallography to have trans-octahedral structures. The BH_4^{-} groups

James Allen Jensen

1988-01-01

385

Theoretical electron densities in transition metal dihydrides  

Microsoft Academic Search

Analysis of the DFT calculated electron density distributions of the first-row transition metal (TM) dihydrides are reported. Nickel dihydride was used to assess the effects of basis set and exchange-correlation functional on calculated density properties, leading to the conclusions that diffuse basis functions and gradient-corrected functionals accurately describe the essential features of NiH2. These calculations indicate a remarkably high degree

James A. Platts

2001-01-01

386

Semiconducting Al transition-metal quasicrystals  

NASA Astrophysics Data System (ADS)

We report on a class of icosahedral aluminum transition-metal (Al-TM) alloys with true semiconducting behavior. Our description of the structure of these icosahedral quasicrystals is based on the six-dimensional Katz-Gratias-Boudard (KGB) model of the face-centered-icosahedral (fci) quasicrystal and its rational approximants. The shell structure of the atomic surfaces in perpendicular space defines the chemical order of aluminum and transition-metal (TM) atoms leading to semiconducting transport properties. In transition-metal aluminides the hybridization between the Al(s,p) and transition-metal d orbitals is responsible for the formation of a semiconducting gap in the electronic spectrum. We have analyzed the electronic charge distribution and observed an enhanced charge density along the Al-TM bonds that is characteristic of covalent bonding. The existence of an energy gap in the electronic spectrum at or in the vicinity of the Fermi level is explicitly demonstrated for several low-order approximants in the hierarchy of Fibonacci approximants which converges to the icosahedral quasicrystals of the fci class, to which also the i-AlPdRe belongs. We predict existence of truly semiconducting quasicrystalline 1/1-approximants. Our results also lead to the prediction of the existence of new semiconducting quasicrystals with specified Al-TM compositions. The possibility of the existence of a semiconducting band gap suggests an explanation for the anomalously high resistivity of the icosahedral AlPdRe quasicrystals. We demonstrate that substitutional defects violating the ideal Al-TM ordering predicted by the KGB model lead to the formation of localized states in the band gap. A real sample of i-AlPdRe thus seems to be a semiconductor with a band gap filled by the localized states.

Kraj?, M.; Hafner, J.

2003-10-01

387

Alternate Transition Metal Complex Based Diene Polymerization  

Microsoft Academic Search

In the last decade, there has been a tremendous increase in the number of reports on transition metal complex-mediated butadiene homo- and copolymerization. While typical classical titanium, nickel, cobalt, and neodymium based catalysts have been almost exclusively applied to the production of high cis-1,4-polybutadiene, alternative catalyst systems are currently being developed which enable tuning of the polybutadiene microstructure and permit

Sven K.-H. Thiele; David R. Wilson

2003-01-01

388

Correlated hybridization in transition-metal complexes.  

PubMed

We apply local orbital basis density functional theory (using SIESTA) coupled with a mapping to the Anderson impurity model to estimate the Coulomb assisted or correlated hybridization between transition-metal orbitals and ligand orbitals for a number of molecular complexes. We find remarkably high values which can have several physical implications including (i) renormalization of effective single-band or multiband Hubbard model parameters for the cuprates and, potentially, elemental iron, and (ii) spin polarizing molecular transistors. PMID:16803115

Hbsch, A; Lin, J C; Pan, J; Cox, D L

2006-05-15

389

Transition metal based borohydrides for hydrogen storage  

NASA Astrophysics Data System (ADS)

Using ab-initio studies based on the density-functional theory, we have calculated binding energies per hydrogen molecule for decomposition reactions of transition metal borohydrides MHxB12H12 to MB12 structures, where M corresponds to Sc, Ti, or V. Depending on the valence of the transition metal, x can be 1, 2, or 3. Crystal structures considered for MB12 included both hypothetical and those found in the international crystallographic structural database. On the other hand, the crystal structure considered for MHxB12H12 belongs to C2/c (space group 15) structure as reported in a previous study [V. Ozolins et al. JACS, 131, 230 (2009)]. Among the structures investigated, Titanium-based metal borohydride structure has the lowest binding energy per hydrogen molecule relative to the cubic TiB12 structure (0.37 eV/H2). Our finding should be contrasted with the binding energy/H2 for simple metal based borohydrides (e.g., CaB12H12 ), which has a value of 1.5 eV/H2, suggesting that transition metals play a significant role in lowering the H2 binding energy in borohydrides.

Jayanthi, Chakram; Liu, Jianjun; Wei, Suhuai; Zhao, Yufeng

2010-03-01

390

Magnetoresistance in ordered and disordered double perovskite oxide, Sr 2FeMoO 6  

Microsoft Academic Search

We have prepared crystallographically ordered and disordered specimens of the double perovskite, Sr2FeMoO6, and investigated their magnetoresistance (MR) behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder X-ray diffraction patterns and reconfirmed by Mssbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate

D. D. Sarma; E. V. Sampathkumaran; Sugata Ray; R. Nagarajan; Subham Majumdar; Ashwani Kumar; G. Nalini; T. N. Guru Row

2000-01-01

391

Ethanol adsorption on transition-metal surfaces: A DFT investigation  

NASA Astrophysics Data System (ADS)

The development of low-cost and long-term stability catalyst compounds for the production of hydrogen from ethanol is one of the main problems to be solved for large scale use of direct- ethanol fuel cells. Steam reforming, which is one of the main routes to obtain hydrogen from ethanol, as well as ethanol oxidation, are critically dependent on the choice of the catalyst devices. Therefore, an atom-level understanding of the interaction of ethanol with catalysts systems is on the first problems to be addressed. In this talk, we will report first- principles calculations based on density functional theory for the adsorption of ethanol on close-packed transition-metal surfaces at the limit of low-coverage. In particular, we will report the following properties, namely, adsorption energy, work function changes, and structural parameters for a large number of substrates, which will be used to build up a simple picture to describe the interaction of ethanol with transition-metal surfaces. This work is supported by FAPESP.

da Silva, Juarez L. F.

2010-03-01

392

Single sublattice endotaxial phase separation driven by charge frustration in a complex oxide.  

