High-pressure synthesis is a powerful technique in searching for new materials. We have found a large number of functional oxides using this technique. BiNiO
3 is a distorted perovskite compound with unusual Bi
1/23+Bi
1/25+Ni
2+O
3 valence state. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi
3+Ni
3+O
3. The same inter metallic charge transfer accompanied with 2.6% volume shrinkage is induced by heating Bi
0.95La
0.05NiO
3 at ambient pressure.
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