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 p and 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.
ASJC Scopus subject areas
- 化学 (全般)