Metastable behavior of the superconducting phase in the ${\mathrm{BaBi}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{O}}_3$ system

The structural phases of the ${\mathrm{BaBi}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pb}}_{\mathit{x}}$ ${\mathrm{O}}_3$ 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≤x≤0.20, orthorhombic for 0.25≤x≤0.65, diphasic (orthorhombic and tetragonal) for 0.70≤x≤0.80, and monoclinic for x=1.0. Superconductivity is observed for samples in the composition range 0.70≤x≤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 phase because the tetragonal-to-orthorhombic transition does not proceed to completion. Samples of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{K}}_{\mathit{y}}$ ${\mathrm{Bi}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pb}}_{\mathit{x}}$ ${\mathrm{O}}_3$ have ${\mathit{T}}_{\mathit{c}}$’s that are inversely proportional to the total dopant concentration, x+y, in the composition region (0.5≤x+y≤0.9), where the material is metallic.