PubMed

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO3 perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO3 network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO3 network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation. PMID:23750709

Demont, Antoine; Sayers, Ruth; Tsiamtsouri, Maria A; Romani, Simon; Chater, Philip A; Niu, Hongjun; Mart-Gastaldo, Carlos; Xu, Zhongling; Deng, Zengqiang; Brard, Yohann; Thomas, Michael F; Claridge, John B; Rosseinsky, Matthew J

2013-07-01

393

Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide  

PubMed Central

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO3 perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO3 network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO3 network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation.

2013-01-01

394

Kinetic model studies of atmospheric droplet chemistry 2. Homogeneous transition metal chemistry in raindrops  

Microsoft Academic Search

Considerable evidence indicates that dissolved transition metal ions (TMI) are capable of catalyzing oxidations in atmospheric water droplets, at least in certain circumstances. Wide variations in the importance of TMI chemistry are expected in these systems because concentrations of transistion metals in water droplets range over at least 4 orders of magnitude. In the present work we perform an extensive

T. E. Graedel; M. L. Mandich; C. J. Weschler

1986-01-01

395

Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide  

Microsoft Academic Search

Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the concentration of cobalt between 0 and 8%. Magnetic microscopy images

Yuji Matsumoto; Makoto Murakami; Tomoji Shono; Tetsuya Hasegawa; Tomoteru Fukumura; Masashi Kawasaki; Parhat Ahmet; Toyohiro Chikyow; Shin-ya Koshihara; Hideomi Koinuma

2001-01-01

396

Studies of the oxygen and oxide redox chemistry at lanthanum-strontium-M oxide (M = chromium to cobalt) perovskite-like electrode materials for solid oxide fuel cells  

NASA Astrophysics Data System (ADS)

The O2 reduction reaction (ORR) is commonly the greatest source of internal resistance in solid oxide fuel cells (SOFC). SOFC operate at 600 to 1000C, depending on the choice of cathode and solid electrolyte materials. Standard cathode materials are Sr doped LaMnO3+/-delta perovskite-like oxides (La1-xSrxMnO3+/-delta), the solid electrolyte is Y2O3 stabilized ZrO2, and the anode material is Ni. In this work, the ORR, the O2 evolution reaction (OER) and the equilibrium O2 redox reactions were examined using Sr doped La1-xSrxMO3+/-delta (M = Cr, Mn, Fe, Co) perovskite-like cathode materials. The role of the solid electrolyte was studied using both YSZ and Gd2O3 doped Ceria (GDC). Three electrode half cell techniques were conducted at 500 to 900C in both air and N2, to quantify the area specific polarization resistance (ASRp) of the O2 redox reaction, as well as the electrochemical capacitance (C). The study of the La1-xSrxMnO3+/-delta oxides (xSr = 0, 0.2 and 0.5) on both YSZ and GDC revealed that the O2 redox ASRp decreased with increasing x Sr, and that the ORR and OER rates at a given xSr, were higher on GDC versus YSZ. The study of the La1-xSrxMO 3-delta (M = Fe, Co) oxides (xSr = 0, 0.2, 0.4, and 0.6) on GDC showed that the O2 redox ASRp decreased by 100x and that the electrochemical capacitance increased by 10,000x as x Sr was varied from 0 to 0.6. Thermogravimetric analysis (TGA) in air revealed that the propensity of these oxides to lose mass (i.e., reduce) at higher temperatures, also increased with xSr. The study of the La1-xSrxCoO3-delta (xSr = 0.6, 0.8 and 1) and the xSr = 1 SrCo1-yFeyO 3-delta oxides on GDC, revealed non-ideal electrochemical behaviour that was attributed to solid state defect ordering processes. The applicability of non-ideal transition state theory was demonstrated through (i) the nonstationary O2 redox rates at the La1-x SrxMnO3+/-delta oxides, (ii) transfer coefficient analysis of the OER and ORR, and (iii) the non-ideal Arrhenius behaviour of the O2 redox rates and oxide capacitance at the La1-xSr xCoO3-delta (xSr = 0.6, 0.8, 1) and xSr = 1 SrCo1-yFeyO3-delta oxides.

Fournier, Joseph Lawrence

397

Giant magnetoresistance and table-like magnetocaloric effect in double perovskite oxide PrSrMnCoO6  

NASA Astrophysics Data System (ADS)

Magnetoresistance and magnetocaloric effect of a double perovskite oxide PrSrMnCoO6 (cubic, Fm3m) has been studied in fields up to 7 T. This compound is semiconductor-like and its electrical resistivity increases by 5 orders while going from 300 to 50 K. Giant magnetoresistance of ~40% is observed at 200 K in 7 T field. PrSrMnCoO6 orders ferromagnetically at ~150 K and shows a maximum magnetic entropy change of ~4.6 J/kg/K for 5 T field change in the temperature range of 110-190 K. This nearly constant magnetocaloric effect over a broad temperature span is highly suitable for Ericsson-cycle magnetic refrigeration.

Nath Mahato, Rabindra; Sethupathi, K.; Sankaranarayanan, V.; Nirmala, R.; Nigam, A. K.; Malik, S. K.

2011-04-01

398

Orbital disordering and the metal-insulator transition with hole doping in perovskite-type vanadium oxides  

NASA Astrophysics Data System (ADS)

Filling-control metal-insulator transitions (MITs) and related electronic phase diagrams have been investigated for hole-doped vanadium oxides Pr1-x Cax V O3 , Nd1-x Srx V O3 , and Y1-x Cax V O3 with perovskite structure. The increase of the doping level x causes the melting of the G -type (and C -type) orbital order, prior to or concomitantly with the MIT, due partly to the doped-hole motion and partly to the random potential arising from the quenched disorder. In particular, the G -type spin- and C -type orbital-ordered phase present in Y1-x Cax V O3 disappears immediately upon hole doping, around x=0.02 . On the other hand, the critical doping level x for the MIT is governed by the electron-correlation strength of the undoped parent compound.

Fujioka, J.; Miyasaka, S.; Tokura, Y.

2005-07-01

399

Electronic Magnetic and Structural Properties of A2VM0O6 Perovskites (A = Ca, Sr)  

SciTech Connect

The perovskites Sr{sub 2}VMoO{sub 6} and Ca{sub 2}VMoO{sub 6} have been synthesized by liquid-mix technique in citrate melts, and their electronic, magnetic and structural properties have been investigated. No signs of V/Mo ordering are seen by synchrotron X-ray powder diffraction, but despite the chemical disorder both oxides are highly conductive and Pauli paramagnetic. Electrical conductivities of these solid solutions are comparable or higher than those reported for polycrystalline AMoO{sub 3} end members. It is suggested that the delocalized metallic conductivity of these compounds with two different transition-metal atoms implies valence equilibrium between the degenerate oxidation-state couples V{sup 4+}Mo{sup 4+} and V{sup 3+}Mo{sup 5+}.

