First-principles study of lattice dynamics and thermodynamic properties of LiInX 2 (X = S, Se, Te)

Abstract

First-principles calculations of lattice dynamics and thermodynamic properties of orthorhombic LiInS 2 and LiInSe 2 and chalcopyrite LiInTe 2 have been performed within density functional perturbation theory using norm conserving pseudopotentials. Theoretical values of phonon mode frequencies are in good agreement with the experimental data available for these crystals obtained by methods of Raman spectroscopy and infrared one. In the whole frequency range a significant decrease of the vibrational frequencies is observed going from LiInS 2 to LiInSe 2 and LiInTe 2, which is a consequence of the anion radius increase. The lattice vibrations of In–X (X = S, Se, Te) bonds are mainly located in the low-frequency and mid-frequency ranges, and the Li–X bond vibrations are dominated in the higher frequency range. The mixed covalent–ionic nature of the three compounds is manifested by Born effective charge data. The vibration patterns of orthorhombic LiInS 2 and chalcopyrite LiInTe 2 were discussed in detail. The temperature dependences of thermodynamic quantities (including the internal energy, free energy, heat capacity, entropy and the Debye temperature Θ D) of all the three compounds were also presented in this paper. It is proved that Debye stiffness increases from LiInTe 2 to LiInSe 2 and LiInS 2.