Elastic and Optoelectronic Properties of Cs2NaMCl6 (M = In, Tl, Sb, Bi)

Abstract

In this article, elpasolite perovskites, Cs2NaMCl6 (M = In, Tl, Sb, Bi), are investigated using density functional theory (DFT). Structural properties like lattice constants and bond lengths are in agreement with the available experimental data. Electronic properties are calculated by several DFT exchange-correlation approximations, and it is found that a modified Becke–Johnson (mBJ) approximation along with the inclusion of spin orbit coupling (SOC) gives the most promising results. The M-site cation decides the nature of the band gap; i.e. direct band gaps are obtained for group IIIA elements (In, Tl), and indirect band gaps are experiential for group VA elements (Sb, Bi). Narrow discrete energy bands are observed in the valence and conduction bands, which make these compounds suitable for scintillation applications. SOC induces splitting of Bi/Sb p orbitals in the conduction band and reduces the band gaps of these double perovskite halides. Obtained values of mechanical parameters confirm that these compounds are ductile and anisotropic. Optical properties, i.e. dielectric functions, energy loss function and refractive index, are also calculated, and interesting variations are found which can play a important role in scintillation and other optoelectronic applications of these materials.