Varied roles of Pb in transition-metal PbMO₃perovskites (M= Ti, V, Cr, Mn, Fe, Ni, Ru)

Different structural chemistries resulting from the Pb 2+ lone-pair electrons in the PbMO 3 perovskites are reviewed. The Pb 2+ lone-pair electrons enhance the ferroelectric transition temperature in PbTiO 3 , stabilize vanadyl formation in PbVO 3 , and induce a disproportionation reaction of Cr IV in PbCrO 3 . A Pb 2+ + Ni IV = Pb 4+ + Ni II reaction in PbNiO 3 stabilizes the LiNbO 3 structure at ambient pressure, but an A-site Pb 4+ in an orthorhombic perovskite PbNiO 3 is stabilized at modest pressures at room temperature. In PbMnO 3 , 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. PbRuO 3 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 32 GPa, a covalent Pb-Ru bond is formed by Pb 2+ + Ru IV = Pb 4+ + Ru II electron sharing.