An investigation of phase stability in the Y2O3-Al2O3 system

The stability of the three known phases in the Y₂O₃-Al₂O₃ pseudo-binary system has been investigated. YAlO₃ (YAP) and Y₄Al₂O₉ (YAM) decompose at elevated temperatures, the products of the reaction being the third compound Y₃Al₅O₁₂ (YAG) and an unknown phase (designated X). The decomposition is most evident in powders but can also be initiated on the surface of bulk single crystals. X-ray diffraction studies have been performed in an attempt to identify the structure and composition of the unknown phase. The thermal decomposition has been found to be surface controlled and an optical and scanning electron microscope study of the associated morphological changes in YAlO₃ indicates that the reaction involves localized surface melting, probably with the loss of oxygen which effectively moves the composition off the binary join.