METASTABLE BEHAVIOR OF THE SUPERCONDUCTING PHASE IN THE BABI1-XPBXO3 SYSTEM

The structural phases of the BaBi1-xPbxO3 system have been studied using neutron powder diffraction. As lead is substituted on the bismuth site, the room-temperature structure is monoclinic for 0.00 less-than-or-equal-to x less-than-or-equal-to 0.20, orthorhombic for 0.25 less-than-or-equal-to x less-than-equal-to 0.65, diphasic (orthorhombic and tetragonal) for 0.70 less-than-or-equal-to x less-than-or-equal-to 0.80, and monoclinic for x = 1.0. Superconductivity is observed for samples in the composition range 0.70 less-than-or-equal-to x less-than-or-equal-to 0.80. The superconducting phase is the tetragonal phase. In situ neutron-powder-diffraction measurements versus temperature show that it is not stable at temperatures below about 425 K. All superconducting samples consist of the metastable tetragonal phase and a semiconducting orthorhombic rhombic phase because the tetragonal-to-orthorhombic transition does not proceed to completion. Samples of Ba1-yKyBi1-xPbxO3 have T(c)'s that are inversely proportional to the total dopant concentration, x + y, in the composition region (0.5 less-than-or-equal-to x + y less-than-equal-to 0.9), where the material is metallic.