γ-Al₂O₃ formation from pulsed-laser irradiated sapphire

High-resolution transmission electron microscopy was used to examine the effect of laser irradiation of sapphire (single crystalline α-Al ₂O₃). It was observed that laser-melted material resolidified as γ-Al₂O₃ epitactically grown on the (0001) plane of α-Al₂O₃ having an orientation relationship (0001)_α//(111)_γ and [011̄0]_α//[11̄0]_γ. The interface between unmelted α-Al₂O₃ and resolidified γ-Al₂O₃ is atomically flat with steps of one to a few close-packed oxygen layers. The high thermal gradient in the α-Al ₂O₃ did not produce any interfacial dislocations. However, pronounced lattice distortions existed in the resolidified γ-Al ₂O₃. It is argued that the very rapid cooling rate present during solidification prevents the ordering of Al atoms that is required for the formation of the stable α-Al₂O₃ phase.