Crystal structure and its role in electrical properties of the perovskite CaPbO₃ synthesized at high pressure

The orthorhombic modification of CaPbO₃ was synthesized from a mixture of Ca₂PbO₄ and PbO₂ at high temperature and high pressure. Its structure was analyzed by Rietveld analysis of neutron diffraction data on the basis of space group Pbnm. It has a distorted perovskite structure of the GdFeO₃ type and a unit cell with dimensions of a = 5.6710 Å, b = 5.8875 Å, and c = 8.1495 Å. The Pb-O bond lengths in each PbO₆ octahedron are comparable to each other, whereas the PbO₆ octahedron tilts around [110]_p and [001]_p axes (p: perovskite subcell) by 18.50° and 20.28°, respectively. These tilt angles, which show great structural distortion in CaPbO₃ containing the smaller Ca²⁺ ion, are much larger than corresponding ones in crystal chemically isotypic SrPbO₃. The electric resistivity of CaPbO₃ at room temperature was as high as 3 × 10³ Ω·cm, which is in sharp contrast to low resistivities observed in other perovskite-type oxides BaPbO₃ and SrPbO₃. The high resistivity of CaPbO₃ is explained as gap formation between 2p(O) nonbonding and 6s(Pb)-2p(O) spσ antibonding bands, which overlap with each other in BaPbO₃. Solid solutions, where Sr²⁺, La³⁺, Nd³⁺, and Y³⁺ ions were partially substituted for Ca²⁺ ions, were also prepared to examine structural and electrical properties in perovskites based on CaPbO₃. Substitution of Sr²⁺ for Ca²⁺ led to reductions in the distortion and gap energy, whereas that of La³⁺, Nd³⁺, and Y³⁺ induced metallic conductivity owing to doping of electron carriers into the antibonding band.