Superconductivity in the Sr-Ca-Cu-O system and the phase with infinite-layer structure.

Abstract

Superconductivity and structure in samples of (Sr,Ca)${\mathrm{CuO}}_{2}$ with the infinite-layer structure, prepared by high-pressure synthesis, have been studied using magnetic susceptibility measurements, small angle x-ray diffraction, and neutron diffraction. It is found that the superconducting (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}100 K) samples in this system are phase impure and contain, in addition to the infinite-layer phase, members of the two homologous series ${\mathrm{Sr}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}+1}$${\mathrm{O}}_{2\mathit{n}}$ (n=3,5,...; orthorhombic), and ${\mathrm{Sr}}_{\mathit{n}+1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+1+\mathrm{\ensuremath{\delta}}}$ (n=1,2,...; tetragonal), as minor phases. Samples with larger phase fractions of the ${\mathrm{Sr}}_{\mathit{n}+1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+1+\mathrm{\ensuremath{\delta}}}$ compounds showed higher superconducting fractions. Phase-pure infinite-layer samples are not superconducting. Based on these results, and results previously published in the literature, it is proposed that the superconductivity in these infinite-layer samples comes from the tetragonal ${\mathrm{Sr}}_{\mathit{n}+1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+1+\mathrm{\ensuremath{\delta}}}$ compounds, not from the phase with the infinite-layer structure.