Observation of superconductivity in (Y_1-xCa_x)Ba₂Cu₃O₆

Substitution of Ca²⁺ for Y³⁺ in YBa₂Cu₃O₆ converts this tetragonal semiconductor into a tetragonal superconductor with aT_c of 50 K. Furthermore, the structure of (Y_0.8Ca_0.2)Ba₂Cu₃O₆, determined by neutron powder diffraction, is fully consistent with partial oxidation of the Cu(2){single bond}O sheets, whereas the linear O{single bond}Cu(1){single bond}O units do not undergo oxidation. Observation of a limited solid solubility for Ca²⁺ in this structure is attributed to a maximum allowed oxidation state for Cu(2). This implies that in the superconductor YBa₂Cu₃O₇ the Cu{single bond}O chains are responsible for theT_c of 90 K, whereas the Cu{single bond}O sheets alone produce superconductivity at 50 K in YBa₂Cu₃O_6.5.