New and Unforeseen Crystal Growth Processes for a Metal Oxide

The synthesis of corundum (α-Al₂O₃) via a layered Al₂O₃-MoO₃ system was directly observed for the first time. This revealed a new crystal growth process with three key features: (1) the formation of an Al₂(MoO₄)₃ intermediate layer through a solid-solid interaction in the temperature range of ∼705-860 °C; (2) the melting of the Al₂(MoO₄)₃ layer between approximately 870 and 890 °C; and (3) the decomposition of Al₂(MoO₄)₃ to corundum between 950 and 1100 °C. This molten intermediate decomposition (MIND) mechanism produced corundum, which was light bluish-gray in color and was defined in CIE (L* a* b*) color space as L* = 76.65, a* = -1.09, and b* = -6.20. The reagents used in this study were the same as those used in MoO₃ flux growth studies on the synthesis of corundum, therefore demonstrating that the previous work only gave a superficial treatment of the mechanism of formation.