The solid solution Pb 0.5Sr 2.5Y 1− xCa xCu 2O 7−δ: Superconductivity and structure

Abstract

The “system” Pb 0.5Sr 2.5Y 1− x Ca x Cu 2O 7−δ has been investigated for its structural and superconducting properties. A pure phase has been isolated for 0 ≤ x ≤ 0.60, whose structure belongs to the [2,2] type, i.e., consists of an intergrowth of double pyramidal copper layers ( m = 2) with double rock salt-type layers containing lead and strontium. The oxides corresponding to 0 ≤ x ≤ 0.40 do not superconduct, whereas the pure phases corresponding to 0.50 ≤ x ≤ 0.60 exhibit critical temperatures deduced from magnetic measurements ranging from 50 to 75 K, with diamagnetic fraction volumes at 4 K ranging from 8 to 65% in agreement with the T C(R = 0) = 53K , previously observed for x = 0.50 from resistivity measurements. The enhancement of superconductivity by annealing under an oxygen flow is demonstrated. The magnetic study of two phase samples 0.7 ≤ x ≤ 0.9 and especially of x = 0.9 shows a diamagnetic fraction volume of 3–4%, corresponding to a T C = 104K , which confirms the possibility of superconductivity up to 100 K in this system. The structural study of these oxides and especially of the nonsuperconducting sample x = 0 allows a comparison to be made with the superconductor x = 0.5. Although they belong to the same structural type, the first one differs from the second one, by the existence of several superstructures which are interpreted in terms of ordering of lead, strontium, and oxygen vacancies in the rock salt-type layer.