Ab initio study of structural, elastic, electronic, optical and thermoelectric properties of cubic Ge-based fluoroperovskites AGeF3 (A = K, Rb and Fr)

Structural, elastic, electronic, optical and thermoelectric properties of fluoroperovskites AGeF(3) (A = K, Rb, and Fr) have been investigated using Density Functional Theory (DFT) implemented in Quantum Espresso code with GGA-PBE functional. Optimized lattice constants are 4.45 angstrom, 4.49 angstrom, and 4.60 angstrom for KGeF3, RbGeF3, and FrGeF3, respectively. The three compounds show semiconductor behavior and have direct energy bandgap (R -> R). Energy gaps are tuned to slightly higher values as K is replaced by Rb and Fr, with energy gaps 1.98 eV, 2.02 eV, and 2.14 eV for the respective compounds. Investigation of their elastic properties reveals that the compounds are mechanically stable and ductile. The calculations of optical properties such as refractive index eta(omega), extinction coefficient k(omega), reflectivity alpha(omega), and absorption coefficient a(w) have all been performed using the dielectric function. The materials show very high absorption similar to 105cm(-1) and low reflection in the ultraviolet region, therefore, these materials are highly suitable for optoelectronic applications. Furthermore, these materials are crucial for thermoelectric applications due to their high electronic figure of merit (ZT) values.