Preferred orientation of NiAl2O4 spinel grown on sapphire

The orientation relationships between the spinel nickel aluminate (NiAl₂O₄) and the aluminum oxide single crystal from which it is grown have been investigated. A variety of sapphire surface orientations were used. The nickel source was primarily nickel vapor, but nickel metal was also used. The large grained spinel layer growing in contact with the sapphire is highly textured, and a thinner fine grained layer on its outer surface is only lightly textured. Two principal textures were observed: for sapphire surfaces oriented near the basal plane, the {111} spinel is parallel to the (0001) sapphire; for surfaces oriented away from the basal plane the {111} spinel is parallel to the {1120} sapphire. In both cases the directions 〈110〉 spinel and 〈1010〉 sapphire are aligned. Considerable twinning and several secondary textures have also been seen.