Optical and thermoelectric properties of square lattice phases of alkali halide compounds

Abstract

The geometrical, vibrational, electronic, optical and thermoelectric properties of the 16 compounds of the I-VII square lattice monolayer structure such as s-MX (M = Li, Na, K, Rb and X = F, Cl, Br, I) are studied in a systematic manner through the first principles. First-principles computations using the density functional theory is applied to analyze the series of group I–VII materials. The frequency dependent phonon dispersion relations show that every member of the group I–VII under consideration are dynamically stable. All the sixteen dynamically stable alkali halides possess band gap ranging from 3.99 eV to 7.03 eV through the calculation from PBE functional, whereas using HSE06 functional, the energy bandgap are obtained in the range of 4.98 eV to 9.20 eV. We obtained the effective mass, deformation potential, relaxation time and mobility for electron and holes for all the considered compounds of group I-VII. The optical properties of the compounds I-VII are also examined in the context of parallel and perpendicular field polarizations. According to the current observation, square lattice phases exhibit anisotropic optical behavior with respect to the plane of polarizations (parallel and perpendicular). The lower the value of reflectivity and absorption coefficient, the greater the dominance in the partially visible and ultraviolet energy ranges, indicating that these materials are transparent in this range and hence suitable for anti-reflection coating. Finally, we computed the thermoelectric characteristics of each I-VII (s-MX) material with respect to temperature. All of the s-MX materials of LiI, KBr, KI, and RbCl display excellent thermoelectric properties over a broad temperature range of 300 K–800 K. The present study indicate that the alkali halides in square lattice phase (s-MX) appears to be prominent materials for future thermoelectric devices.