SYNTHESIS AND CRYSTALLIZATION OF YTTRIUM-ALUMINUM-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 st ructure of the resulting powders was determined by powder X-ray diffra ction techniques. The phase stabilities of the crystalline phases were investigated as functions of the glycine-to-nitrate ratio, the yttriu m-to-aluminium ratio, and the heat-treatment conditions. Heat treatmen t for short durations resulted in incompletely crystalline powders tha t consisted of a mixture of Y4Al2O9, YAlO3, and Y3Al5O12 phases, regar dless of the chemical composition of the amorphous combustion product. However, heat treatment for longer durations or higher temperature ge nerated both pure-phase, monoclinic Y4Al2O9 and Y3Al5O12 with the garn et structure. Prolonged heat treatment at high temperature failed to g enerate pure-phase orthorhombic YAlO3. Subsequent analysis revealed a sluggish, complex crystallization process involving the formation and decomposition of several phases.