Ba2−xKxPb1−yBiyO4−δ: Layered oxides with insulating or conducting properties and possible superconducting properties

Superconductivity below 14 K has been found by magnetic-susceptibility measurements in a family of layered oxides ${\mathrm{Ba}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{Pb}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{Bi}}_{\mathit{y}}$ ${\mathrm{O}}_{4\mathrm{-}\mathrm{\delta }}$. The structure is isomorphous of tetragonal ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ but these materials do not contain any magnetic ions. Compounds of this family show a transition from insulator (for example, ${\mathrm{Ba}}_2$ ${\mathrm{PbO}}_4$, x=0,y=0) to conductor by injection of charge carriers induced by substitution of monovalent potassium onto the divalent barium site. The carrier concentrations are estimated from the screening of phonons by the plasmon deduced from the analysis of infrared-reflection spectra. In the tetragonal structure, ${\mathit{T}}_{\mathit{c}}$ is found twice higher than in the perovskite with the same bismuth concentration. This is achieved in spite of the fact that the carrier concentration remains low, and that the percentage of bismuth y does not exceed 15% in this series of experiments. A thermal treatment at 450 °C under oxygen improves conducting properties in the normal phase but, at least for low bismuth rate, is detrimental to superconductivity. These materials are expected to help clarify the role played by phonons and possible highly anharmonic fluctuations in the mechanism of superconductivity of oxides since here no magnetic interaction can play any part to mediate Cooper pairing as it is often invoked in copper compounds.