Polarized single crystal absorption spectra and luminescence spectra of ${\mathrm{Cr}}^{4+}$-doped ${\mathrm{Ca}}_2$${\mathrm{GeO}}_4$ and ${\mathrm{Mg}}_2$${\mathrm{SiO}}_4$ are presented and discussed. The absorption spectra are analyzed using the angular overlap model (AOM). The agreement between the experimental and the AOM-calculated energies of the ligand field states is satisfactory. The ligand field parameters 10Dq and B are 8950 ${\mathrm{cm}}^{\mathrm{-}1}$ and 540 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{Ca}}_2$${\mathrm{GeO}}_4$ and 10 100 ${\mathrm{cm}}^{\mathrm{-}1}$ and 560 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{Mg}}_2$${\mathrm{SiO}}_4$, respectively. The ${}_{}{}^3{}_{}{}^{}$${\mathit{A}}_2$${\to }^1$${\mathit{A}}_1$ transition appears as a Fano antiresonance in the spectrum of ${\mathrm{Cr}}^{4+}$-doped ${\mathrm{Mg}}_2$${\mathrm{SiO}}_4$. The luminescence spectra are assigned to a transition from an excited triplet state, which is definitely not mixed with ${}_{}{}^1{}_{}{}^{}$E contributions in ${\mathrm{Ca}}_2$${\mathrm{GeO}}_4$. The luminescence spectra are surprisingly sharp. Possible explanations are discussed. The luminescence of ${\mathrm{Cr}}^{4+}$-doped ${\mathrm{Ca}}_2$${\mathrm{GeO}}_4$ is much less quenched at room temperature than in the ${\mathrm{Mg}}_2$${\mathrm{SiO}}_4$ host. Finally, the spectroscopic properties of ${\mathrm{Cr}}^{4+}$-doped materials are compared with those of other 3${\mathit{d}}^2$ ions in tetroxo coordination. © 1996 The American Physical Society.

• Received 27 September 1995

DOI:https://doi.org/10.1103/PhysRevB.53.2367