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 C a2PbO4 and PbO2 at high temperature and high pressure. Its structure was an alyzed by Rietveld analysis of neutron diffraction data on the basis of spa ce group Pbnm. It has a distorted perovskite structure of the GdFeO3 type a nd a unit cell with dimensions of a = 5.6710 Angstrom, b = 5.8875 Angstrom, and c = 8.1495 Angstrom. 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 degrees and 20.28 degr ees, respectively. These tilt angles, which show great structural distortio n in CaPbO3 containing the smaller Ca2+ ion, are much larger than correspon ding ones in crystal chemically isotypic SrPbO3. The electric resistivity o f CaPbO3 at room temperature was as high as 3 x 10(3) Omega.cm, which is in sharp contrast to low resistivities observed in other perovskite-type oxid es BaPbO3 and SrPbO3. The high resistivity of CaPbO3 is explained as gap fo rmation between 2p(O) nonbonding and 6s(Pb)-2p(O) sp sigma antibonding band s, which overlap with each other in BaPbO3. Solid solutions, where Sr2+, La 3+, Nd3+, and Y3+ ions were partially substituted for Ca2+ ions, were also prepared to examine structural and electrical properties in perovskites bas ed on CaPbO3. Substitution of Sr2+ for Ca2+ led to reductions in the distor tion and gap energy, whereas that of La3+, Nd3+, and Y3+ induced metallic c onductivity owing to doping of electron carriers into the antibonding band.