G. V. Subba Rao

Infrared and Electronic Spectra of Rare Earth Perovskites: Ortho-Chromites, -Manganites and -Ferrites

The electronic and ir spectra of rare earth perovskites of the general formula LnZO3, where Ln is the rare earth ion or yttrium and Z is Cr, Mn, or Fe, have been studied in detail. The results have been discussed in terms of crystallography, magnetic properties, covalency of Ln—O and Z—O bonds, and Goodenoughs one electron energy diagrams. In all these compounds the rare earth ions do not markedly affect the electronic transitions of the transition metal ions; the 3d electrons clearly exhibit localized behavior. Both the electronic and ir spectra of the LnZO3 perovskites are comparable to the spectra of the corresponding transition metal sesquioxides, Z2O3.

Electrical transport in rare earth ortho-chromites, -manganites and -ferrites

Abstract Rare earth ortho-chromites, -manganites and -ferrites are p -type semiconductors with conductivities in the range 10 −4 –10 −1 ohm −1 cm −1 . The conductivity in each series of perovskites decreases with the increasing atomic number of the rare earth. The ionic contribution to conductivity is small in all the three series of solids. None of these solids exhibits intrinsic behavior up to ∼1000°C. The conductivity behaviors of these rare earth compounds reflect the known crystallographic, dielectric and magnetic transitions in these materials. Seebeck coefficients in these compounds are large, typical of narrow-band materials; the Seebeck coefficients show marked changes at temperatures where magnetic and dielectric transitions occur. The electrical transport properties of all the three series of rare earth compounds are essentially controlled by the d -electrons of the transition elements which show localized behavior. This conclusion is in agreement with the results from optical spectra and the predictions of Goodenough. In all these compounds small polarons seem to be responsible for the conduction.

Electrical transport in rare-earth oxides☆

Abstract Sesquioxides and nonstoichiometric oxides of rare-earths (Ln) exhibit electrical conductivities in the range 10 −9 –10 −1 Ω −1 cm −1 . The sesquioxides exhibit mixed conduction with some contribution from ionic conductivity and major contribution from electronic conductivity. Seebeck coefficient data as well as the oxygen partial pressure dependence of conductivity indicate that LnO x compounds are mixed valence semiconductors where oxides with 1.50 ⩽ x ⩽ 1.75 are p -type semiconductors and oxides with 1.75 x ⩽ 2.00 are n -type semiconductors. The conductivity of LnO x (Ln = Pr or Tb) goes through a maximum at x ≈ 1.75; Seebeck coefficients are sensibly constant with temperature and approach zero value at x ≈ 1.75. Employing the conductivities and Seebeck coefficients, transport parameters have been calculated. The mechanism of conduction in these oxides can be understood in terms of the hopping model and the small polaron theory. Fully ionised cation vacancies seem to be the predominant defects contributing to the defect structure in rare-earth sesquioxides.

Semiconductor based photoelectrochemical cells for solar energy conversion—An overview

An overview of the semiconductor based photoelectrochemical (pec) cells for solar energy conversion is presented.pec cells are of two types: photoelectrolysis cells and photovoltaic cells. The principles involved, electrode and electrode/electrolyte interface characteristics, experimental methods of investigation and energy conversion efficiency are discussed in detail. Up-to-date data on variouspec cells are also presented and discussed.

Electrical properties of Ln2Mo2O7 pyrochlores (Ln=SmYb,Y)☆

Cubic oxide pyrochlores, Ln2Mo2O7, Ln=Smue5f8Yb,Y, (Nd1−xAx), A=Er and Yb; 0 < x < 1.0, have been synthesized and electrical properties were examined in the range 77-600K. Semiconductor behavior is observed for Ln=Euue5f8Yb, Y but the resistivity and Ea are low. Sm2Mo2O7 and Nd1−xYbxMo2O7 (0.05 ≤ x ≤ 0.1) exhibit semimetallic or metallic behavior. The mechanism of conduction in these compounds is explained on the basis of band model proposed by Sleight et al.