Karen,P.; Moodenbaugh, A.; Goldberger, J.; Santhosh, P.; Woodward, P.

2006-01-01

400

Transition-metal phosphate colloidal spheres.  

PubMed

The music of the spheres: Transition-metal phosphate colloidal spheres with one metal (Mn, Fe, Co, Ni, and Cu; see picture) or more than one metal (such as Fe-Ni, Co-Cu, Fe-Co-Cu, Fe-Co-Ni-Cu, Mn-Fe-Co-Ni-Cu, and Mn-Fe-Co-Ni-Cu-Zn) were synthesized in solution at low temperature. Porous and hollow iron phosphate spheres were prepared by adjusting the pH value of the reaction. PMID:19472253

Chen, Chen; Chen, Wei; Lu, Jun; Chu, Deren; Huo, Ziyang; Peng, Qing; Li, Yadong

2009-01-01

401

Surface resonances at transition metal dichalcogenide heterostructures  

NASA Astrophysics Data System (ADS)

Layered transition metal dichalcogenides do generally not exhibit characteristic electronic surface states localized perpendicular to the layers. Employing van der Waals epitaxy together with angle-resolved photoemission spectroscopy we show how surface-layer-derived electronic states can be generated on these materials. For a heterojunction consisting of one HfS2 epilayer adsorbed on bulk WSe2, purely two-dimensional behavior as well as three-dimensional coupling of the epilayer to substrate bulk states is observed despite a large lattice mismatch between epilayer and substrate. The experimental results are discussed in the context of electronic structure calculations.

Kreis, C.; Werth, S.; Adelung, R.; Kipp, L.; Skibowski, M.; Vo, D.; Krger, P.; Mazur, A.; Pollmann, J.

2002-04-01

402

Magnetic properties of transition-metal nitrides  

NASA Astrophysics Data System (ADS)

The structural stability of transition-metal nitrides (TMN's) and their magnetic properties in different phases are investigated using first-principles calculations. The early TMN, ScN-CrN, are found to have rocksalt as equilibrium structure at ambient pressure while the later ones (MnN, FeN, and CoN) prefer zincblende. However, the early ones can also adopt the zincblende structure under tensile strain. The tendency towards magnetism is stronger in the rocksalt phase than in the zincblende phase. Antiferromagnetic versus ferromagnetic ordering in the different phases and the relevance of the results to TM-doped GaN are discussed.

Lambrecht, Walter R. L.; Miao, M. S.; Lukashev, Pavel

2005-05-01

403

Multivariate data analysis approach to understand magnetic properties of perovskite manganese oxides  

SciTech Connect

Here we apply statistical multivariate data analysis techniques to obtain some insights into the complex structure-property relations in antiferromagnetic (AFM) and ferromagnetic (FM) manganese perovskite systems, AMnO{sub 3}. The 131 samples included in the present analyses are described by 21 crystal-structure or crystal-chemical (CS/CC) parameters. Principal component analysis (PCA), carried out separately for the AFM and FM compounds, is used to model and evaluate the various relationships among the magnetic properties and the various CS/CC parameters. Moreover, for the AFM compounds, PLS (partial least squares projections to latent structures) analysis is performed so as to predict the magnitude of the Neel temperature on the bases of the CS/CC parameters. Finally, so-called PLS-DA (PLS discriminant analysis) method is employed to find out the most influential/characteristic CS/CC parameters that differentiate the two classes of compounds from each other. - Graphical abstract: Statistical multivariate data analysis techniques are applied to detect structure-property relations in antiferromagnetic (AFM) and ferromagnetic (FM) manganese perovskites. For AFM compounds, partial least squares projections to latent structures analysis predict the magnitude of the Neel temperature on the bases of structural parameters only. Moreover, AFM and FM compounds are well separated by means of so-called partial least squares discriminant analysis method.

Imamura, N. [Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503 (Japan); Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, FI-02015 TKK (Finland); Mizoguchi, T. [Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503 (Japan); Yamauchi, H. [Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503 (Japan); Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, FI-02015 TKK (Finland); Karppinen, M. [Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503 (Japan); Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, FI-02015 TKK (Finland)], E-mail: maarit.karppinen@tkk.fi

2008-05-15

404

Structures of ordered tungsten- or molybdenum-containing quaternary perovskite oxides  

SciTech Connect

The room temperature crystal structures of six A{sub 2}MMoO{sub 6} and A{sub 2}MWO{sub 6} ordered double perovskites were determined from X-ray and neutron powder diffraction data. Ba{sub 2}MgWO{sub 6} and Ba{sub 2}CaMoO{sub 6} both adopt cubic symmetry (space group Fm3-bar m, tilt system a{sup 0}a{sup 0}a{sup 0}). Ba{sub 2}CaWO{sub 6} has nearly the same tolerance factor (t=0.972) as Ba{sub 2}CaMoO{sub 6} (t=0.974), yet it surprisingly crystallizes with I4/m symmetry indicative of out-of-phase rotations of the MO{sub 6} octahedra about the c-axis (a{sup 0}a{sup 0}c{sup -}). Sr{sub 2}ZnMoO{sub 6} (t=0.979) also adopts I4/m symmetry; whereas, Sr{sub 2}ZnWO{sub 6} (t=0.976) crystallizes with monoclinic symmetry (P2{sub 1}/n) with out-of-phase octahedral tilting distortions about the a- and b-axes, and in-phase tilting about the c-axis (a{sup -}a{sup -}c{sup +}). Ca{sub 2}CaWO{sub 6} (t=0.867) also has P2{sub 1}/n symmetry with large tilting distortions about all three crystallographic axes and distorted CaO{sub 6} octahedra. Analysis of 93 double perovskites and their crystal structures showed that while the type and magnitude of the octahedral tilting distortions are controlled primarily by the tolerance factor, the identity of the A-cation acts as the secondary structure directing factor. When A=Ba{sup 2+} the boundary between cubic and tetragonal symmetries falls near t=0.97, whereas when A=Sr{sup 2+} this boundary falls somewhere between t=1.018 and t=0.992. - Graphical abstract: A survey of the tolerance factor of 41 Mo/W- and 52 Nb/Ta-containing quaternary perovskites plotted as a function of the difference between the two six-coordinate M-cation ionic radii. Compounds with cubic symmetry are represented by diamonds, those with tetragonal symmetry are represented by squares, those with I2/m monoclinic symmetry are represented by Multiplication-Sign , and those with P2{sub 1}/n monoclinic symmetry are represented by triangles. White symbols represent compositions where A=Ba{sup 2+}, gray symbols represent compositions where A=Sr{sup 2+}, and black symbols represent where A=Ca{sup 2+}. The filled circle represents rhombohedral Ba{sub 2}BiTaO{sub 6} (t=0.961; space group-R3-bar ; tilt system-a{sup -}a{sup -}a{sup -}). References for the compounds included in this figure are listed in the Supporting Information File. Black-Small-Square Highlights: Black-Right-Pointing-Pointer Reports the structures of six double perovskites containing either Mo{sup 6+} or W{sup 6+}. Black-Right-Pointing-Pointer Compounds with similar tolerance factors (t) exhibited different symmetries. Black-Right-Pointing-Pointer This effect was observed in Sr{sub 2}ZnMO{sub 6} and Ba{sub 2}CaMO{sub 6} (M=Mo or W). Black-Right-Pointing-Pointer Contains a structural survey of 93 double perovskites. Black-Right-Pointing-Pointer Octahedral tilting distortions are controlled by t and the nature of the A-cation.

