New and Unforeseen Crystal Growth Processes for a Metal Oxide.

The synthesis of corundum (α-Al 2 O 3 ) via a layered Al 2 O 3 -MoO 3 system was directly observed for the first time. This revealed a new crystal growth process with three key features: (1) the formation of an Al 2 (MoO 4 ) 3 intermediate layer through a solid-solid interaction in the temperature range of ∼705-860 °C; (2) the melting of the Al 2 (MoO 4 ) 3 layer between approximately 870 and 890 °C; and (3) the decomposition of Al 2 (MoO 4 ) 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 MoO 3 flux growth studies on the synthesis of corundum, therefore demonstrating that the previous work only gave a superficial treatment of the mechanism of formation.