Pressure-Induced Enhanced Optical Absorption in Sulvanite Compound Cu3TaX4 (X = S, Se, and Te): An ab Initio Study.

Ab initio study on the family of ternary copper chalcogenides Cu3TaX4 (X = S, Se, and Te) is performed to investigate the suitability of these compounds to applications as photovoltaic absorber materials. The density functional theory based full potential linearized augmented plane wave method (FP-LAPW method) is employed for computational purposes. The electronic structure and optical properties are determined including electron-electron interaction and spin-orbit coupling (SOC), within the generalized gradient approximation plus Hubbard U (GGA+U) and GGA+U+SOC approximation. The large optical band gaps of Cu3TaS4 and Cu3TaSe4 considered ineffective for absorber materials, and also the hole effective mass has been modulated through applied pressure. These materials show extreme resistance to external pressure, and are found to be stable up to a pressure range of 10 GPa, investigated using phonon dispersion calculations. The observed optical properties and the absorption coefficients within the visible-light spectrum make these compounds promising materials for photovoltaic applications. The calculated energy and optical band gaps are consistent with the available literature and are compared with the experimental results where available.

Electronic and thermoelectric properties of Nd-doped Ce-filled skutterudites.

The electronic and thermoelectric properties of Nd-doped Ce-filled skutterudites (CeFe4P12, CeFe4As12, and CeOs4P12) were explored using full-potential linearized augmented plane waves (FP-LAPW). The exchange-correlation between the electrons was treated with the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) and the Coulomb repulsion term (U) between the electrons for the highly correlated system was also considered. The energy band structures revealed the semiconducting nature with energy gaps of 0.42 eV, 0.25 eV and 0.22 eV for CeFe4P12, CeFe4As12, and CeOs4P12, respectively. The phonon dispersion curve displayed the forbidden gap between the optical and acoustic modes in CeFe4P12 and CeOs4P12. The analysis of n-type and p-type doping on pure alloys suggest enhanced thermoelectric behavior in p-type doping on pure alloys and hence the addition of Nd at the central cage atomic site generates flat and dense bands at EF and also opens an optical band gap in doped CeOs4P12. Moreover, the Nd atom introduces strong phonon scattering and hence reduces the lattice thermal conductivity (KL) substantially from 6.79 W m-1 K-1 to 3.47 W m-1 K-1 for CeFe4P12, 3.63 W m-1 K-1 to 1.97 W m-1 K-1 for CeFe4As12 and 6.43 W m-1 K-1 to 2.58 W m-1 K-1 for CeOs4P12 at room temperature. A considerably amplified figure of merit has been observed for the doped sample materials with the highest value of 0.72 at 800 K for doped CeFe4P12 with the highest Seebeck coefficient of 215.51 µV K-1.

Investigation of mechanical, thermodynamical, dynamical and electronic properties of RuYAs (Y=Cr and Fe) alloys.

Investigation of structural, dynamical, mechanical, electronic and thermodynamic properties of RuYAs (Y= Cr and Fe) alloys have been performed from the first principle calculations. Among the three structural phases, 'α' phase is found to be energetically favorable for both the RuCrAs and RuFeAs compounds. The computed cohesive energies and phonon dispersion spectra indicate the structural and dynamical stabilities of both the compounds. Mechanical stability of these compounds are studied using elastic constants. The Pugh's ratio predicts RuFeAs to be more ductile than RuCrAs. The RuCrAs alloy, on the other hand, is found to be a stiffer, harder and highly rigid crystal with stronger bonding forces than the RuFeAs. Furthermore, the thermodynamical properties have also been estimated with respect to the temperature under different pressures using the quasi-harmonic Debye model. In order to account for the effect of the highly correlateddtransition elements in the system we incorporated the GGA +Uapproximations. Within the GGA +Uapproach, the electronic structure reveals the half-metallicity for both compounds, which follows the Slater-Pauling rule. The charge density and electron localized function reflect the covalent bonding among the constituent atoms. Bader analysis reveals that the charge transfer takes place from Cr/Fe to Ru and As atoms in both approximations. Both Raman and infrared active modes have been identified in the compounds.