Crystal structure of (Hg 1− yPb y) 2Ba 2(Y 1− xCa x)Cu 2O 8− δ superconducting compounds by neutron powder diffraction

Abstract

Three Pb-doped (Hg,Pb) 2Ba 2(Y 1− x Ca x )Cu 2O 8− δ samples with x Ca=0, 0.30 and 0.50 were synthesized at high pressure–high temperature. The use of simple oxides instead of precursor pre-reacted mixtures allows first to obtain better superconducting properties and second to fix precisely the oxygen content to O 8.0 ( δ=0). This allows us to isolate the Ca effects on the structure without any additional perturbations due to changes in the oxygen composition from one sample to another. The structure was refined from neutron powder diffraction data and confirms previous observations from synchrotron data. The main effect of Pb is to fill up the oxygen vacancies of the (Hg,Pb) 2O 2− δ charge reservoir (CR). Ca substitution makes the CR thicker and consequently increases the Cu–O2 apical distance while it decreases the buckling angle of the CuO 2 superconducting planes.