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, CrMg2SiO4, at 1.2 mu m has been att ributed to the presence of Cr4+. The goal of this work was to determin e the Cr4+/Cr3+ ratios Of crystals grown at different oxygen partial p ressures and to compare them with a model based on point defect thermo dynamics. Cr:forsterite crystals were, grown by the floating zone tech nique 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 Cr4+/C r3+ ratios, as well as the distribution of the Cr3+ cations on the M1 and M2 sites, were determined by electron paramagnetic resonance techn iques. The fraction of Cr present as Cr4+ ions as a function of the ox ygen activity of the growth environment was well described by the mode l.