IRON AND SPINEL PRECIPITATION IN IRON-DOPED SAPPHIRE

Single crystals of Al2O3 containing 0.5 wt% Fe were exposed to low po2 atmospheres at 1500-degrees-C to produce precipitate phases. Analytical TEM identified the precipitate phases as spinel (hercynite) and iron, with the following orientation relationships: [001]Fe parallel-to [22BAR01]s with {11BAR0}Fe parallel-to {112BAR0}s, [111]Fe parallel-to [101BAR0]s with {01BAR1}Fe parallel-to (0001)s, and [11BAR0]H parallel-to [0110]s with {111}H parallel-to (0001)s. The three phase fields observed- (Fe,Al)2O3, spinel + alpha-Al2O3, and iron + alpha-Al2O3-are in accordance with phase stability diagrams. Precipitation kinetics indicate that oxygen is mobile in the reduced region of the crystal.