Cs2XF6 (X=Si,Ge) compounds: Common and different features as uncovered by the first-principles calculations

Results of ab initio calculations of the electronic, optical and elastic constants for Cs2GeF6 and Cs2SiF6 are reported for the first time. Both compounds are the direct band gap dielectrics, with the calculated band gaps 6.926 eV (Cs2SiF6) and 6.417 eV (Cs2GeF6). Analysis of the calculated elastic constants and Cauchy condition shows both crystals as slightly covalent compounds. However, with increased pressure chemical bonds in Cs2GeF6 turn to be more ionic, whereas in Cs2SiF6 the covalent character of chemical bonds is enhanced. Pressure dependence of the chemical bond lengths and lattice constants was determined and represented as the second power functions of external pressure. Different trends in the values of the Mulliken charges for both compounds were found. The obtained results are applicable to the analysis of the luminescence properties of impurity ions at varying pressure or to the microscopic studies of crystal field effects in these crystals.