Bismuth and lead are main group elements, but these have charge degree of freedom stemming from 6s² (Bi³⁺ and Pb²⁺) and 6s⁰ (Bi⁵⁺ and Pb⁴⁺) electronic configurations. Stereochemical 6s² lone pairs of Bi³⁺ and Pb²⁺ induce polar distortions as typically observed in PbTiO₃ and BiFeO₃. These are coupled with charge variation and magnetism of transition metals resulting in various functionalities. 

From left to right in the periodic table, BiCrO₃ to BiCoO₃ are all Bi³⁺M³⁺O₃. However, BiNiO₃ has an unusual Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ charge distribution. An intermetallic charge transfer between Bi⁵⁺ and Ni²⁺ takes place under pressure leading to the Bi³⁺Ni³⁺O₃ high-pressure phase. BiNiO₃ decomposes on heating at 500 K, but La substitution for Bi or Fe substitution for Ni destabilizes the Bi charge disproportionation and (Bi,La)³⁺(Ni,Fe)³⁺O₃ appears on heating at an ambient pressure. Because of the contraction of Ni-O bond owing to the oxidation of Ni²⁺ to Ni³⁺, negative thermal expansion, shrinkage of volume on heating, is observed.

Similar charge distribution changes are observed three times in PbMO₃. PbVO₃ is Pb²⁺V⁴⁺O₃ like Pb²⁺Ti⁴⁺O₃, but PbCrO₃, PbMnO₃ and PbFeO₃ were found to be Pb²⁺_0.5Pb⁴⁺_0.5M³⁺O₃. PbCoO₃ has turned out to be Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂. PbNiO₃ has a valence distribution of Pb⁴⁺Ni²⁺O₃. Namely, PbMO₃ changes from Pb²⁺M⁴⁺O₃ to Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ (average valence state of Pb³⁺M³⁺O₃) to Pb²⁺_0.25Pb⁴⁺_0.75Co²⁺_0.5Co³⁺_0.5O₃ (Pb^3.5+Co^2.5+O₃) and to Pb⁴⁺M⁴⁺O₃ according to the order in the periodic table and the depth of d level.

Among these compounds, BiCoO₃ and PbVO₃ have PbTiO₃-type polar structures with enhanced c/a ratios exceeding 1.2 because of the stereochemical activities of 6s² one pairs of Bi³⁺ and Pb²⁺ and d_xy orbital ordering of Co³⁺ and V⁴⁺. Giant negative thermal expansions in PbVO₃ derivatives will also be discussed.