Day, Bradley E.; Bley, Nicholas D.; Jones, Heather R.; McCullough, Ryan M. [Department of Chemistry, Millikin University, 1184 W. Main Street, Decatur, IL 62522 (United States); Eng, Hank W. [Department of Natural Sciences, San Antonio College, 1300 San Pedro Avenue, San Antonio, TX 78212 (United States); Department of Natural Sciences, St. Philip's College, 1801 Martin Luther King Drive, San Antonio, TX 78203 (United States); Porter, Spencer H.; Woodward, Patrick M. [Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, OH 43210 (United States); Barnes, Paris W., E-mail: pbarnes@millikin.edu [Department of Chemistry, Millikin University, 1184 W. Main Street, Decatur, IL 62522 (United States)

2012-01-15

405

Monte Carlo Methods in Transition Metal Alloys  

NASA Astrophysics Data System (ADS)

Giant moments of several Bohr magnetons are formed in transition metal alloys where the matrix is palladium or platinum. The interaction between these giant moments produces a phase transition from paramagnetism to ferromagnetism, when the alloy is below the Curie temperature. These giant moments can be measured mainly by neutron diffraction, although several characteristics can be determined by magnetization measurements. In this work, several magnetic properties of these alloys are presented, based on calculations made mainly by Monte Carlo simulation of these properties. A localized moment model is used to simulate the formation of magnetization clouds and thier transformation as the temperature is raised. The simulation allows the calculation of the critical temperatures of ferromagnetism, which are then compared with experimental measurements. In several of these alloys, unpolarized diffuse neutron scattering measurements show large forward peaks that would indicate giant moments larger than those obtained by magnetization measurements. We calculated, using Monte Carlo simulation, the diffuse neutron cross sections for these alloys and reproduced the neutron data. We find a significant quasielastic contribution to the scattering that can not be attributed to the magnetization cloud. The calculation methods were applied to several dilute and concentrated transition metal alloys. The results indicate that the methods and models used are valid for a large group of Pd and Pt based alloys.

Parra, Rixio

1997-08-01

406

Controlling surface reactivities of transition metals by carbide formation  

Microsoft Academic Search

The surface reactivities of transition metals, including the Groups 46 early transition metals as well as the late transition metals of the 3d series, can often be modified by the formation of a carbide overlayer. More importantly, the reactivities of carbide-modified surfaces frequently demonstrate strong similarities to those of the Pt-group (Pt, Pd, Ir, Rh, Ru and Os) metals. In

Jingguang G Chen; Bernd Frhberger; Joseph Eng; Brian E Bent

1998-01-01

407

Bonding and phase transitions in transition metal dichalcogenide layer compounds  

Microsoft Academic Search

The electronic structure of the transition metal dichalcogenide layer compounds is studied using hybridised orbitals appropriate to the transition metal atom coordination. Non-bonding orbitals on the transition metal atoms give the conduction electron bands which are responsible for the charge-density-wave phase transitions in the VB compounds, and the band structure of 1T- and 2H-TaS2 is found using tight binding. The

J. E. Inglesfield

1980-01-01

408

First-principles study of electromechanical and polar properties in perovskite oxides and half-Heusler semiconductors  

NASA Astrophysics Data System (ADS)

This thesis discusses electromechanical and polar properties in two well-known classes of materials, perovskite oxides and half-Heusler compounds, using first-principles calculations. Certain features of the ab initio codes, such as the capability to calculate polarization based on the modern theory of polarization, or to apply a finite electric field, are central to the problems presented in this thesis. Hence these formalisms are discussed, following a brief opening section on the basic methodology of density-functional theory. The first problem presented in this thesis concerns the nonlinear piezoelectric response of ferroelectric PbTiO3 for the case of a polarization-enhancing electric field applied along the tetragonal axis. The dependence of the c/a ratio on electric field is found to be almost linear in the range up to 500 MV/m, contrary to what expected from Landau-Devonshire theory, but in qualitative agreement with a recent experiment. In the second problem we study the energy landscape and ferroelectric states of double perovskites of the form AA'BB'O6 in which the atoms on both the A and B sites are arranged in rock-salt order. If a ferroelectric instability occurs, the energy landscape will tend to have minima with the polarization along tetrahedral directions, leading to a rhombohedral phase, or along Cartesian directions, leading to an orthorhombic phase. We are not aware of compounds naturally occurring in this structure, although they might be synthesized experimentally. In the final problem, we use a first-principles rational-design approach to search a large materials family, half-Heusler compounds to identify semiconductors, and then compute their piezoelectric properties. This previously-unrecognized class of piezoelectrics may benefit greatly from calculations such as those presented here. Our work may provide guidance for experimental verification of existing compounds and for the experimental realization of other potential candidates.

Roy, Anindya

409

Magnetic endohedral transition-metal-doped semiconducting-nanoclusters.  

