Synthesis of Layered Perovskite Oxides, ACa2-xLaxNb3-xTix010 (A = K, Rb, Cs), and Characterization of New Solid Acids, HCa2-xLaxNb3-xTixOlo (0 < x d 2), Exhibiting Variable Bronsted Acidity?

Layered perovskite oxides of the formula ACa~,La,Nb3-,Ti,010 (A = K, Rb, Cs and 0 < x d 2) have been prepared. The members adopt the structures of the parent ACazNb3010. Interlayer alkali cations in the niobium-titanium oxide series can be ion-exchanged with Li+, Na+, NH4+, or H+ to give new derivatives. Intercalation of the protonated derivatives with organic bases reveals that the Bronsted acidity of the solid solution series, HC~ ~ , L ~ ,N~ ~ , T ~ ,dOep~eOnd, s on the titanium content. While the x = 1 member (HCaLaNbzTiOlo) is nearly as acidic as the parent HCazNb3010, the x = 2 member (HLazNbTizOlo) is a weak acid hardly intercalating organic bases with pKa - 11.3. The variation of acidity is probably due to an ordering of Nb/Ti atoms in the triple octahedral perovskite slabs, [Ca~,La,Nb~,Ti,0~0], such that protons are attached to NbO6 octahedra in the x = 1 member and to Ti06 octahedra in the x = 2 member.

(Ba3MTiMO9)-Ti-III-O-V (M-III = Fe, Ga, Y, Lu, M-V = Nb, Ta, Sb) perovskite oxides Synthesis, structure and dielectric properties

We describe the synthesis structures and dielectric properties of new perovskite oxides of the formula (Ba3MTiMO9)-Ti-III-O-V for M-III = Fe Ga Y Lu and M-V = Nb Ta Sb While M-V = Nb and Ta oxides adopt disordered/partially ordered 3C perovskite structures where M-III/Ti/M-V metal-oxygen octahedra are corner connected the M-V = Sb oxides show a distinct preference for the 6H structure where Sb-V/Ti-IV metal-oxygen octahedra share a common face forming (Sb Ti)O-9 dimers that are corner-connected to the (MO6)-O-III octahedra The preference of antimony oxides (Sb-V 4d(10)) for the 6H structure which arises from a special Sb-V-O chemical bonding that tends to avoid linear Sb-O-Sb linkages unlike Nb-V/Ta-V d(0) atoms which prefer similar to 180 degrees Nb/Ta-O-Nb/Ta linkages - is consistent with the crystal chemistry of M-V-O oxides in general The dielectric properties reveal a significant difference among Mill members All the oxides with the 3C structure excepting those with Mill = Fe show a normal low loss dielectric behaviour with epsilon = 20-60 in the temperature range 50-400 degrees C the M-III = Fe members with this structure (M-V = Nb Ta) display a relaxor-like ferroelectric behaviour with large E values at frequencies <= 1 MHz (50-500 degrees C) (C) 2010 Elsevier Masson SAS All rights reserved

Ab initio molecular orbital calculations on ion pair-water complexes of metal halides and oxides

Ab initio MO calculations are performed on a series of ion-molecular and ion pair-molecular complexes of H2O + MX (MX = LiF, LiCl, NaCl, BeO and MgO) systems. BSSE-corrected stabilization energies, optimized geometrical parameters, internal force constants and harmonic vibrational frequencies have been evaluated for all the structures of interest. The trends observed in the geometrical parameters and other properties calculated for the mono-hydrated contact ion pair complexes parallel those computed for the complexes of the individual ions. The bifurcated structures are found to be saddle points with an imaginary frequency corresponding to the rocking mode of water molecules. The solvent-shared ion pair complexes have high interaction energies. Trends in the internal force constant and harmonic frequency values are discussed in terms of ion-molecular and ion-pair molecular interactions.

A study of cubic bismuth oxides of the type bi(26-X)m(X)O(40-delta) (m=ti, mn, fe, co, ni or pb) related to gamma-bi2O3

A structural investigation of cubic oxides (space group I23) of the formula Bi(26-x)M(x)O(40-delta) (M = Ti, Mn, Fe, Co, Ni and Pb) related to the Y-Bi2O3 phase has been carried out by the Rietveld profile analysis of high-resolution X-ray powder diffraction data in order to establish the cation distributions. Compositional dependence of the cation distribution has been examined in the case of Bi26-xCoxO40-delta (1 < x < 16). The study reveals that in Bi(26-X)M(X)O(40-delta) with M = Ti, Mn, Fe, Co or Pb, the M cations tend to occupy tetrahedral (2a) sites when x < 2 while the octahedral (24f) sites are shared by the excess Co or Ni cations with Bi atoms when x > 2. Also experimental magnetic moments of Mn, Co and Ni derivatives have been used to establish the valence state and distribution of these cations.

