Mullites with Cr contents up to about 12 wt% (6.5 mol%) Cr2O3 and with Cr plus Fe contents up to about 8 wt% (4.5 mol%) Cr2O3 and 8 wt% (4.5 mol%) Fe2O3 were synthesized from oxide powder mixtures by means of solid-state sintering. Microchemical analyses, X-ray powder diffractometry, and electron paramagnetic resonance spectroscopy were used to characterize the structural mechanisms of Cr and Cr plus Fe doping. The studies yielded evidence for substitution of Cr3+ for Al3+ in the Al(1)O6 octahedra of the structure and for Cr3+ incorporation at octahedrally coordinated interstices, respectively. Although the solubility of trivalent cations in mullite generally is limited to about 6.5 oxide mol%, Cr-rich mullites annealed with Fe-rich silicate melts contain up to about 9 mol% (Cr,Fe)2O3. This extension of the substitution limit is explained by part of the Cr3+ ions entering interstitial sites, leaving space for Fe3+ to enter the octahedra.

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