Substitution of chromium for univalent copper in superconducting Pb₂Sr₂(Ca,Y)Cu₃O_8+δ

Abstract

Following considerations of geometry and the similarity between chromate and carbonate groups in terms of size and charge, we have investigated the possibility of replacing the two-coordinate Cu^I in superconducting lead cuprates of the general formula Pb₂Sr₂(Ca, Y)Cu₃O₈ by Cr. A high-resolution electron microscopy study coupled with energy dispersive X-ray analysis on small crystals of the title phases suggests that between 10 and 15% of the Cu^I can be replaced by Cr. While from the present structural study using HRTEM and Rietveld refinement of X-ray powder data we are unable to precisely obtain the oxidation state and oxygen coordination of Cr, we suggest in analogy with Cr substitution in other similar cuprates that in the title phases Cu^IO₂ rods are partially replaced by tetrahedral CrO²⁻₄ groups. Infrared spectroscopy supports the presence of CrO²⁻₄ groups. The phases Pb_1.75Sr₂Ca_0.2Y_0.8Cu₃O_8+δ and Pb_1.75Sr₂Ca_0.2Y_0.8Cu_2.85Cr_0.15O_8+δ are superconducting as-prepared, but the substitution of Cr for Cu^I results in a decrease of the T_C as well as the superconducting volume fraction.