A (GGA+PBE) investigation of MGeBr3 (M = Rb, Cs, Fr) bromide perovskites: structural, electronic, and optical characteristics

First-principles DFT calculations by utilizing FP-LAPW under GGA+PBE method are performed to investigate the structural, electronic and optical characteristics of bromide perovskites MGeBr3 (M = Rb, Cs, Fr). It is found that the cubic structure (Pm-3m) and optimized lattice constants are in good agreement with the previous data. Our GGA+PBE results reveal that MGeBr3 show nonmagnetic semiconductor behavior with direct bandgap (Eg = 0.925 eV (M = Rb), 0.898 eV (M = Cs), 0.952 eV (M = Fr)) along the L–L symmetry direction. Formation energy, octahedral ration and tolerance factor for MGeBr3 have also been calculated. The 2-D charge densities confirm that the chemical bonds (Ge2+–Br- ) and (M+–Br- ) follow the covalent and ionic bonding types. Moreover, we have calculated and discussed the optical parameters, dielectric constants, absorption, conductivity and refractivity. The calculated electronic and optical properties show the narrow band-gap, high absorption and semiconductor nature making these inorganic materials suitable for optoelectronics applications.