fig2

Figure 2. (A) Schematic phase diagram for RNiO₃ as functions of temperature T and pressure P. PI, AFI, PM, and CD represent paramagnetic insulating, antiferromagnetic insulating, paramagnetic metallic, and charge disproportionated phases. (B) Distortions of the NiO₆ octahedron and the electronic configurations for the charge disproportionated Ni²⁺ and Ni⁴⁺ sites without oxygen 2p holes. The yellow and red circles indicate oxygen and transition metal ions, respectively. (C) Energy per unit cell for ferromagnetic and antiferromagnetic (A, G, and C type) states calculated by mean-filed approximation for the perovskite-type d-p model with d⁷ as a function of breathing distortion (the ratio between the long and average Ni-O bond length). Without the distortion, each site takes the low spin d⁷ state with S = 1/2. With the distortion, the expanded site becomes d⁸ (S = 1) and the compressed site becomes d⁶ (S = 0). (D) Electronic configuration of d⁸L. (E) Electronic configuration of d⁸L².