TlInGe2S6, A Prospective Nonlinear Optical Material: First-Principles DFT Calculations of the Electronic Structure and Optical Properties

Abstract

We report on results of first-principles density functional theory (DFT) calculations of the total density of states (DOS) and the partial densities of states of a quaternary TlInGe2S6 compound, a promising nonlinear optical material. The calculations are made using the augmented plane wave + local orbitals method employing the WIEN2k package. The DOS curves are calculated in different approaches for exchange-correlation potentials, and it has been established that the best coincidence with the experiment is achieved when modified Becke–Johnson exchange potential is used with correction parameter U and an account of the spin–orbit splitting effect. The present DFT calculations present that the S 3p states are the primary contributors to the TlInGe2S6 valence band, making the main input to its top and upper portion. The primary contributors to the central portion of the valence band of TlInGe2S6 are the Tl 6s and Ge 4p states, while its bottom is generated due to contributions of mainly the In 5s states with slightly smaller contributions of the Ge 4p states as well. The bottom of the conduction band is dominated by the unoccupied Ge 4s states. The conduction band minimum and the valence band maximum are located at the L point of the first Brillouin zone resulting in a direct band gap of the TlInGe2S6 compound. The key optical constants are elucidated for TlInGe2S6 based on the present DFT calculations.