Ramaswamy Murugan

Electronic and structural properties of zinc chalcogenides ZnX (X=S, Se, Te)

The structural phase transformations of ZnS, ZnSe and ZnTe under high pressure are studied by tight binding linear muffin tin orbital (TB-LMTO) method. A simple cubic 16 (SC16) phase is observed in all three zinc chalcogenides ZnX (X=S, Se, Te) and the stability of the high pressure phases is also presented. In ZnS and ZnSe, the sequence of transformation is similar to zincblende (ZB)→SC16→rock salt (RS), but in ZnTe the transformation sequence is different, namely the SC16 phase is observed above the cinnabar phase. The ground state properties of the phases of zinc chalcogenides ZnX (X=S, Se, Te) are also calculated.

Synthesis and characterization of LiNiyCo1−yPO4 (y=0–1) cathode materials for lithium secondary batteries

In this work series of LiNiyCo1−yPO4 (y=0, 0.2, 0.4, 0.6, 0.8 and 1) phospho olivines were synthesized by solution co-precipitation technique and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and impedance spectroscopic analysis. The XRD patterns of LiNiyCo1−yPO4 (y=0.2, 0.4, 0.6 and 0.8) revealed that they are essentially single phase and have an Olivine-type XRD patterns similar to those of their parent compounds LiCoPO4 and LiNiPO4. An increase in wave number for most of the dominant infrared bands in PO4 vibrational region for the substitution of Co by Ni in LiCoPO4 indicated the strengthening of both the P-O and Li/Ni-O bonds.

Investigation of structural changes in the phase transformations of γ-Bi2MoO6

In this work, extensive thermo-Raman investigations have been carried out on a ?(L)-Bi2MoO6 sample (`(L) indicating the low-temperature polymorph) in an effort to gain an understanding of the structural evolution and phase transformation involved during a dynamical thermal process. The room temperature Raman spectrum of the ?(L)-Bi2MoO6 phase indicated the presence of corner-sharing distorted MoO6 octahedra. The spectral variations, shift in the band positions, decrease in intensities and broadening of the Mo-O modes observed in the thermo-Raman spectra and the thermal evolution of the Mo-O bonds in the temperature interval from 25 to 610??C strongly suggest that the transformation from the ?(L)-Bi2MoO6 to the ?(I)-Bi2MoO6 phase (`(I) indicating the intermediate-temperature polymorph) involves a gradual anisotropic thermal response of Mo-O bonds initially, upon heating, and further appreciable increase in the octahedral distortion from 310??C onwards - until the structure is on the verge of becoming composed of distorted tetrahedra. The spectral variation in the thermo-Raman spectra observed for the transformations to ?(H)-Bi2MoO6 (`(H) indicating the high-temperature polymorph) starting at around 620??C and ending at 660??C revealed the sluggish nature of the transformations and also clearly indicated the transformation from highly distorted MoO6 octahedra to distorted tetrahedra. The comparison of the profile of the Raman spectrum of the ?(H)-Bi2MoO6 phase measured at 720??C and that measured after cooling down to 25??C clearly rules out the possibility of the formation of a ?-Bi2Mo2O9} phase structure consisting of Mo6+ in tetrahedral coordination at high temperature.

Ionic conductivity and Raman investigations on the phase transformations of Na4P2O7

Abstract The ionic conductivity and thermo-Raman spectra of anhydrous sodium pyrophosphate Na 4 P 2 O 7 were measured dynamically in the temperature range from 25 to 600xa0°C with a heating rate of 2xa0°C min −1 to understand the structural evolution and phase transformation involved. The DSC thermogram was also measured in the same thermal process for the phase transformation investigation. The spectral variations observed in the thermo-Raman investigation indicated the transformation of Na 4 P 2 O 7 from low temperature phase ( e ) to high temperature phase ( α ) proceeded through pre-transitional region from 75 to 410xa0°C before the major orientational disorder at 420xa0°C and minor structural modifications at 511, 540 and 560xa0°C. The activation energies and enthalpies of the proposed phase transformations were determined. The possible mechanism for temperature dependent conductivity in Na 4 P 2 O 7 was discussed with the available data.

ELECTRONIC STRUCTURE, MAGNETIC ORDERING AND PHASE STABILITY OF LiFeX (X = P, As and Sb) UNDER PRESSURE

Electronic band structure calculations were performed on the nonmagnetc (NM) and antiferromagnetic (AFM) phases of LiFeX (X = P, As and Sb) compounds using ab initio method. The crystal structure of these compounds is well tetragonal P4/nmm structure (space group = 129). Self-consistent calculations were performed by planewave pseudo-potential, density functional based method using PWSCF-Quantum Espresso code. To study the electronic structure and magnetic ordering, the total energies of these compounds have been computed as a function of reduced volumes and fitted with Brich Murnaghan equation. The estimated lattice parameters are in good agreement with available experimental data. The calculated Fe magnetic moment for LiFeSb is larger than LiFeAs and LiFeP. The obtained electron–phonon coupling constant (λ) for the NM phase are very weak when compared to that of AFM phase of LiFeX compounds. Present calculations reveal that the electron–phonon coupling constant λ decreases as a function of pressure.

First principle study on electronic structure, structural phase stability, optical and vibrational properties of Ba2ScMO6 (M = Nb, Ta)

First principle calculations are performed to investigate the electronic structure, structural phase stability, optical and vibrational properties of double perovskite oxide semiconductors namely Ba2ScMO6 (M = Nb, Ta) in the cubic symmetry using WIEN2k. In order to study the ground state properties of these compounds, the total energies are calculated as a function of reduced volumes and fitted with Brich Murnaghan equation. The estimated ground state parameters are comparable with the available experimental data. Calculations of electronic band structure on these compounds reveal that both Ba2ScNbO6 and Ba2ScTaO6 exhibit a semiconducting behavior with a direct energy gap of 2.78 and 3.15 eV, respectively. To explore the optical transitions in these compounds, the real and imaginary parts of the dielectric function, refractive index, extinction coefficient, reflectivity, optical absorption coefficient, real part of optical conductivity and the energy-loss function are calculated at ambient pressure and an...

Anion reorientation in anhydrous Na3PO4 during the phase transformation

The transport phenomena in alkali-metal super ionic conductors based on Na3PO4 structure are of particular interest due to their potential technological application. Differential thermal analysis (DTA), differential scanning calorimetry (DSC), Raman spectroscopy and temperature dependent electrical conductivity measurements were carried out to probe the nature of the phase transformation involved in anhydrous Na3PO4. The changes in spectral profile of the v3 mode and the line width of v1 mode of PO43− observed in the temperature interval from 331 to 345 °C revealed the high degree of disorder nature during the α-γNa3PO4 phase transformation.