PubMed

Endohedral first-row transition-metal-doped TM@Zn(i)S(i) nanoclusters, in which TM stands for the first-row transition-metals from Sc to Zn, and i=12, 16, have been characterized. In these structures the dopant metals are trapped inside spheroidal hollow semiconducting nanoclusters. It is observed that some of the transition metals are trapped in the center of mass of the cluster, whereas others are found to be displaced from that center, leading to structures in which the transition metals display a complex dynamical behavior upon encapsulation. This fact was confirmed by quantum molecular dynamics calculations, which further confirmed the thermal stability of endohedral compounds. In the endohedrally-doped nanoclusters in which the transition-metal atom sits on the center of mass, the host hollow cluster structure remains undistorted after dopant encapsulation. Conversely, if the encapsulated transition-metal atom is displaced from the center of mass, the host hollow cluster structure suffers a very tiny distortion. Additionally, it is found that there is negligible charge transfer between the dopant transition-metal atom and its hollow cluster host and, after encapsulation, the spin densities remain localized on the transition-metal atom. This allows for the atomic-like behavior of the trapped transition-metal atom, which gives rise to their atomic-like magnetic properties. The encapsulation free energies are negative, suggesting that these compounds are thermodynamically stable. PMID:18668495

Matxain, Jon M; Formoso, Elena; Mercero, Jose M; Piris, Mario; Lopez, Xabier; Ugalde, Jesus M

2008-01-01

410

Multivariate data analysis approach to understand magnetic properties of perovskite manganese oxides  

NASA Astrophysics Data System (ADS)

Here we apply statistical multivariate data analysis techniques to obtain some insights into the complex structure-property relations in antiferromagnetic (AFM) and ferromagnetic (FM) manganese perovskite systems, AMnO3. The 131 samples included in the present analyses are described by 21 crystal-structure or crystal-chemical (CS/CC) parameters. Principal component analysis (PCA), carried out separately for the AFM and FM compounds, is used to model and evaluate the various relationships among the magnetic properties and the various CS/CC parameters. Moreover, for the AFM compounds, PLS (partial least squares projections to latent structures) analysis is performed so as to predict the magnitude of the Nel temperature on the bases of the CS/CC parameters. Finally, so-called PLS-DA (PLS discriminant analysis) method is employed to find out the most influential/characteristic CS/CC parameters that differentiate the two classes of compounds from each other.

Imamura, N.; Mizoguchi, T.; Yamauchi, H.; Karppinen, M.

2008-05-01

411

Photovoltaic effect in micrometer-thick perovskite-type oxide multilayers on Si substrates  

NASA Astrophysics Data System (ADS)

Micrometer-thick perovskite-type multilayer heterostructures with ten thousands of unit cells, [SrNb0.05Ti0.95O3/La0.9Sr0.1MnO3]3, have been fabricated on Si substrates. The structure exhibited nonlinear and rectifying current-voltage characteristics. Stable and temporal photovoltaic effects in the multilayer have been experimentally studied by using different wavelength light sources from ultraviolet to infrared, respectively. The photovoltage and photocurrent responsivities as a function of wavelength displayed that two cutoff wavelengths occurred corresponding to the band gaps of SrNb0.05Ti0.95O3 and Si and La0.9Sr0.1MnO3. Under the pulsed laser illumination the nanosecond response characterization of the present multilayer structure was obtained. Based on the band structure of the multilayers, a possible mechanism of photovoltaic process was proposed.

Liu, Hao; Zhao, Kun; Zhou, Na; Lu, Huibin; He, Meng; Huang, Yanhong; Jin, Kui-Juan; Zhou, Yueliang; Yang, Guozhen; Zhao, Songqing; Wang, Aijun; Leng, Wenxiu

2008-10-01

412

Electronic structure of perovskite oxides, LaMO[sub 3] (M = Ti-Ni), from high-energy electron spectroscopic investigations  

SciTech Connect

We study the electronic structure of the LaMO[sub 3] (M = Ti-Ni) perovskite oxides as probed by various high-energy electron spectroscopies. These spectroscopic studies, in conjunction with model many-body calculations, provide important information concerning the ground state character of these oxides. In particular, the on-site Coulomb interaction strength, U[sub dd], the charge transfer energy, [Delta], and the transfer integral between the metal 3d and oxygen 2p states, t, have been estimated. These parameters are found to exhibit a systematic variation across the series. Electronic structure of the perovskites is discussed in terms of electron-removal and electron-addition spectra for the parent compound as well as for doped LaMO[sub 3], with specific reference to semiconductor-metal transitions.

Sarma, D.D.; Chainani, A. (Indian Institute of Science, Bangalore (India))

1994-07-01

413

New Cyclosiloxanolate Cluster Complexes of Transition Metals  

Microsoft Academic Search

New cyclosiloxanolate transition metal cluster complex derivatives were prepared. PhSiO2K reacted with NiX2 (X2 =Cl2 or acac) to give K2{[?6?(PhSiO2)6]2[?3?(OH)]2Ni4K4}, a mixed group 1group 10 metal complex. PhSiO2Na reacted with Ni(NH3)6I2 to give Na{[?6?(PhSiO2)6]2Ni6(?6?I)} as the first example of encapsulated I? ion in siloxanolate complexes. The macrocyclic Na4{[?12?(PhSiO2)12]Cu4} complex reacted with ?6?(1,3,5?C7H8)Cr(CO)3 to give the heterobimetallic adduct Na4{[?12?(PhSiO2)12]Cu4} [Cr(CO)3]3 as

Cecilia Mortal; Andrea Caneschi; Andrea Cornia; Eliano Diana; Silvia Faranda; Valrie Marvaud; Maddalena Pizzotti; Olga I. Shchegolikhina; Claudia Zucchi; Gyula Plyi

2007-01-01

414

Surface segregation trends in transition metal alloys  

NASA Astrophysics Data System (ADS)

In this work, we revisit the problem of predicting the surface segregation trends in binary transition metal alloys from the knowledge of the basic features of the pure component d-band electronic structure within tight-binding approximation. In contrast to previous trend studies, the present one includes, within the fourth-moment approximation (FMA) of the tight-binding scheme, both the difference in the average band energies (diagonal disorder) and the difference in the band widths (off-diagonal disorder) of the two components. We show that treating on the same footing these two effects is essential for a correct prediction of surface segregation. The presented study, giving a natural link between the electronic structure and mixing/segregation properties, is particularly interesting and useful in the context of the possibility of building efficient new energy models based on FMA for use in large (time) scale atomistic simulations.

