The structural, mechanical, thermal, electronic and optical properties of halide perovskites Cs2TiX6 (X = Cl, Br, I): First-principles investigations

The structural, mechanical, thermal, electronic and optical properties of inorganic Pb-free double perovskites Cs2TiX6 (X = Cl, Br, I) compounds are investigated. Using the first-principles with full-potential linearized augmented plane wave (FP-LAPW) method, supported the density functional theory, and therefore the gener-alized gradient approximation (GGA-PBE) and Hybrid exchange-correlation functionals with and without spin -orbit coupling. The computation of the lattice constants showed an honest agreement between our results and the experimental results and the theoretical data available. Using the three independent elastic constants C11, C12 and C44, various mechanical and thermal properties are obtained such as: bulk modules B, shear modules G, Young's modules Y, Pough's ratio B/G, Frantesvich ratio G/B, Poisson's ratio upsilon, Shear anisotropy character A and Debye temperature Theta D for Cs2TiI6, Cs2TiBr6 and Cs2TiCl6 respectively. Our calculated elastic constant results show that Cs2TiI6, Cs2TiBr6 and Cs2TiCl6 are mechanically stable, anisotropic and ductile in nature. Furthermore, the Cs2TiCl6 compound has the highest Debye temperature, which indicates the highest melting temperature. The electronic band structures reveal the semiconductor conductivity of the three compounds, with a band gap suitable for photovoltaic applications, within the range of 1.5-2.96 eV. Finally, Cs2TiCl6 (X= Cl, Br, I) HPs compounds exhibit excellent optical properties like optical absorption, conductivity and refraction index.