Optical spectroscopy of Cr4+-doped Ca2GeO4 and Mg2SiO4.

Abstract

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 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 540 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{Ca}}_{2}$${\mathrm{GeO}}_{4}$ and 10 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 560 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{Mg}}_{2}$${\mathrm{SiO}}_{4}$, respectively. The $^{3}$${\mathit{A}}_{2}$${\ensuremath{\rightarrow}}^{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. \textcopyright{} 1996 The American Physical Society.