Los, J. H.; Mottet, C.; Trglia, G.

2013-10-01

415

Transition metal chemistry in crossed molecular beams  

NASA Astrophysics Data System (ADS)

Bimolecular state-selective chemistry has been performed using the technique of crossed molecular beams. Systems of current study involve transition metal atoms (M) interacting with small hydrocarbons, in particular: Mo + CH4, Zr + C2H4, and V + C2H4. Atomic metal reactants are prepared state-specifically by laser excitation followed by radiative decay. Nascent products were allowed to drift to a triply differentially pumped detector where they were ionized by either conventional electron impact or VUV photoionization and counted to obtain product angular and velocity distributions. C-H bond activation was observed for the former two systems, but not for the V systems. This behavior is rationalized in terms of the electronic configurations of the atomic reactants.

Willis, Peter A.; Stauffer, Hans U.; Hinrichs, Ryan Z.; Davis, H. Floyd

1998-05-01

416

Tunable band gaps in transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We investigate band-gap tuning in transition-metal dichalcogenide bilayers by external electric fields applied perpendicular to the layers. Using density functional theory, we show that the fundamental band gap of MoS2, MoSe2, MoTe2, and WS2 bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic. We interpret our results in the light of the Giant Stark Effect and obtain a robust relationship, which is essentially characterized by the interlayer spacing, for the rate of change of band gap with applied external field. Our study expands the known space of layered materials with widely tunable band gaps beyond the classic example of bilayer graphene and suggests potential directions for fabrication of novel electronic and photonic devices.

Ramasubramaniam, Ashwin; Naveh, Doron; Towe, Elias

2012-02-01

417

Radiation damage of transition metal carbides  

SciTech Connect

In this grant period we have investigated electrical properties of transition metal carbides and radiation-induced defects produced by low-temperature electron irradiation in them. Special attention has been given to the composition VC[sub 0.88] in which the vacancies on the carbon sublattice of this fcc crystal order to produce a V[sub 8]C[sub 7] superlattice. The existence of this superlattice structure was found to make the crystal somewhat resistant to radiation damage at low doses and/or at ambient temperature. At larger doses significant changes in the resistivity are produced. Annealing effects were observed which we believe to be connected with the reconstitution of the superlattice structure.

Dixon, G.

1991-01-01

418

Voltage-induced Metal-Insulator Transitions in Perovskite Oxide Thin Films Doped with Strongly Correlelated Electrons  

NASA Astrophysics Data System (ADS)

We have observed a reversible metal-insulator transition in perovskite oxide thin films that can be controlled by charge trapping pumped by a bipolar voltage bias. In the as-fabricated state, the thin film is metallic with a very low resistance comparable to that of the metallic bottom electrode, showing decreasing resistance with decreasing temperature. This metallic state switches to a high-resistance state after applying a voltage bias: such state is non-ohmic showing a negative temperature dependence of resistance. Switching at essentially the same voltage bias was observed down to 2K. The metal-insulator transition is attributed to charge trapping that disorders the energy of correlated electron states in the conduction band. By increasing the amount of charge trapped, which increases the disorder relative to the band width, increasingly more insulating states with a stronger temperature dependence of resistivity are accessed. This metal-insulator transition provides a platform to engineer new nonvolatile memory that does not require heat (as in phase transition) or dielectric breakdown (as in most other oxide resistance devices).

Wang, Yudi; Gil Kim, Soo; Chen, I.-Wei

2007-03-01

419

Electrical and magnetic properties of manganates with perovskite-related structure  

NASA Astrophysics Data System (ADS)

Multi-metal transition metal oxides with perovskite and related structures are known to exhibit a variety of interesting electrical and magnetic properties. In the work presented in this dissertation two systems were chosen to highlight the influence of lattice distortion and structure on those properties. The first system investigated was a series of charge ordered Ruddlesden-Popper (layered perovskite) manganates with a composition LnSr2Mn 2O7, where Ln is a rare earth. The degree of distortion of the structure was modified by changing the rare earth cation of the perovskite (La, Nd and Gd). Thin films were deposited on single crystal substrates by nebulized spray pyrolysis. It was observed that by reducing the dimensionality of the structure the magnetic exchange becomes antiferromagnetic, replicating the behavior of three-dimensional perovskites with higher distortion. It was further observed that the charge ordered behavior of all films was insensitive to magnetic fields up to 7 T, and also that application of stronger current while measuring transport properties (100 muA) would eliminate the charge ordering signature in the measurement. Both these results establish a clear connection between dimensionality of the structure and the level of distortion of the parent perovskite structure. The second system chosen was BiMnO3, one of the few perovskites known to be ferromagnetic (Tc = 105 K) which has recently been indexed in a polar C2 (monoclinic) space group consistent with ferroelectricity. The synthesis of this compound involves high pressures and temperatures. The samples were prepared both by cubic anvil cell and a Paris-Edinburgh setup. Ferroelectric hysteresis loops were measured confirming that BiMnO3 is the first true multiferroic in the perovskite family, i.e. simultaneously exhibits ferroelectricity and ferromagnetism. Samples of this oxide were also synthesized by nebulized spray pyrolysis and pulsed laser deposition, both conventional ambient pressure techniques. The magnetic structure of BiMnO3 was determined through neutron diffraction and it revealed a collinear arrangement of the spins along [010], the unique axis of the monoclinic cell. A careful observation of six Mn-O-Mn pathways in the crystal structure showed that four are ferromagnetic and two weakly antiferromagnetic, supporting the belief that the particular orbital ordering observed in BiMnO3 is responsible for its ferromagnetism.

