Metal Ion Substitution Chemistry in the YBaCuO System as a Probe of the Superexchange Pathway

Abstract

The effect of substitution for Cu by aluminum, nickel, and zinc in the YBaCuO superconducting system is described in detail. All three metals lower the Tcvalue and make the superconducting transition less abrupt. However, the strength of the effect varies with the metal, and this is attributed to their different reactivities and site preferences. The most dramatic effect is with zinc, which completely quenches the superconductivity when as few as 10% of the Cu sites are replaced. It is postulated that under these synthetic conditions zinc prefers the five-coordinated “Cu(2)” site, which is presumed to lie on the main superconducting pathway. Nickel has a stronger affinity for the four-coordinated “Cu(1)” sites and is apparently found in both sites at high dopant levels. Magic angle spinning NMR (MAS NMR) and other data indicate that A1 prefers the four-coordinate Cu(1) site for initial substitution, although aluminum ions may also aggregate at the twinning boundaries. We suggest the hypothesis that substitution of the Cu(1) sites eliminates their contribution to the superconductivity and thereby makes the system like the related 40 K superconductors, which lack the Cu(1) sites. © 1988, American Chemical Society. All rights reserved.