Balan Palanivel

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.

Li+ transport properties of W substituted Li7La3Zr2O12 cubic lithium garnets

Lithium garnet Li7La3Zr2O12 (LLZ) sintered at 1230 °C has received considerable importance in recent times as result of its high total (bulk + grain boundary) ionic conductivity of 5 × 10−4 S cm−1 at room temperature. In this work we report Li+ transport process of Li7−2xLa3Zr2−xWxO12 (x = 0.3, 0.5) cubic lithium garnets. Among the investigated compounds, Li6.4La3Zr1.7W0.3O12 sintered relatively at lower temperature 1100 °C exhibits highest room temperature (30 °C) total (bulk + grain boundary) ionic conductivity of 7.89 × 10−4 S cm−1. The temperature dependencies of the bulk conductivity and relaxation frequency in the bulk are governed by the same activation energy. Scaling the conductivity spectra for both Li6.4La3Zr1.7W0.3O12 and Li6La3Zr1.5W0.5O12 sample at different temperatures merges on a single curve, which implies that the relaxation dynamics of charge carriers is independent of temperature. The shape of the imaginary part of the modulus spectra suggests that the relaxation processes are non-Deb...

Superconductivity of WC in the NaCl-Type Structure under Pressure

Here we report the theoretical calculations of band structure and superconductivity of WC in the NaCl-type structure. The effect of pressure on the band structure is obtained using the linear muffin-tin orbital method. The superconducting transition temperature is calculated using McMillans formula. The value of T c at ambient conditions is 5.98 K which increases on compression. The increase in T c values with pressure may be due to continuous transfer of s electrons from the carbon site to d states of the tungsten site. The calculated value of T c at ambient pressure and the positive pressure coefficient of T c are in agreement with other transition metal carbides and nitrides.

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...

First-principle study on lithium intercalated antimonides Ag3Sb and Mg3Sb2

First-principle calculations based on density functional theory have been performed to investigate the negative electrode behaviors, structural changes, and electronic and bonding properties of lithium intercalated antimonides Ag3Sb and Mg3Sb2. Initial intercalation of lithium to orthorhombic Ag3Sb led to form cubic Li2AgSb. Lithium insertion to hexagonal Mg3Sb2 results in cubic LiMgSb. Further insertion of lithium with the intercalated compounds Li2AgSb and LiMgSb results in to the formation of alkali antimonide Li3Sb. The structural transformation of both antimonides Ag3Sb and Mg3Sb2 followed by the insertion of Li+ ends with the formation of Li3Sb with cubic phase. The computed band structures along high symmetry directions of the Brillouin zone, and total and partial density of states clearly illustrate that the intercalation of lithium with Ag3Sb and Mg3Sb2 changes their metallic nature into semiconductor. From the charge density calculations, it is observed that the covalent bond nature in the parent phases Ag3Sb and Mg3Sb2 changed into ionic bond in the Li+ intercalated phases Li2AgSb, LiMgSb, and Li3Sb.

Electronic and optical properties of AgMX2 (M= Al, Ga, In; X= S, Se, Te)

The Electronic properties of chalcopyrite AgMX2 (M=Al, Ga, In; X=S, Se, Te) compounds are studied theoretically by means of full potential Linear Muffin Tin Orbital Method. The equilibrium volume, bulk modulus and indirect energy band gap for the compounds are calculated and compared with the available data and found to be in good agreement. The structural as well as optical properties like dielectric functions, degree of anisotropy and refractive index are also calculated.

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.

First principle calculations on structural, electronic and transport properties of Li2TiS3 and Li3NbS4 positive electrode materials

First principle calculations based on density functional theory have been performed on lithium containing transition metal sulfides Li2TiS3 and Li3NbS4 which are recently identified as novel positive electrode materials for rechargeable Li+ batteries. The calculations were performed to investigate the structural stability, electronic and transport properties of Li2TiS3 and Li3NbS4 along with their corresponding delithiated phases LiTiS3 and Li2NbS4. In this study it has been observed that these lithium containing sulfur materials maintain their face-centered cubic structure upon extraction of Li+. To calculate the structural stability and volume change due to lithium extraction, the total energies of Li2TiS3, Li3NbS4 and their corresponding delithiated phases LiTiS3 and Li2NbS4 have been computed by applying full potential linearized augmented plane wave (FP-LAPW) method implemented in WIEN2K. The equilibrium structural parameters for all the phases were determined by achieving total energy convergence. These electrode materials exhibit very small percentage of volume change with change in Li+ concentration which accounts for excellent structural stability. The computed band structure along high symmetry lines in the Brillouin zone, total and partial density of states clearly reveals that the extraction lithium from these electrode materials does not change their metallic nature. The electronic conductivities of both lithiated and delithiated phases have been calculated by employing BoltzTrap which can be interfaced with WIEN2K. The topological distributions of electron charge density at various critical points within the system were analyzed with the use of CRITIC code which is based on Bader’s theory of atoms in molecules (AIM). From the charge density calculations, it was observed that, there is strong ionic bond and weak covalent bond between atoms of the compounds Li2TiS3 and Li3NbS4. But the ionic bond nature was found to decrease in the delithiated phases LiTiS3 and Li2NbS4. The calculated values of electronic conductivities and discharge voltages for both electrodes are found to be in accordance with the recent experimental reports.

Ab initio calculation of structural stability, electronic and optical properties of Ag2Se

The structural stability, electronic and optical properties of Ag2Se compound is studied using ab initio packages. Ag2Se is found to crystallize in orthorhombic structure with two different space groups i.e. P212121 (No. 19) and P2221 (No. 17). For this compound in these two space groups, the total energy has been computed as a function of volume. Our calculated results suggest that the P212121–phase is more stable than that of the P2221–phase. The band structure calculation show that Ag2Se is semimetallic with an overlap of about 0.014 eV in P212121–phase whereas is metallic in nature in P2221–phase. Moreover, the optical properties including the dielectric fuction, energy loss spectrum are obtained and analysed.