Corundum-to-spinel structural phase transformation in alumina

Several polymorphs exist in alumina (Al2O3), and they transform to a stable α-phase with a hexagonal corundum structure on thermal annealing. This structural change is irreversible as a function of temperature, and transformation of corundum to another metastable crystalline phase has never been observed by heat treatments. In this study, we irradiated single crystals of Al2O3 with Zr ions and obtained an irradiated microstructure consisting of a buried α-Al2O3 layer surrounded on top and bottom by layers of a defect cubic spinel Al2O3 phase. We examined the thermal stability of this microstructure using transmission electron microscopy and X-ray diffraction. We found that the corundum phase completely transforms to the spinel phase following annealing at 1173 K for 1 h: the thermodynamically stable phase transforms to the metastable phase by heat treatments. We discuss this unusual structural change within the context of our results as well as previous observations.