CO CLUSTER FORMATION AT REDUCED TEMPERATURES IN MICRODOPED Y1-ZCAZBA2CU3O6+Y(CO-57)

An investigation of the Co distribution in microdoped 57Co (ppm)Y1-zCazBa2Cu3O6+ y, z = 0.00 and z = 0.05, for different annealing treatments at reduced temperatures has been made with 57Co-emission Moössbauer spectroscopy. 57Co was first diffused into the lattice in oxygen at 650°C for 9 hr, 800°C for 0.5 hr, and 400°C for 3 hr (treatment [O]). This treatment resulted in Co substitution principally on the Cu(1) site in four-, five-, and sixfold coordination (denoted (1)4, (1)5, and (1)6, respectively). The (1)5 and (1)6 coordinations indicate cobalt atoms can cluster even at the ppm level of doping. Samples were next annealed at 700°C in N2 (treatment [N]); they were finally annealed at 350°C in O2 for 4 hr (treatment [NO]). Cobalt atoms migrate to Cu(1) from Cu(2) during treatment [N], and the Co in Cu(1) sites retain at least a (1)4 configuration even though most of the oxygen is lost from the Cu(1)Oy planes. An analysis of the data allows the drawing of a number of conclusions about the oxygen coordination at Co atoms and the Co-atom mobility within the Cu(1)Oy planes as a function of temperature and atmosphere. We conclude that suitable heat treatment should allow the formation of flux-pinning centers at Co-atom clusters without serious degradation of Tc, especially for z > 0.05. © 1992 Academic Press, Inc. All rights reserved.