The structure of the 44 K superconductor (Y_1-xCa_x)Ba₂Cu₃O_6+δ (δ ≤ 0.2)

Replacing Y(III) by Ca(II) in (Y_1-xCa_x)Ba₂Cu₃O₆ induces superconductivity. A solid-solution limit exists, as is evidenced by the lack of variation in unit cell parameters for values of x > 0.3. At low nominal doping levels (x < 0.3), materials prepared by solid-state techniques consist of a complex intergrowth of Ca-rich and Ca-poor 123-type phases, as well as YBa₂Cu₃O₆ and BaCuO₂. At higher nominal doping levels the Ca content of the 123-type phase becomes constant (x ∼ 0.30). Structural refinements using data collected by X-ray and neutron powder diffractometry are consistent with Ca-doping causing oxidation only of Cu in the sheet, leaving the copper in the so-called chain site as Cu(I). Many of the structural changes occurring in this system upon oxidation, such as a flattening of the CuO₂ sheets, a contraction of the in-plane CuO distance, and a decrease in the CuCu distance between planes, also occur in the structurally related superconducting system, Pb₂Sr₂(Y_1-xCa_x)Cu₃O₈.