Dislocation Structures in Single‐Crystal Al 2 O 3

Chemical polishing and etching techniques were used to reveal the dislocation structures of sapphire and ruby crystals grown by the flame‐fusion and flux techniques. The average density of edge dislocations lying in prism planes was 3.0 × 10 5 per cm 2 , which could be changed only slightly by chromium additions and annealing at 2000°C. An average basal dislocation density of 2 × 10 5 per cm 2 decreased 35 to 80% on annealing. Crystal orientation (i.e., angle between the c axis and the growth axis) showed no effect on dislocation density but a pronounced effect on subboundary arrangement and density. The substructure of 0° crystals was more complex than that of 90° crystals; 60° crystals possessed a structure intermediate between 0° and 90°. Principal observations included (1) prismatic and basal slip on all as‐grown crystals; (2) profuse basal slip, readily polygonized on annealing; (3) dislocation densities of flux crystals lower than those of Verneuil crystals; and (4) a rare form of basal twinning, composition plane , on all flux crystals.