FIGURE 1. View along the [001] direction of the relaxed structures from the ab initio calculations. The two types of layers are indicated by light and dark shading of the SiO₆-octahedra in each layer. Magnesium atoms are shown as black circles and unit cells are indicated by thin solid lines. (a) 1 x 1 (perovskite); (b) 2 x 1; (c) 3 x 1; (d) 4 x 1; (e) 5 x 1; (f) 2 x 2; (g) 3 x 3; (h) x 0 (CaIrO₃). We also give the calculated fractional coordinates of the 2 x 1 and the 3 x 1 type structures as they are used to model the observed diffraction patterns.

2 x 1: Mg: 0.057, , 0.538; 0.269, , 0.083; 0.730, , 0.917; Si: 0, 0, 0; 0.330, 0.502, 0.555; 0.380, , 0.820; O: 0.792, 0.560, 0.140; 0.207, 0.060, 0.860; 0.306, , 0.429; 0.458, 0.555, 0.243; 0.644, , 0.313.

3 x 1: Mg: 0.042, , 0.514; 0.303, , 0.073; 0.559, , 0.989; 0.799, , 0.895; Si: 0, 0, 0; , 0, ; 0.247, 0.501, 0.583; O: 0.985, , 0.111; 0.230, , 0.455; 0.480, , 0.368; 0.7352, , 0.287; 0.845, 0.559, 0.125; 0.906, 0.443, 0.647; 0.343, 0.556, 0.283; 0.407, 0.444, 0.801.