Phase selection in precursor-derived yttrium aluminum garnet and related Al2O3-Y2O3 compositions

Al2O3-Y2O3 powders were synthesized in the range of 25-55% Y2O3 by reverse coprecipitation of nitrate Solutions. All starting powders were amorphous and formed primary yttrium aluminum garnet (YAG) upon crystallization. X-ray diffraction detected only garnet in compositions of 30-40% Y2O3 after heat treatment at 1250 degrees C. Compositions of 45-55% Y2O3 established a metastable YAG + Y4Al2O9 microstructure. The YAG phase field was metastably extended away from its stoichiometry, as indicated by a systematic increase in lattice parameter with Y2O3 content. Although some Al2O3 enrichment was achieved, YAG appears to tolerate greater off-stoichiometry on the Y2O3-rich side. Possible defect structures accommodating the solubility extension were examined. Phase selection results indicate that compositional inhomogeneity is not the only reason behind the appearance of hexagonal YAlO3, which is frequently reported during YAG synthesis.