Mat. Res. Bull., Vol. 22, pp. 1581-1588, 1987. Printed in the USA. 0025-5408/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd. z~'rr~rs OF Ag/Cu SUBSTITUTI£~ IN YBa2Cu307 S U P E R S David Cahen, Ziona Moisi, and Michael Schwartz Dept. of Structural Chemistry The Weizmann Institute of Science, Rehovot, 76100. Israel. (Received August 25, 1987; Refereed) ABSTRACT: We have substituted up to ca. 2 at.% of the Cu in ortk~rhombic YBa2Cu$O7 by Ag. This substitution leads to a wider temperature range xor the resistive transition to the superconducting state and a decrease in the fraction of the sample that is superconductive, as compared to the Ag-free material. Introduction of up to ca. i. 5 at.% Ag causes a slight lattice expansion. Higher nominal levels of Ag bring about the formation of additional phases, which, however, still show zero resistance, albeit at lower temperatures. We use solid state chemical arguments to infer that Ag substitutes for Cu in the CuO4 chains of the structure, in which case our results suggest that optimal superconducting behaviour is obtained when these chains are intact. MATERIALS INDEX: silver, copper, oxides, yttrium, barium Introduction Superconductivity in the Y-Ba-Cu-O system [I] has been shown to be due to the YBa2Cu307_x (x <0.5) phase [2]. The commonly accepted structure of this cc~pound (cf. howe~er ref.~ for alternate proposals) shows two distinct types of Cu, formally Cux and Cu~+. (Because the compound is metallic above Tc, the use of integer valencies for the Cu atoms is largely a matter of convenience). One kind of Cu Lxzcurs in CuO5 pyramids, with Cu slightly displaced from the basal plane (CuZ+), the other one in CuO4 squares (~S+). Their arrangement in the structure results in Cu-O planes and Cu-O chains, resp. [4]. A number of findings suggest that this arrangement of Cu and 0 in planes and chains is crucial for superconducting (SC) behaviour. Thus electrical anisotropy has been measured on single crystals [5]. Only min~r effects have been reported, in terms of SC character, when Y, which is situated between the Cu-O planes, 1581