Bi and Pb have a unique 6s⁰ and 6s² electron configuration that creates charge degrees of freedom. Due to the lack of this 6s¹ electron configuration, a property called valence skipper, Bi takes 3+ and 5+ and Pb takes 2+ and 4+. In particular, for perovskite compounds containing Bi or Pb at the A site, the valence state changes according to the depth of the d-orbitals of the transition metal ions corresponding to the order in the periodic table of the elements due to the close relationship between the 6s level of Pb or Bi and the 3d level of the 3d transition metal ions. For BiMO₃, M= Cr, Mn, Fe, Co, the state is Bi³⁺M³⁺O₃, while BiNiO₃ has a specific valence state of Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃. PbMO₃ has Pb²⁺_0.5Pb⁴⁺_0.5MO₃ for M= Ti and V, Pb²⁺_0.5Pb⁴⁺_0.5M³⁺O₃ for M= Cr and Fe, PbCoO₃ has Pb²⁺_0.25Pb⁴⁺_0.75Co²⁺_0.5Co³⁺_0.5O₃, PbNiO₃ has Pb⁴⁺Ni²⁺O₃. In BiNiO₃ and PbMO₃ (M = Cr, Fe and Co), Bi and Pb become charge disproportionated in the 6s⁰ and 6s² states, and temperature- and pressure-induced elimination of charge disproportionation and charge transfer phase transitions occur. These charge transfer phase transition causes leads to unique physical properties such as negative thermal expansion.