ИСТИНА

Tetravalent chromium doped forsterite Cr4+:Mg2SiO4 (Cr-Fo) single crystals are used as the active media of tunable and femtosecond solid-state lasers. Such lasers have great potential of wide application in various fields, including telecommunications, medicine, LIDARs, spectroscopy, photochemistry, etc. Some important drawbacks of Cr-Fo as laser crystal are caused by the presence of chromium in parasitic oxidation states in the crystal, e.g., Cr2+, and Cr3+ ions. Our study was aimed to test the possibility to diminish the contents of octahedrally coordinated Cr2+, and Cr3+ in the crystals, and to raise the contents of the useful tetrahedrally coordinated Cr4+ by usage of non-stoichiometric melts for the crystals growth, and by impact of a prolonged high-temperature oxidizing annealing of the as-grown crystals. We have successfully grown by Czochralski high-quality Cr-Fo single crystals from the melts containing 0.25 – 2 wt. % excess of MgO. Such excess should promote the formation of vacancies in Si4+ sub-lattice. This, in turn, should promote Cr4+ introduction into the crystal, and inhibit Cr2+, and Cr3+ introduction. The prolonged high-temperature oxidizing annealing of Cr4+:Mg2SiO4 should lead to the oxidation of Cr2+, and Cr3+ ions for Cr4+ and, possibly, for more oxidized chromium ions. The dependence of Cr4+, Cr2+, and Cr3+ concentrations in the studied crystals on the melt composition is given in the presentation, as well as the evolution dynamics of Cr4+, Cr2+, and Cr3+ concentrations in the Cr-Fo crystals, during their prolonged high-temperature oxidizing annealing. The concentrations of each chromium oxidation state were determined from the polarized optical absorption spectra of the crystals.