Varied roles of Pb in transition-metal PbMO(3) perovskites (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) [Image: see text] a covalent Pb–Ru bond is formed by Pb(2+) + Ru(IV) = Pb(4+) + Ru(II) electron sharing.