The structure of the 44 K superconductor (Y 1− xCa x)Ba 2Cu 3O 6+δ (δ ≤ 0.2)

Abstract

Replacing Y( III) by Ca( II) in (Y 1− x Ca x )Ba 2Cu 3O 6 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 YBa 2Cu 3O 6 and BaCuO 2. 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 CuO 2 sheets, a contraction of the in-plane CuO distance, and a decrease in the CuCu distance between planes, also occur in the structurally related superconducting system, Pb 2Sr 2(Y 1− x Ca x )Cu 3O 8.