Metal-insulator transition and superconductivity in Sr- and La-substituted BaPb_1-xBi_xO₃

Compositional boundaries of the metal-insulator transition (MIT) in Sr- and La-substituted BaPb_1-xBi_xO₃ (BPBO) were determined by dc susceptibility, electrical resistivity and thermoelectric power (TEP) measurements. By the 20% substitution of Sr for Ba, a Bi compositional range for metallic (superconducting) phase considerably extended to the Bi-rich region (x≈0.5), though BPBO exhibited metal (superconductivity) only below x = 0.3. On the other hand, by the 10% La substitution, a Bi range for metal (superconductivity) slightly decreased to the Bi-poor region (x≈0.25). Transport properties indicated that the MIT takes place at the lattice parameter a_p≈4.29 angstroms, irrespective of the substitution elements. This suggests that the cell size determining the Bi-O orbital overlap and/or a magnitude of the band split, is one of the essential factors of the MIT in BPBO.