V. K. Lakhani

Structural and elastic properties of Ca-substituted LaMnO3at 300 K

The compositional dependence of structural and elastic properties of Ca2+ substituted manganite perovskite system with a general formula La1−xCaxMnO3 (x = 0.00, 0.25, 0.33, 0.50 and 1.00) have been studied by means of x-ray diffraction, scanning electron microscopy and ultrasonic pulse transmission technique at 300 K. The elastic moduli of the specimens have also been computed and these are corrected to the void free state. The lattice parameters calculated theoretically are in agreement with those determined experimentally. The structural–elastic properties correlations have been studied. The observed enhancement of elastic constants up to x = 0.33 and reduction with a further increase in calcium concentration has been explained in the light of change in the strength of interatomic bonding caused by change in interatomic distances, microstructure and electronic configuration. The applicability of the heterogeneous metal mixture rule for theoretical estimation of elastic constants has been tested.

Raman and Mossbauer Spectroscopy and X-ray Diffractometry Studies on Quenched Copper–Ferri–Aluminates

Four spinel ferrite compositions of the CuAl(x)Fe(2-x)O4, x = 0.0, 0.2, 0.4, 0.6, system prepared by usual double-sintering ceramic route and quenched (rapid thermal cooling) from final sintering temperature (1373 K) to liquid nitrogen temperature (80 K) were investigated by employing X-ray powder diffractometry, (57)Fe Mossbauer spectroscopy, and micro-Raman spectroscopy at 300 K. The Raman spectra collected in the wavenumber range of 100-1000 cm(-1) were analyzed in a systematic manner and showed five predicted modes for the spinel structure and splitting of A1g Raman mode into two/three energy values, attributed to peaks belonging to each ion (Cu(2+), Fe(3+), and Al(3+)) in the tetrahedral positions. The suppression of lower-frequency peaks was explained on the basis of weakening in magnetic coupling and reduction in ferrimagnetic behavior as well as increase in stress induced by square bond formation on Al(3+) substitution. The enhancement in intensity, random variation of line width, and blue shift for highest frequency peak corresponding to A1g mode were observed. The ferric ion (Fe(3+)) concentration for different compositions determined from Raman spectral analysis agrees well with that deduced by means of X-ray diffraction line-intensity calculations and Mossbauer spectral analysis. An attempt was made to determine elastic and thermodynamic properties from Raman spectral analysis and elastic constants from cation distribution.

High temperature thermoelectric power study on calcium substituted lanthanum manganites

Compositional and temperature dependent thermoelectric power measurements have been performed on the polycrystalline manganite perovskite system: La1?xCaxMnO3+? (x = 0.00, 0.25, 0.33, 0.50 and 1.00), in the paramagnetic state (T > Tc) (300?673?K). The Seebeck coefficient (?) has been used to calculate the Fermi energy, the activation energy (E?), the Mn4+-ion concentration and the oxygen content (?), for different compositions. The observed decrease in E? with increase in Ca-concentration and a large value of E? as compared with a relatively high temperature sintered material of the same compositions have been explained in the light of the decrease in the interatomic distances and grain growth with Ca-substitution. It is shown that [Mn4+][Mn3+] ion concentration is not the only factor that governs conduction in the manganite perovskites. The significantly small values of ? (?22??V?K?1) at high temperature can be explained on the basis of (Mn3+?Mn3+) charge disproportionation into stable Mn2+?Mn4+pairs in the system rather than on the basis of nominal Mn3+/4+ valence arguments. Finally, nearly temperature independent variation of ?, charge-carrier concentration at high temperature and low value of mobility are hallmarks of small polaron hopping.

CCNR type high field instability in Ti4+-substituted Mn–Zn ferrites

Polycrystalline samples of the spinel ferrite system Zn0.3Mn0.7+xTixAl0.1Fe1.9−2xO4 (x = 0.0–0.3) have been studied for electrical switching in the temperature range from 300 to 700 K and applied voltage range from 0 to 400 V. High electric field induced resistance switching of nearly 200% has been observed for all the compositions. The dependence of temperature and applied electric field strength on switching has been examined. The electric field and temperature required for switching increase with increase in Mn–Ti concentration. The results have been interpreted considering previous models and have been ascribed to the existence of space charge limiting current in the system. The switching observed, is of current-controlled negative resistance (CCNR)-type.

Crystal Defects and Cation Redistribution Study on Nanocrystalline Cobalt-Ferri-Chromites by Positron Annihilation Spectroscopy

Positron lifetime and Doppler broadening measurements were carried out on nanocrystalline (grain size ~60–65 nm) samples of the Cr3

Positron and Positronium Annihilation as Probes to Identify Cation Substitution Effects in CoCrxFe2-xO4

CoCrxFe2-xO4 samples with x = 0.0 to 2.0 were prepared by air oxidation of aqueous suspension containing Co2+, Cr3+ and Fe3+ ions and characterized by x-ray diffraction. Positron lifetime and coincidence Doppler broadening (CDB) measurements indicated three distinct stages of positron trapping in defects. Initially the vacancy-type defects located at the A-sites (tetrahedral) trapped positrons but, with the substitution of Fe by Cr in low concentrations (x <= 0.7), positrons are trapped by defects at the B-sites (octahedral). Mossbauer spectroscopic results indicated the cationic distribution at B-sites to be stoichiometry-dependent and, till x = 0.7, the deficiency of Fe3+(B) ions was compensated by interchange of Fe3+(A) ions with Co2+(B) ions. Between x = 0.9 and 1.7, the substitution resulted in continued decrease of Fe3+(B) ions and the structure got fully transformed into a normal spinel configuration during x = 1.8 to 2.0.

Observation of bimodality in nanocrystalline cobalt - ferri - chromites

In this communication we present detail analysis of particle size distribution curves, differential size distribution and cumulative undersize distribution, recorded for nano particles of spinel ferrite system, CoCrxFe2-xO4 (x = 0.0, 1.1 and 2.0), synthesized by chemical co-precipitation technique. It is found that the distribution is bimodal as well as not mono disperse. Observed bimodality has been explained in the light of Ostwald ripening. Various parameters have been determined and the role of Cr3+ - substitution in governing signature of distribution patterns has been discussed.

Improvement in elastic properties of CuAl0.4Fe1.6O4 spinel ferrite by rapid thermal cooling

The elastic properties of spinel ferrite composition, CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}, quenched from final sintering temperature of 1373 K to liquid nitrogen temperature (∼ 80K) have been studied by means of X-ray powder diffractometry and pulse echo-overlap technique (9 MHz) at 300 K. The magnitude of elastic constants is found to enhance by 15% compared to slowly-cooled counterpart. The observed mechanical strengthening has been discussed in the light of compressive stress on the surface, with tensile stresses at interior regions and corresponding changes in structural parameters. The B{sub o}/G{sub o} ratio indicates the brittle nature of CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}.

Improvement in elastic properties of CuAl0.4Fe 1.6O4 spinel ferrite by rapid thermal cooling

The elastic properties of spinel ferrite composition, CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}, quenched from final sintering temperature of 1373 K to liquid nitrogen temperature (∼ 80K) have been studied by means of X-ray powder diffractometry and pulse echo-overlap technique (9 MHz) at 300 K. The magnitude of elastic constants is found to enhance by 15% compared to slowly-cooled counterpart. The observed mechanical strengthening has been discussed in the light of compressive stress on the surface, with tensile stresses at interior regions and corresponding changes in structural parameters. The B{sub o}/G{sub o} ratio indicates the brittle nature of CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}.