The world of perovskite oxides: From dielectrics to superconductors

Some aspects of the properties of oxides of perovskite and K2 NiF4 structures are presented. Some of the interesting aspects discussed are intergrowths, orthorhombicity of superconducting cuprates and importance of holes on oxygen.

Surface electronic structure of the high-Tc oxides

Electron energy loss spectroscopy (EELS) has been employed to monitor surface conductivity changes in YBa2Cu3O7 as a function of temperature. Concomitant use of x-ray photoelectron spectroscopy (XPS) establishes that the formation of oxygen dimers with lowering of temperature is accompanied by a simultaneous increase of surface conductivity.

The hole spin model for oxides

A model of mobile 0-holes hybrized with Cu-spins on a square lattice is examined. A variational groundstate wavefunction which interpolates smoothly between n.n. RVB and Néel limits gives a Néellike minimum. A hole in an AF lattice polarizes it locally and becomes quite mobile. Two n.n. holes attract. Finally we speculate how holes can stabilize a spin liquid state.

Interatomic Auger transitions in transition-metal oxides

Interatomic L3(M)M23(M)V(O) and L3(M)V(O)V(O) Auger transitions of some transition-metal oxides are reported for the first time. The interatomic mode of decay becomes progressively more dominant (relative to the intra-atomic mode) as the metal d level gets depleted or as the oxidation state of the metal increases. The usefulness of interatomic Auger transitions in studying oxidation of metals has been examined.

An Electron-Microscopic Investigation Of Oxides Related To Beta-Alumina

High resolution electron microscopic (HREM) investigation of potassiumbeta-alumina and the related gallate and ferrite has revealed that whereas the aluminate and gallate are highly disordered, consisting of random sequence ofbeta andbetaPrime units, the ferrite is more ordered. The aluminate and gallate are sensitive to electron beam irradiation exhibiting beam-induced damage similar to sodiumbetaPrime-alumina. Significantly, the ferrite is beamstable, the difference in behaviour amongst these related oxides arising from the different mechanisms by which alkali metal nonstoichiometry is accommodated. Barium hexaaluminate and hexaferrite are both highly ordered; specimens prepared by the barium borate flux method exhibit a new radic3a×radic3a superstructure of the hexagonal magnetoplumbite cell.

Oxides supported carbon air-electrodes for alkaline solution power devices

Porous carbon oxygen-reducing electrodes incorporated with perovskite oxide catalysts are reported. It has been possible to fabricate high-performance oxygen-reducing electrodes by introducing La0.5Sr0.5CoO3 and La0.99Sr0.01NiO3 with the activated coconut-shell charcoal; these electrodes could sustain load currents as high as 1 A cm−2 without serious degradation. A model to explain oxygen-reducing activity of these oxides has been proposed.

Electron spin resonance studies of d5 ions (Fe3+ and Mn2+) in lead oxide-lead halide glasses

Electron spin resonance (ESR) of d5 ions (Fe3+ and Mn2+) has been investigated in PbO---PbF2 and PbO---PbCl2 glasses in wide ranges of composition. ESR spectra of d5 ions in these glasses exhibit significant differences which we have attributed to at least three important causes: (i) The ionic potentials of Fe3+ and Mn2+ are different. Hence Fe3+ ions tend to acquire their own environment while Mn2+ ions take up substitutional (Pb2+ ion) positions. (ii) The sizes and nephelauxetic behaviours of O2- and F- ions are similar. Thus even when there is a mixed anionic coordination, the environment of Mn2+ ions is highly symmetrical in oxyfluoride glasses. The Mn2+ spectra in oxychloride glasses are considerably different. (iii) Increase in halide ion concentration increases the ionicity of lead-ligand bonding and favours a more symmetrical environment around dopant ions in halide-rich glasses. The features in ESR spectra have been interpreted in the light of known behaviour of d5 ions in glasses and also in the context of known structural features of PbO---PbX2 glasses. Dopant ions appear to cluster at high concentrations although isolated low-symmetry sites are still observed. Effects of crystallization and annealing upon ESR spectra have also been investigated.

Infrared and Mössbauer spectroscopic study of the metal-insulator transition in some oxides of perovskite structure

The IR spectra of some LaNi1−xBxO3 (B = Cr, Fe, and Co) compounds having perovskite structure have been studied in the range 1000−300 cm−1. An investigation of the changes in the metal-oxygen stretching frequency as x → xc from the insulating side has been carried out. An important feature is that as x → xc the vibrational features in the infrared spectra disappear when the resistivity is not, vert, similar10−1 Ω cm which is of two orders of magnitude more than the value of varrho0 at which the temperature coefficient of resistance changes sign. Mössbauer studies on Fe-containing samples with various conductivities show that the isomer shift decreases as conductivity increases which is indicative of larger Fe---O overlap.

Reinvestigation of the antifungal activities of some aromatic N-oxides

Dinitroquinoline-N-oxide, 4-nitroquinoline N-oxide and a series of 4-substituted pyridine N-oxides have been subjected toMINDO/3 treatment in order to understand their antifungal activities. The photoelectron spectra and the nature of the N-oxide bond are discussed.

The study of oxidation state of manganese in lead oxyhalide glasses by optical spectroscopy

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Infrared spectra of some oxides of K2NiF4 and related structures

The i.r. spectra of some Ln2BO4 and LnSrBO4 compounds (Ln = La, Pr, Nd, Sm or Gd;B = Fe, Al, Co or Cu) with K2NiF4 or related structures have been studied in the range 800-300 cm−1. The BO6 octahedra in compounds with K2NiF4 structure are elongated. The assignment of the bands in terms of internal modes of sheets of bridged BO6 octahedra or square-planar BO4 sheets has been considered. The observed spectra are correlated with those of solid solutions of these oxides and of LnBO3 perovskites. Unusually high stretching frequencies found in some of the oxides are discussed in terms of the short B---O bonds in the basal plane and the Ln---O bonds along the c axis.