Moreira Dos Santos, Antonio Francisco

420

Aspects of the formation and mobility of protonic charge carriers and the stability of perovskite-type oxides  

Microsoft Academic Search

Proton conducting acceptor-doped perovskite-type alkaline earth cerates, zirconates, niobates and titanates have been investigated experimentally and by numerical simulations. For all cubic perovskites the concentration of protonic defects almost reaches the acceptor dopant concentration under appropriate conditions, and the mobility of protonic defects fall into a narrow range. Any symmetry reduction, however, leads to a reduction of the concentration and

K. D Kreuer

1999-01-01

421

Room-temperature electronic phase transitions in the continuous phase diagrams of perovskite manganites  

PubMed

Highly correlated electronic systems--such as transition-metal oxides that are doped Mott insulators--are complex systems which exhibit puzzling phenomena, including high-temperature superconductivity and colossal magnetoresistivity. Recent studies suggest that in such systems collective electronic phenomena are important, arising from long-range Coulomb interactions and magnetic effects. The qualitative behaviour of these systems is strongly dependent on charge filling (the level of doping) and the lattice constant. Here we report a time-efficient and systematic experimental approach for studying the phase diagrams of condensed-matter systems. It involves the continuous mapping of the physical properties of epitaxial thin films of perovskite manganites (a class of doped Mott insulator) as their composition is varied. We discover evidence that suggests the presence of phase boundaries of electronic origin at room temperature. PMID:10963590

Yoo; Duewer; Yang; Yi; Li; Xiang

2000-08-17

422

Reproducible switching effect in thin oxide films for memory applications  

NASA Astrophysics Data System (ADS)

Thin oxide films with perovskite or related structures and with transition metal doping show a reproducible switching in the leakage current with a memory effect. Positive or negative voltage pulses can switch the resistance of the oxide films between a low- and a high-impedance state in times shorter than 100 ns. The ratio between these two states is typically about 20 but can exceed six orders of magnitude. Once a low-impedance state has been achieved it persists without a power connection for months, demonstrating the feasibility of nonvolatile memory elements. Even multiple levels can be addressed to store two bits in such a simple capacitor-like structure.

Beck, A.; Bednorz, J. G.; Gerber, Ch.; Rossel, C.; Widmer, D.

2000-07-01

423

Transition metal catalysis in the generation of natural gas  

Microsoft Academic Search

Carbonaceous sedimentary rocks containing transition metals are known to be catalytic in the conversion of hydrogen and n-alkenes into natural gas, but the source of activity is unclear. The evidence presented here supports transition metals as the active agents. Various metal compounds in the pure state show the same levels of catalytic activity as sedimentary rocks and the products are

Frank D. Mango

1996-01-01

424

Lattice Dynamics of Transition Metals in the Resonance Model.  

National Technical Information Service (NTIS)

In this paper the authors reformulate the lattice dynamics of the transition metals in the framework of the recently proposed transition-metal model-potential (TMMP) method in order to study the nonlocal effects arising from the strong energy dependence o...

B. A. Oli A. O. E. Animalu

1975-01-01

425

Aromaticity and Antiaromaticity in Transition-Metal Systems  

SciTech Connect

Aromaticity is an important concept in chemistry primarily for hydrocarbon compounds, but it has been extended to compounds containing transition-metal atoms. Recent findings of aromaticity and antiaromaticy in all-metal clusters have stimulated further researches in describing the chemical bonding, structures, and stability in transition-metal clusters and compounds on the basis of aromaticity and antiaromaticity, which are reviewed here. The presence of d-orbitals endows much more diverse chemistry, structure, and chemical bonding to transition-metal clusters and compounds. One interesting feature is the existence of a new type of ?-aromaticity, in addition to ?- and ?-aromaticity that are only possible for main group compounds. Another striking characteristic in the chemical bonding of transition-metal systems is the multi-fold nature of aromaticity, antiaromaticity, or even conflicting aromaticity. Separate sets of counting rules have been proposed for cyclic transition-metal systems to account for the three types of ?-, ?-, and ?-aromaticity/antiaromaticity. The diverse transition-metal clusters and compounds reviewed here indicate that multiple aromaticity and antiaromaticity may be much more common in chemistry than one would anticipate. It is hoped that the current review will stimulate interest in further understanding the structure and bonding, on the basis of aromaticity and antiaromaticity, of other known or unknown transition-metal systems, such as the active sites of enzymes or other biomolecules, which contain transition-metal atoms and clusters.

Zubarev, Dmitry Y.; Averkiev, Boris B.; Zhai, Hua Jin; Wang, Lai S.; Boldyrev, Alexander I.

2008-01-14

426

Characterization of electrically conductive transition metal dichalcogenide lubricant films  

Microsoft Academic Search

Groups VB and VIB transition metal dichalcogenides with layered structures, are intrinsic solid lubricants, and constitute a class of materials with unique and unusual properties based on their extreme anisotropy. The primary objective of this investigation was to conduct a comprehensive study on the tribological and electrical properties of burnished and sputtered transition metal dichalcogenide films, and characterize the performance

Harish C. Waghray

1997-01-01

427

Transition Metal Doped ZnO for Spintronics.  

National Technical Information Service (NTIS)

In this project, the properties of transition metal (TM) -doped ZnO will be investigated. The project focuses on two activities. First, the properties of ZnO doped with transition metals (Mn, Co, or Cr) and deep level impurities (Cu, As, Sn) is explored. ...

D. Norton

2007-01-01

428

Effect of transition metal elements addition on the properties of nitrogen-doped TiO2 photocatalysts  

NASA Astrophysics Data System (ADS)

Nitrogen-doped TiO2 (TiO2-xNy) nanoparticles with and without adding the transition metal ions, such as Co+ and Ni+, were synthesized by thermohydrolysis of TiCl3. The samples were characterized by X-ray diffraction, specific surface area determination, UV-vis diffuse reflectance spectroscopy. The photocatalytic activity of the sample was investigated by employing the oxidative destruction of nitric oxide as a probe reaction using a continuously flowing gases-solid system. Although the doping of transition metal ion in the lattice of titania was not useful to improve the photocatlytic activity, loading of transition metal oxides on the surface of titania resulted in an improvement of the photoreactivity of the nitrogen-doped TiO2. The beneficial effect was explained by an increased separation efficiency of the photogenerated electron-hole pairs.

Li, H.; Zhang, P.; Yin, S.; Wang, Y.; Dong, Q.; Guo, C.; Sato, T.

2012-01-01

429

Activation of methane by transition metal-substituted aluminophosphate molecular sieves  

DOEpatents

Aluminophosphate molecular sieves substituted with cobalt, manganese or iron and having the AlPO{sub 4}-34 or AlPO{sub 4}-5, or related AlPO{sub 4} structure activate methane starting at approximately 350{degree}C. Between 400 and 500{degree}C and at methane pressures {le} 1 atmosphere the rate of methane conversion increases steadily with typical conversion efficiencies at 500{degree}C approaching 50% and selectivity to the production of C{sub 2+} hydrocarbons approaching 100%. The cobalt substituted AlPO{sub 4}-34 structure (CoAPSO-34) produces ethylene, ethane, propylene, and propane in varying proportions, depending on reaction conditions. The cobalt-substituted AlPO{sub 4}-5 (CoAPSO-5) produces polypropylene in very high yield, with ethane, ethylene, and propane also produced. The activation mechanism is based on reduction of the transition metal (III) from of the molecular sieve to the transition metal(II) form with accompanying oxidative dehydrogenation of the methane. Reoxidation of the transition metal(II) form to the transition metal (III) form can be done either chemically (e.g., using O{sub 2}) or electrochemically. 4 figs.

