The stability, electronic, thermal, and optical properties of silver halide (AgX: X = F, Cl, Br, and I) semiconductors: Ab-initio study

Abstract

The structural properties, mechanical and dynamic stability, electronic, thermal and optical properties of silver halide AgX (X = F, Cl, Br, and I) compounds in the rock-salt phase within GGA and HSE approximations have been studied. The calculations are performed using the QUANTUM-ESPRESSO package which is based on the density functional theory (DFT) and the pseudo-potential method. The calculated elasticity parameters illustrated that the rock-salt phase complies with the mechanical stability conditions at ambient pressure. Based on the results of the mechanical calculations, it can be concluded that AgI is more rigid than other silver halide compounds, while the AgF compound is more malleable than other silver halide compounds. Also, against other examined silver halides, the AgI compound has an isotropic nature. The phonon scattering diagram has proved the dynamic stability of silver halide compounds in the studied rock-salt phase. Evaluation of the electronic results indicates that silver halide compounds in both GGA and HSE approximations have an indirect bandgap in the L−Γ path. In comparison to GGA, the HSE calculation improves the amount of bandgap toward a better agreement with experimental results. At room temperature, the hole mobility of silver halides is much less than the electron mobility. The decreasing trend of the effective mass of electrons in silver halides with the atomic radius of halide in the rock-salt phase leads to a faster carrier transfer rate in AgI, AgBr, AgCl, and AgF compounds, respectively. Moreover, by increasing the atomic radius of halide atom X, the Debye temperature θD and stiffness decrease, and inversely the heat capacity Cv increases. The increasing entropy relative to temperature refers to the endothermic properties of silver halide compounds. In both GGA and HSE approximations, the AgF compound has the highest absorption coefficient peak with the maximum optical absorption in the ultraviolet regime of the electromagnetic spectrum. The absorption coefficients of the silver halide compounds are of the order of 105 cm−1 which is comparable to the absorption coefficient of silicon crystals. The latter property along with a high stability and high electron mobility, makes silver halides attractive photocatalytic materials in the NaCl structure.