Diffusion path during internal displacement reactions in multi-component oxides: Reaction between Fe and (Co,Mg)TiO3 solid solution at 1273 K

The reaction between Fe foil and a disc of ilmenite solid solution (Co-0.48 Ni-0.52) TiO3 was studied at 1273 K. At the metal/oxide interface, the displacement reaction, Fe + (Co,Mg)TiO3 = Co + (Fe,Mg)TiO3 occurs, resulting in an ilmenite solid solution containing three divalent cations. Ferrous ions diffuse into the oxide solid solution and cause the precipitation of Co-Fe alloy as discrete particles inside the oxide matrix. The morphology of the product layer was characterized by SEM. Only two phases, alloy and ilmenite, were detected in the reaction zone. This suggests that the local flux condition imposed by ilmenite stoichiometry (Co + Fe + Mg):Ti = 1:1] was satisfied during the reactive diffusion: (J(Co) + J(Fe) + J(Mg)) = J(Ti). The composition of the alloy and the oxide was determined using EPMA as a function of distance in the direction of diffusion. Although Mg does not participate in the displacement reaction, its composition in the ilmenite phase was found to be position dependent inside the reaction zone. The up-hill diffusion of inert Mg is caused by the development of chemical potential gradients as a result of displacement reaction. The evolution of composition gradients inside the reaction zone and the diffusion path in a ternary composition diagram of the system CoTiO3-FeTiO3-MgTiO3 are discussed. (C) 2010 Elsevier B.V. All rights reserved.

Rearrangements in the cerium(IV) and manganese(III) oxidations of substituted naphthalenes and the nih shift mechanism

Ceric ammonium sulphate oxidation of 1- and 1,4- disubstituted naphthalenes gives 2- and/or 2,3- disubstituted 1,4- naphthoquinones through migration of substituents (D, Br, Ph). Similar rearrangements are also observed in the manganese(III) oxidation and also in the anodic oxidation of these substrates. The results are consistent with the proposal that these oxidations go through the formation of radical cation followed by reaction with H2O and further oxidation of the radical to the carbocationic intermediate on the way to the corresponding 1,4-naphthoquinone. Oxidation of 1,4-diphenylnaphthalene gives 2,3-diphenyl-1,4-naphthoquinone or 4-hydroxy- 2,4- diphenyl - 1(4)R - naphthalenone. The results are in accordance with the conclusion that such rearrangements do not require prior formation of arene oxide intermediates, originally proposed for the NM shift mechanism.

AM(1-x)Al(x)O(3-x) (A=Na or K; M=Nb or Ta): New anion-deficient Perovskite oxides exhibiting oxide ion conduction

Anion-deficient perovskite oxides of the formula AM(1-x)Al(x)O(3-x) (A = Na or K; M = Nb or Ta) have been prepared for 0 < x less than or equal to 0.5. Diffraction experiments reveal that while the potassium compounds adopt orthorhombic/cubic perovskite structures similar to the parent KNbO3/KTaO3, the sodium compound, NaNb0.5Al0.5O2.5, possesses a brownmillerite/LaSr-CuAlO5-like superstructure. Al-27 NMR spectra show an exclusive tetrahedral oxygen coordination for AI(III) in Na-Nb0.5Al0.5O2.5 (I) and both tetrahedral and octahedral coordination for Al(III) in KNb0.5Al0.5O2.5 (II). The results suggest a long-range and short-range ordering of oxide ion vacancies in I and II respectively. Electrical conductivity measurements show a significant oxide ion conduction for KNb1-xAlxO3-x, with the conductivity increasing with x up to x = 0.5. The differences in the Arrhenius plots of the ionic conductivity of I and II have been rationalized in terms of the long-range and short-range ordering of oxide ion vacancies in the anion-deficient perovskite oxides.

