SOL-GEL SYNTHESIS OF CHROMIUM-DOPED FORSTERITE

Pale purple, polycrystalline, chromium-doped forsterite powder was pre pared at low temperature from a magnesium silicate sol. The chromium d opant, introduced as either chromium(II) acetate or chromium(VI) oxide , was combined in solution to maximize homogeneity. Electronic absorpt ion spectroscopy indicated dopant ions, Cr(III) and Cr(VI), were intim ately bound to the magnesium silicate network. At a Cr/Si mole ratio o f 0.04 or less, the dried xerogel formed only a single phase, forsteri te, after calcination in humid air to 560-degrees-C, and was fully cry stalline above 800-degrees-C. The residual carbon content of the xerog el was decreased by the use of humidified air during calcination. FTIR spectroscopy indicated that the dopant occupied both octahedral and t etrahedral sites. In the first attempts to grow single crystals of chr omium-doped forsterite, coarse-grained samples were obtained by meltin g and solidification of the polycrystalline powder under an oxygen atm osphere by the crucible-free, floating zone method. Observation of a b road ESR peak for Cr4+ at room temperature and of typical near IR emis sion spectra indicated that the Cr4+/Cr3+ ratio in the melt grown samp les was higher than that of commercial single crystals, grown under a more reducing atmosphere.