(a) Crystal structure of simple perovskite ABO₃ with a Pm-3m space group. Atomic sites: A 1a (0, 0, 0), B 1b (0.5, 0.5, 0.5), O 3c (0, 0.5, 0.5); (b) crystal structure of B-site ordered double perovskite A₂BB′O₆ with an Fm-3m space group. Atomic sites: A 8c (0.25, 0.25, 0.25), B 4a (0, 0, 0), B′ 4b (0.5, 0.5, 0.5), O 24e (x, 0, 0); (c) crystal structure of A-site ordered quadruple perovskite with an Im-3 space group. Atomic sites: A 2a (0, 0, 0), A′ 6b (0, 0.5, 0.5), B 8c (0.25, 0.25, 0.25), O 24g (x, y, 0).