Synthesis and crystallization of yttrium-aluminium garnet and related compounds

Amorphous oxide combustion products with compositions corresponding to Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ were synthesized by the glycine-nitrate process and heat-treated to induce crystallization. The crystalline structure of the resulting powders was determined by powder X-ray diffraction techniques. The phase stabilities of the crystalline phases were investigated as functions of the glycine-to-nitrate ratio, the yttrium-to-aluminium ratio, and the heat-treatment conditions. Heat treatment for short durations resulted in incompletely crystalline powders that consisted of a mixture of Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ phases, regardless of the chemical composition of the amorphous combustion product. However, heat treatment for longer durations or higher temperature generated both pure-phase, monoclinic Y₄Al₂O₉ and Y₃Al₅O₁₂ with the garnet structure. Prolonged heat treatment at high temperature failed to generate pure-phase orthorhombic YAlO₃. Subsequent analysis revealed a sluggish, complex crystallization process involving the formation and decomposition of several phases.