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 °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.