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

Abstract

The lasing of Cr-doped forsterite, Cr:Mg 2SiO 4, at 1.2 μm has been attributed to the presence of Cr 4+. The goal of this work was to determine the Cr 4+ Cr 3+ 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% O 2, 80% Ar + 20% O 2, 33% Ar + 67% O 2, and 100% O 2 at 1 atm total pressure. The Cr 4+ Cr 3+ ratios, as well as the distribution of the Cr 3+ cations on the M1 and M2 sites, were determined by electron paramagnetic resonance techniques. The fraction of Cr present as Cr 4+ ions as a function of the oxygen activity of the growth environment was well described by the model.