(a) Side view of the stacking structure. Atomic position indices a, b and c: in-plane positions of O ions and Ga ions in the GaO6 unit; α, β and γ: Ga ions in the GaO4 unit. (b) In-plane structure of octahedral Ga ions at the c positions on the underlying O ions at the a positions. The shortest unit translation vector, , and the second shortest vectors, and , are shown. (c) Possible dissociation of dislocations with Burgers vectors of and shown in (b). VGa represents the vacancy site in the Ga layer in the GaO6 unit.