Electronic structure of early 3d-transition-metal oxides by analysis of the 2p core-level photoemission spectra

The electronic structures of a wide range of early transition-metal (TM) compounds, including Ti and V oxides with metal valences ranging from 2+ to 5+ and formal d-electron numbers ranging from 0 to 2, have been investigated by a configuration-interaction cluster model analysis of the core-level metal 2p x-ray photoemission spectra (XPS). Inelastic energy-loss backgrounds calculated from experimentally measured electron-energy-loss spectra (EELS) were subtracted from the XPS spectra to remove extrinsic loss features. Parameter values deduced for the charge-transfer energy Delta and the d-d Coulomb repulsion energy U are shown to continue the systematic trends established previously for the late TM compounds, giving support to a charge-transfer mechanism for the satellite structures. The early TM compounds are characterized by a large metal d-ligand p hybridization energy, resulting in strong covalency in these compounds. Values for Delta and U suggest that many early TM compounds should be reclassified as intermediate between the charge-transfer regime and the Mott-Hubbard regime.

Mössbauer effect and electron paramagnetic resonance studies on zinc borate glasses containing transition metal oxides

Studies have been carried out in glasses containing Fe2O3, V2O5, and Fe2O3 + V2O5. Mossbauer studies in the ZnO-B2O3-Fe2O3 system show that iron is present as Fe3+ with tetrahedral coordination and that the isomer shift and the quadrupole splitting decrease with increase of Fe2O3 Content; similarly, the isomer shift and quadrupole splitting are also found to decrease with increasing ZnO. On the other hand, in the Na2O-ZnO-B2O3-Fe2O3 system, the isomer shift increases with Na2O or ZnO while the quadrupole splitting is fairly insensitive. Electron paramagnetic resonance in the ZnO-B2O3-Fe2O3 system shows signals at g = 4.20 and 2.0, whose intensity and linewidth show strong dependence on Fe2O3 content. In the ZnO-B2O3-V2O5 system, electron paramagnetic resonance shows that vanadium is present as the vanadyl complex, and the hyperfine coupling constants, A(parallel-to) and A(perpendicular-to) decrease with increasing V2O5 content; on the other hand, g(parallel-to) decreases and g(perpendicular-to) increases slightly, indicating an increase in tetragonal distortion. Zinc borate glasses containing Fe2O3 + V2O5 do not show the hyperfine structure of V4+ due to the interaction between Fe3+ and V4+

Giant magnetoresistance in lanthanum strontium yttrium manganese oxides (La(1-x)Sr(x-z)YuMnO(3))

Electron transport and magnetic properties of several compositions of the La1-xSx-zYzMnO3 system have been investigated in order to explore the effect of yttrium substitution on the magnetoresistance and related properties of these manganates. Yttrium substitution lowers the T-c and the insulator-metal transition temperature, while increasing the peak resistivity. A comparison of the properties of La1-xSrx-zYzMnO3 with the corresponding La1-xCax-zYzMnO3 compositions shows that the observed properties can be related to the average size of the A-site cations.

Giant magnetoresistance in transition metal oxides

Some materials exhibit large changes in electrical resistance in the presence of a magnetic field, and this change can be used in applications from sensor technology to magnetic data storage. In their Perspective, Rao and Cheetham discuss magnetoresistance in perovskite manganates, where the effect is unusually strong. Much has been learned about these materials, and this understanding is driving the search for new materials with even more impressive properties.

Normal state tunneling conductance of perovskite oxides: Implications for high-Tc superconductors

We have investigated tunneling conductances in disordered, normally conducting perovskite oxides close to the metal�insulator transition. We show that the normal state tunneling conductance of perovskite oxides can be cast in a general form G(V) = G0[1 + curly logical orV/V*curly logical orn] with 1?n?0.5 and where V* is an intrinsic energy scale. The exponent n graduall y increases from 0.5 to 1 as the metal-insulator (M-I) transition is approached. In the high-Tc Bi(2212) cuprates, the normally observed, linear G(V)(n=1) can be made sub-linear (n<1) by substitution of Ca with Y. From the similarity of the linear conductances, we suggest proximity to the M-I transition as a likely cause for this G(V)logical or, bar below V dependence. In systems showing linear conductances (nreverse similar, equals1), we find that ?G/?Vreverse similar, equalsG?0 with ?reverse similar, equals 1 and the intrinsic energy scale V*reverse similar, equals25�75 meV in the different oxides investigated.