First principles study of the structural, mechanical and optical properties of Li6PS5X (X= Cl, I) argyrodite compounds

Phosphorus-containing argyrodite are now gaining high attention for energy storage devices. Here, the electronic, structural, optical and elastic properties of Li6PS5X (X = Cl, I) argyrodites by utilizing density functional theory (DFT) have been studied. Trans-Blaha approach of the modified Becke–Johnson (TB-mBJ) potential is used to get accurate band gap energy values where the (PBEsol-GGA) are used for the calculations of the structural parameters of Li6PS5Cl and Li6PS5I compounds with lattice parameters of 10.31 Å and 10.36 Å respectively. The mBJ-functional shows energy bandgap of Li6PS5Cl to be 3.11 eV and for Li6PS5I with band gap of 2.81 eV. The static refractive index for Li6PS5Cl and Li6PS5I are 2.23 and 1.90. Elastic constants of the cubic compounds C11, C12 and C44 and different moduli like Young modulus, Shear moduli alongwith different parameters like Kleinman's parameters, first and second Lame constants, Hardness, Melting temperature, Chung-Buessem anisotropy index, universal anisotropic index, acoustic behavior and its anisotropy, Poisson ratios. Mechanically the compounds determined as brittle. The current study has reported many new results on the argyrodite compounds which maybe interesting for future experimental studies and further theoretical exploration of the family of these compounds.