Influence of chemical bond length changes on the crystal field strength and “ligand–metal” charge transfer transitions in Cs2GeF6 doped with Mn4+ and Os4+ ions

Detailed study of dependence of the crystal field strength 10Dq and lowest charge transfer (CT) energies for different interionic distances in Cs2GeF6:Mn4+ and Cs2GeF6:Os4+crystals is presented. The calculations were performed using the first-principles discrete-variational Dirac–Slater (DV-DS) method. As a result, the functional dependencies of 10Dq and lowest CT energy on the metal–ligand distance R were obtained without any fitting or semiempirical parameters. It was shown that 10Dq depends on R as 1/Rn, with n=4.0612 and 4.3874 for Cs2GeF6:Mn4+ and Cs2GeF6:Os4+, respectively. Two approximations (linear and quadratic) are obtained for the dependence of the lowest CT energy on R; CT energy decreases when R increases with dE(CT)/dR=−638 and −1080 cm−1/pm for Cs2GeF6:Mn4+ and Cs2GeF6:Os4+, respectively, if the linear approximation is used. These values can be used for estimations of the lowest CT energies for Mn4+ and Os4+ ions in other hosts with fluorine ligands. Estimations of the electron-vibrational interaction (EVI) constants, Huang–Rhys parameters, and Stokes shifts for all the above-mentioned crystals were performed using the obtained 10Dq and E(CT) functions.