V. S. Sastry

The pressure-amorphized state in zirconium tungstate: a precursor to decomposition

In contrast to widely accepted view that pressure-induced amorphization arises due to kinetic hindrance of equilibrium phase transitions, here we provide evidence that the metastable pressure-amorphized state in zirconium tungstate is a precursor to decomposition of the compound into a mixture of simple oxides. This is from the volume collapse ΔV across amorphization, which is obtained for the first time by measuring linear dimensions of irreversibly amorphized samples during their recovery to the original cubic phase upon isochronal annealing up to 1000 K. The anomalously large ΔV of 25.7 ± 1.2% being the same as that expected for the decomposition indicates that this amorphous state is probably a precursor to kinetically hindered decomposition. A P–T diagram of the compound is also proposed.

Superconductivity in Ru substituted polycrystalline BaFe 2-x Ru x As 2

The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples of BaFe2-xRuxAs2 for nominal Ru content in the range of x=0.75 to 1.125. A systematic suppression of the spin density wave transition temperature (TSDW) precedes the appearance of superconductivity in the system. A phase diagram is proposed based on the measured TSDW and superconducting transition temperature (TC) variations as a function of Ru composition. Band structure calculations, indicate introduction of electron carriers in the system upon Ru substitutiom. The calculated magnetic moment on Fe shows a minimum at x=1.0, suggesting that the suppression of the magnetic moment is associated with the emergence of superconductivity. Results of low temperature and high field Mossbauer measurements are presented. These indicate weakening of magnetic interaction with Ru substitution

Wide band gap tunability of bulk Cd1−xCaxO

We report the lattice parameter variation and the band gap tuning of CdO with Ca by preparing thermodynamically stable Cd1−xCaxO solid solution in the entire composition range 0≤x≤1. The functional dependence of the lattice parameter on Ca concentration is found to deviate from Vegard’s law. The principal band gap is found to vary nonmonotonically over a wide range, from 1.4 to 3.9 eV for 0≤x≤0.8. First principles density functional theory calculations, using full potential linearized augmented plane wave methods also predict a nonlinear variation for the lattice parameter and the optical band gap with Ca concentration. From these calculations, contributions from volume deformation, electron transfer, and structural relaxation are estimated and the results are compared with experiments.

Charge state modification in Mn site substituted CMR manganites: strong deleterious influence on the ferromagnetic?metallic state

The effect of charge state modification at the Mn site on the physical properties of CMR manganites is reported. With a view to avoiding additional complexity of local spin coupling effects, Mn site substitution of La0.67Ca0.33MnO3 is carried out with appropriate diamagnetic ions—Zn2+, Zr4+, Ta5+ and W6+—of different valence states. The substitution results in size changes of the unit cell and enhanced local structural distortions, which increase in the order Zn, Zr, Ta and W. The ground state is ferromagnetic–metallic below a certain critical concentration xc of the substituents, beyond which the magnetic ground state shows a glassy behaviour. The phase transition temperatures (TMI and Tc) decrease with substitution, but to different extents. The observed suppression rates of the Curie temperature, Tc, of ~39 K/at.% and ~45 K/at.% respectively for Ta5+ and W6+ substituted compounds are the highest reported in the Mn site substituted CMR manganites. Besides the modification of majority carrier concentration due to the increased (decreased) Mn3+ concentration and enhanced local structural effects, the local electrostatic potential of the substituents seems to contribute to the unusually strong reduction in the itinerant ferromagnetism and the observed glassy states.

Electrical and magnetoresistivity studies in chemical solution deposited La(1−x)CaxMnO3 thin films

High quality magnetoresistive La(1−x)CaxMnO3 thin films have been prepared by the chemical solution deposition technique. A solution of propionate precursors of lanthanum, calcium, and manganese in propionic acid was used for this purpose. Films of varying compositions (x varying from 0.1 to 0.4) were spin coated on to LaAlO3(100) and SrTiO3(100) substrates at room temperature and pyrolyzed in the temperature range 600–850 °C. For fixed compositions, annealing at higher temperatures shifts the insulator–metal transition temperature (TI–M) to higher values accompanied by a reduction in the resistivity values. The TI–M variation for different x values was found to be less pronounced in the compositions x=0.2, 0.3, and 0.4. Typical TI–M values of 283 K and 290 K were obtained for La0.7Ca0.3MnO3 coated on LaAlO3 and SrTiO3 substrates, respectively, when annealed at 850 °C. The substrate effect was found to be more pronounced for the x value 0.1 which showed two peaks (one at 271 K and another at 122 K) in the...

