Substitution of Chromium for Univalent Copper in Superconducting Pb2Sr2(Ca,Y)Cu3O8+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 CuIin superconducting lead cuprates of the general formula Pb2Sr2(Ca, Y)Cu3O8by 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 CuIcan 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 CuIO2rods are partially replaced by tetrahedral CrO2−4groups. Infrared spectroscopy supports the presence of CrO2−4groups. The phases Pb1.75Sr2Ca0.2Y0.8Cu3O8+δand Pb1.75Sr2Ca0.2Y0.8Cu2.85Cr0.15O8+δare superconducting as-prepared, but the substitution of Cr for CuIresults in a decrease of theTCas well as the superconducting volume fraction.