Iton, L.E.; Maroni, V.A.

1990-03-16

430

Computational study of oxygen vacancy defects in perovskite oxides, SrTiO3 and LaAlO3  

NASA Astrophysics Data System (ADS)

Insulating perovskite oxides, SrTiO3 (STO) and LaAlO3 (LAO) have attracted a lot of attention due the observed two dimensional electron gas at the interface of these two insulating oxides. Oxygen vacancies, which are easily incorporated in these oxide systems during various processes such as growth, annealing and redox reactions, form an n-type defect in these systems. Hence they are responsible for modifications of various physical properties such as conductivity and optical properties to name a few. As such, there is still a lack of comprehensive theoretical understanding of these important defects in these materials. In this work we present first principles calculations of neutral (V^0) and charged (V^+, V^++) oxygen vacancies in STO and LAO. Density functional theory within the local density approximation proves insufficient in reliably predicting the defect levels due to their well know band gap problem. We therefore employ the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional to study these defects. Band gaps of 3.01 eV and 5.0 eV are obtained for bulk STO and LAO which are in excellent agreement to experimental values. In STO we find a localized V^0 defect state at 0.7 eV below the conduction band edge (CBE) while in LAO it is a deep lying level at 2.13 eV below the CBE. A band alignment of the defect levels in these two materials shows that upon stabilizing the singly ionized state, charge transfer could take place from the V^+ LAO state to the V^+ STO state which lies at 1.19 eV below the former.

Mitra, Chandrima; Lin, Chungwei; Robertson, John; Demkov, Alexander

2012-02-01

431

Electronic, magnetic and structural properties of A {sub 2}VMoO{sub 6} perovskites (A=Ca, Sr)  

SciTech Connect

The perovskites Sr{sub 2}VMoO{sub 6} and Ca{sub 2}VMoO{sub 6} have been synthesized by liquid-mix technique in citrate melts, and their electronic, magnetic and structural properties have been investigated. No signs of V/Mo ordering are seen by synchrotron X-ray powder diffraction, but despite the chemical disorder both oxides are highly conductive and Pauli paramagnetic. Electrical conductivities of these solid solutions are comparable or higher than those reported for polycrystalline AMoO{sub 3} end members. It is suggested that the delocalized metallic conductivity of these compounds with two different transition-metal atoms implies valence equilibrium between the degenerate oxidation-state couples V{sup 4+}Mo{sup 4+} and V{sup 3+}Mo{sup 5+}. - Graphical abstract: Unit cell of Ca(V{sub 0.5}Mo{sub 0.5})O{sub 3}.

Karen, P. [Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern N-0315, Oslo (Norway)]. E-mail: pavel.karen@kjemi.uio.no; Moodenbaugh, A.R. [Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973-5000 (United States); Goldberger, J. [Department of Chemistry, Ohio State University, 100 West 18th Avenue, Columbus, OH 43210-1185 (United States); Santhosh, P.N. [Department of Chemistry, Ohio State University, 100 West 18th Avenue, Columbus, OH 43210-1185 (United States); Woodward, P.M. [Department of Chemistry, Ohio State University, 100 West 18th Avenue, Columbus, OH 43210-1185 (United States)

2006-07-15

432

Concomitant Ordering in First Row Transition Metal Fluorides.  

NASA Astrophysics Data System (ADS)

Type of compounds investigated. First row transition metal fluorides with a general formula, A_ {x}M^{2+}_{x}M ^{'3+}_{1-x}F _3, where A = alkali metal ion, M,M ^' = First row transition metal ions, and x = 0-1 are reported to form, ReO_3 , perovskite, GdFeO_3, tetragonal tungsten bronze (TTB), hexagonal tungsten bronze (HTB), and, modified pyrochlore type structures. Compounds of the type LiM^{II}M^{III }F_{6} have been reported to form rutile and trirutile type structures. These compounds display ionic, electronic, and magnetic ordering. Formation of TTB structure and linear trimers. The TTB structure demonstrates ordering between M^ {2+} and M^{3+} ions. Consequently, a variety of TTB fluorides have been tailored that display anitferro-, ferri- or ferro -magnetism. By substituting a diamagnetic ion for either M^{2+} or M^ {3+}, the diluted paramagnetic ions form linear trimers. These linear trimers are useful for the study of 180^circ M-F-M interactions. Structural and magnetic properties of several quaternary TTB fluorides are reported. The Series K_{rm x}CrF _3. The series K_{rm x}CrF_3 (x ~ 0.4-0.6) was reported to form the hexagonal BaTa_2O_6 type structure. It was believed that the TTB structure did not form due to the Jahn-Teller ion Cr^ {2+}. The unit cell did not show any evidence of distortion. Using superior synthesis and characterization techniques, we report formation of three phase in the series K_{rm x}CrF _3 (x ~ 0.42-0.58). In the lower range the structure is a hexagonal BaTa _2O_6 type. In the middle composition range the structure is an orthorhombic distortion of the hexagonal unit cell. At higher values of x, formation of a TTB structure is observed. As expected, the tetragonal unit cell is distorted to orthorhombic due to cooperative Jahn-Teller ordering. Magnetic data support these observations. Derivation of symmetry trees. It is observed that the symmetry of a crystal is lowered as concomitant ordering sets in. This observation in conjunction with the Symmetry Principle has been used to derive Symmetry Trees (or, symmetry lowering diagrams) for the systems mentioned above. These Symmetry Trees experimentally confirm Landau and Lifshitz's contention that ordering reduces symmetry. They also provide experimental evidence for the Special Symmetry Evolution Principle, (the degree of symmetry of the state of an isolated system cannot decrease during evolution but either remains constant or increases). In several cases, where experimental determination of a unique space group is not feasible, the symmetry tree approach is implemented to determine the space group.

Shah, Ajay Vinodbhai