Varied roles of Pb in transition-metal PbMO3 perovskites (M = Ti,V, Cr,Mn, Fe,Ni, Ru)

Different structural chemistries resulting from the Pb²⁺ lone-pair electrons in the PbMO₃ perovskites are reviewed. The Pb²⁺ lone-pair electrons enhance the ferroelectric transition temperature in PbTiO₃, stabilize vanadyl formation in PbVO₃, and induce a disproportionation reaction of Cr^IV in PbCrO₃. A Pb²⁺ + Ni^IV = Pb⁴⁺ + Ni^II reaction in PbNiO₃ stabilizes the LiNbO₃ structure at ambient pressure, but an A-site Pb⁴⁺ in an orthorhombic perovskite PbNiO₃ is stabilized at modest pressures at room temperature. In PbMnO₃, a ferroelectric displacement due to the lone pair electron effect is minimized by the spin–spin exchange interaction and the strong octahedral site preference of the Mn^IV/III cation. PbRuO₃ is converted under pressure from the defective pyrochlore to the orthorhombic (Pbnm) perovskite structure where Pb–Ru interactions via a common O −2p orbital stabilize at low temperature a metallic Imma phase at ambient pressure. Above P_c a covalent Pb–Ru bond is formed by Pb²⁺ + Ru^IV = Pb⁴⁺ + Ru^II electron sharing.