Optical spectroscopy of Cr4+-doped Ca2GeO4 and Mg2SiO4

Polarized single crystal absorption spectra and luminescence spectra of Cr4+-doped Ca2GeO4 and Mg2SiO4 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 cm(-1) and 540cm(-1) for Ca2GeO4 and 10 100 cm(-1) and 560 cm(-1) for Mg2SiO4, respectively. The (3)A(2)-->(1)A(1) transition appears as a Fano antiresonance in the spectrum of cm Cr4+-doped Mg2SiO4. The luminescence spectra are assigned to a transition from an excited triplet state, which is definitely not mixed with (1)E contributions in Ca2GeO4. The luminescence spectra are surprisingly sharp. Possible explanations are discussed. The luminescence of Cr4+-doped Ca2GeO4 is much less quenched at room temperature than in the Mg2SiO4 host. Finally, the spectroscopic properties of Cr4+-doped materials are compared with those of other 3d(2) ions in tetroxo coordination.