Oxygen activity dependence of the chromium (IV) population in chromium-doped forsterite crystals grown by the floating zone technique

The lasing of Cr-doped forsterite, Cr:Mg2SiO4, at 1.2 μm has been attributed to the presence of Cr4+. The goal of this work was to determine the Cr4+Cr3+ ratios of crystals grown at different oxygen partial pressures and to compare them with a model based on point defect thermodynamics. Cr:forsterite crystals were grown by the floating zone technique with gas atmospheres of 100% Ar, 99% Ar + 1% O2, 80% Ar + 20% O2, 33% Ar + 67% O2, and 100% O2 at 1 atm total pressure. The Cr4+Cr3+ ratios, as well as the distribution of the Cr3+ cations on the M1 and M2 sites, were determined by electron paramagnetic resonance techniques. The fraction of Cr present as Cr4+ ions as a function of the oxygen activity of the growth environment was well described by the model.