First-Principles Calculations of Pressure Effects on the Structural, Electronic, Elastic, and Thermodynamic Properties of Rb₂MF₆ (M = Si, Ni, Pd) Crystals

Three double perovskite crystals Rb₂MF₆ (M = Si, Ni, Pd) are studied using the first-principles methods. Their structural, electronic, elastic, and thermodynamic properties are calculated; effects of external hydrostatic pressure in the range from 0 to 10 GPa are analyzed. A remarkable feature of all studied compounds is the complicated structure of the valence bands, which consist of several sub-bands separated by gaps that may produce intraband luminescence. Dependencies of the band gaps, unit cell volumes, and chemical bond lengths on pressure are calculated. It is observed that the Rb–F bonds are about 20 times more compressible than the M–F bonds. The influence of partially filled 3d orbitals of Ni in Rb₂NiF₆ and 4d orbitals of Pd in Rb₂PdF₆ on the electronic properties of these materials is analyzed by calculating the crystal field strength parameter 10Dq at varying hydrostatic pressure. Estimations of the Debye temperature from the calculated elastic constants yield the values of 198 K (Rb₂SiF₆), 199 K (Rb₂NiF₆), and 180 K (Rb₂PdF₆). The results of this investigation highlight the importance of systematic first-principles calculations of physical properties for isostructural compounds and expands the existing databases of physical properties of double perovskite crystals.