Iron and Spinel Precipitation in Iron‐Doped Sapphire

Single crystals of Al 2 O 3 containing 0.5 wt% Fe were exposed to low p o 2 atmospheres at 1500°C to produce precipitate phases. Analytical TEM identified the precipitate phases as spinel (hercynite) and iron, with the following orientation relationships: 〈001〉 Fe ‖〈2 2 01〉 s with {1 1 0} Fe ‖{11 2 0} s , 〈111〉 Fe ‖〈10 1 0〉 s with {0 1 1} Fe ‖ (0001) s , and 〈1 1 0〉 H ‖〈01 1 0〉 s with {111} H ‖ (0001) s . The three phase fields observed — (Fe, Al) 2 O 3 , spinel +α‐Al 2 O 3 , and iron +αAl 2 O 3 — are in accordance with phase stability diagrams. Precipitation kinetics indicate that oxygen is mobile in the reduced region of the crystal.