La-substitution effects on transport properties of APbO_3-δ (A = Ba, Sr and Ca)

Partial substitutions of La for alkaline-earth elements of perovskite-type APbO_3-δ (A = Ba, Sr, and Ca) were studied in order to examine carrier doping effects on the APbO_3-δ system. The solubility limit of La was found to be about 10% for every alkaline-earth element. The temperature dependencies of electrical resistivity and the Seebeck coefficient for A_0.9La_0.1PbO_3-δ samples indicated that they were metallic conductors, irrespective of the ionic size of the alkaline-earth elements. This is in good contrast with the resistivity of APbO_3-δ, which is strongly influenced by the ionic size of A, via the distortion of PbO₆ octahedra. The Seebeck coefficients for the A_1-xLa_xPbO_3-δ (x = 0 and 0.1) samples were negative, and absolute values were reduced by the La substitution, which implies that electron carriers were introduced into the conduction band. Superconductivity was not observed in these compounds down to 1.8 K, although the carrier density expected, based on the Seebeck coefficient, was comparable to the value for the superconducting BaPb_1-xBi_xO₃.