Figure 3.

(a) z coordinate of oxygen for O3- (z_O3) and P2-type (z_P2) structure against a-axis lattice constant (a). Blue and red circles represent z_O3 at 300 and 700 K, respectively. Light green and pink triangles represent z_P2 at 300 and 700 K, respectively. The broken lines represent z calculated by the hard sphere model with minimum Madelung energy for O3 and P2-type compounds ($z_{\mathrm{O}3}^{\mathrm{calc}}$ and $z_{\mathrm{P}2}^{\mathrm{calc}}$) against a, respectively. (b) The ratio of interlayer distance (d) to a against a. Light blue and purple circles represent d/a of O3-type compounds at 300 and 700 K, respectively. Green and yellow triangles represent d/a of P2-type compounds at 300 and 700 K, respectively. The broken line represents d/a calculated by the hard sphere model with minimum Madelung energy (d^calc/a^calc) against a. The inset of Fig. 3(b) shows a schematic view of the local atomic configuration around M.