Metal atom incorporation studies on the phases with NZP structure: □NbTiP3O12

Abstract A wide variety of electropositive elements of the periodic table can be inserted into the vacant sites in the host framework structure of hexagonal NbTiP 3 O 12 (an analog of nasicon) to give rise to isostructural phases. Synthesis, characterization, and preliminary data on the structure, IR spectra, and electrical resistivity are presented. Possible areas for further exploration are delineated.

Phase transitions and conductivity anomalies in solid solutions of VO2 with TiO2, NbO2, and MoO2

The solid solutions V1−xTixO2 (0·02<-x<-0·4) which are monoclinic at room temperature (with the monoclinicity decreasing with increasing x) transform to rutile structures at Tt. The Tt, H as well as the conductivity jump (at Tt) decrease with increase in x. Incorporation of 2 at.% of Nb (or Mo) lowers the Tt of VO2 considerably, while 10% Nb stabilizes the high-temperature rutile structure. The high temperature rutile phases of all the solid solutions show semiconducting behavior indicating that the conductivity anomalies correspond to semiconductor-semiconductor transitions.

Synthesis and solid state studies on Mb2Sb2O7 and (Mn1−xCdx)2Sb2O7 pyrochlores

Abstract Pyrochlore oxides of the type Mn 2 Sb 2 O 7 and (Mn 1− x Cd x ) 2 Sb 2 O 7 have been synthesized by high-temperature solid state reactions and characterized by X-ray diffraction and chemical analysis. X-Ray diffraction studies showed that the compound Mn 2 Sb 2 O 7 has a rhombohedrally distorted pyrochlore structure. In the solid solutions (Mn 1− x Cd x ) 2 Sb 2 O 7 , the phases with x ≥ 0.6 are cubic. Magnetic and 121 Sb Mossbauer studies indicate that all the Mn and Sb are present in the +2 and +5 state occupying A and B sites, respectively, in the pyrochlore structure. Electrical measurements indicate that the compounds are insulators or semiconductors exhibiting p -type behavior. The stoichiometry and probable cause of the rhombohedral distortion in Mn 2 Sb 2 O 7 and solid solutions are discussed.

Critical state model and the magnetic behaviour of high Tc superconductors

Intragrain critical current densities (Jc) are usually inferred from magnetization measurements using Beans critical state model. Using this model results are presented for the case of Jc decaying exponentially with field and explanations are given for the observed changes in the shape of the magnetization hysteresis curve as (1) the sample temperature is changed; (2) the sample dimension is changed; (3) the maximum field to which the sample is cycled is varied; and (4) the field direction is changed for a single crystal sample. Some of the observed thermomagnetic history effects in the magnetization of the high temperature superconductors are also explained. Finally, d.c. and low-frequency a.c. magnetization data on various Rga2Cu3O7 superconductors are presented where features originating from intergranular and intragrain regions are identified and explained.

Metal atom incorporation studies on AxMo6S8 Chevrel phases

Abstract Group III a metals and Nb, Hg and Pb containing A x Mo 6 S 8 Chevrel phases have been synthesized by low temperature reaction (420–430°C) frorn A metal and Mo 6 S 8 . Some of these phases are hitherto unknown. Electrolysis method of preparation of A x Mo 6 S 8 reported by Schollhorn does not yield the desired phases when A = Pb or Ag. Stoichiometry, thermal stability, electrical and superconducting properties of the compounds have been examined. In the Pb x Mo 6 S 8 system, a series of phases with varying x have been prepared and studied. Group III a metal and Nb-compounds are found to be semiconductors with low energies of activation exhibiting p -type behavior in the range 77–300 K. Hg-phase behaves as a metal or a degenerate semiconductor and does not become superconducting above 4.2 K. X-Ray, resistivity and T c studies show that for x ⩽ 0.8 in Pb x Mo 6 S 8 , the compounds behave more like Mo 6 S 8 and only for x ≅ 1.0 or (Pbue5f8Bi) 1.0 , the inherent behavior of the ternary Chevrel phase is exhibited. Seebeck coefficients are small and positive for Mo 6 S 8 , Cu and Pb-containing phases in the range 77–300 K. The results are discussed in the light of available band structure of these materials.