Sol-Gel Synthesis of Chromium-Doped Forsterite

Abstract

Pale purple, polycrystalline, chromium-doped forsterite powder was prepared at low temperature from a magnesium silicate sol. The chromium dopant, introduced as either chromium(II) acetate or chromium(VI) oxide, was combined in solution to maximize homogeneity. Electronic absorption spectroscopy indicated dopant ions, CrIII and CrVI, were intimately bound to the magnesium silicate network. At a Cr/Si mole ratio of 0.04 or less, the dried xerogel formed only a single phase, forsterite, after calcination in humid air to 560 °C, and was fully crystalline above 800 °C. The residual carbon content of the xerogel was decreased by the use of humidified air during calcination. FTIR spectroscopy indicated that the dopant occupied both octahedral and tetrahedral sites. In the first attempts to grow single crystals of chromium-doped forsterite, coarse-grained samples were obtained by melting and solidification of the polycrystalline powder under an oxygen atmosphere by the crucible-free, floating zone method. Observation of a broad ESR peak for Cr4+ at room temperature and of typical near IR emission spectra indicated that the Cr4+/Cr3+ ratio in the melt grown samples was higher than that of commercial single crystals, grown under a more reducing atmosphere. © 1993, American Chemical Society. All rights reserved.