ESR study of spin–lattice correlated clusters in single crystalline Nd0.7Sr0.3MnO3

We report electron spin resonance measurements in single crystalline Nd0.7Sr0.3MnO3 that provide evidence for the existence of spin–lattice correlated clusters above and below TC (= 205 K). The linewidth of the paramagnetic spectrum indicates the presence of strong electron–phonon interaction rather than the spin only interaction seen in other manganites. The gradual increase observed in the g value above TC is attributed to the presence of orbital correlations. The observation of ferromagnetic resonance (FMR) spectra only below 185 K (TC = 205 K) and noisy features in the FMR spectra above 140 K are ascribed to the strong competition between localization due to lattice distortions and delocalization of charge carriers. The magnetocrystalline anisotropy of the sample splits the FMR line into two below 185 K. An additional splitting observed in the FMR line is attributed to the existence of spin–lattice correlated (insulating) clusters within the ferromagnetic (metallic) phase. The influence of these clusters on the spin dynamics above and below TC is discussed.

Superconductivity in Ru substituted BaFe2-xRuxAs2

The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples of BaFe2-xRuxAs2 for nominal Ru content in the range of x=0.75 to 1.125. A systematic suppression of the spin density wave transition temperature (TSDW) precedes the appearance of superconductivity in the system. A phase diagram is proposed based on the measured TSDW and superconducting transition temperature (TC) variations as a function of Ru composition. Band structure calculations, indicate introduction of electron carriers in the system upon Ru substitutiom. The calculated magnetic moment on Fe shows a minimum at x=1.0, suggesting that the suppression of the magnetic moment is associated with the emergence of superconductivity. Results of low temperature and high field Mossbauer measurements are presented. These indicate weakening of magnetic interaction with Ru substitution

Effect of heterovalent ion substitution on the crystalline structure and magnetic ordering of SrRuO3

Structural and magnetic studies have been carried out on SrRu1−xMxO3 (M=Ga:x=0.1; Cu and Zn: x=0.1 and 0.2) in order to understand the dependence of the ferromagnetic transition temperature of SrRuO3 on heterovalent magnetic and nonmagnetic cation substitution at the Ru site. Powder x-ray diffraction has been carried out to analyze the crystal structure using the Rietveld refinement procedure. Tc decreases more rapidly for divalent cation dopants as compared to the trivalent cations. The refinement results show that there are distortions of the RuO6 octahedra in the doped compounds, with little change in the lattice parameters. However, the direction and magnitudes of the tilt of the octahedra for all the compounds do not show correlation with the Tc. Therefore, the change of Tc is attributed to the charge and spin disorders introduced by the heterovalent dopant cations. The filled eg orbitals in Ga, Cu, and Zn ions are expected to have weak hybridization with the oxygen p orbitals as compared to the pris...

Pressure-induced frustration in charge ordered spinel AlV2O4

AlV2O4 is the only spinel compound so far known that exists in the charge ordered state at room temperature. It is known to transform to a charge frustrated cubic spinel structure above 427 ° C. The presence of multivalent V ions in the pyrochlore lattice of the cubic spinel phase brings about the charge frustration that is relieved in the room temperature rhombohedral phase by the clustering of vanadium into a heptamer molecular unit along with a lone V atom. The present work is the first demonstration of pressure-induced frustration in the charge ordered state of AlV2O4. Synchrotron powder x-ray diffraction studies carried out at room temperature on AlV2O4 subjected to high pressure in a diamond anvil cell show that the charge ordered rhombohedral phase becomes unstable under the application of pressure and transforms to the frustrated cubic spinel structure. The frustration is found to be present even after pressure recovery. The possible role of pressure on vanadium t2g orbitals in understanding these observations is discussed.

Raman Scattering of Cadmium Oxide: in B1 Phase

Raman spectra of polycrystalline Cadmium Oxide (CdO) have been measured between 100 and 2000 cm−1 over the temperature range 80 to 300 K. The observed modes are reconciled with the results of the earlier reports. The observed Raman modes are ascribed to the structural disorder (static/dynamic (thermal)) and the resonance Raman processes. The Raman modes are assigned to forbidden longitudinal optical excitations, which are allowed under resonance conditions by a Frohlich interaction mechanism, and two‐phonon excitations.