EFFECT OF ANNEALING ON THE LOCAL MICROSTRUCTURE AND TC IN Y1-ZCAZBA2(CU0.90CO0.10)3O6+Y

An investigation of the Co distribution in Y1-zCazBa2(Cu0.90Co0.10)3O6+y, z=0.00 and 0.05, for different annealing treatments has been made with Co-57 emission Mossbauer spectroscopy. Samples prepared by conventional ceramic techniques were first annealed in oxygen at 650-degrees-C for 15 h, 800-degrees-C for 1 h, and 400-degrees-C for 3 h (treatment [O]); they were next annealed at 700-degrees-C in N2 (treatment [N]); they were finally annealed at 350-degrees-C in O2 (treatment [NO]). After [O], the Co atoms for z=0.00 were distributed nearly 50-50 on Cu(1) and Cu(2) sites with evidence for Co-atom clustering in the Cu(1)O(y) planes; for z=0.05, over 80% of the Co were on the Cu(1) sites, and, more important, Co-atom clustering was evident. During treatment [N], Co atoms migrated form Cu(2) sites to the cobalt clusters in the Cu(1)O(y) planes. After [NO], the Co-atom clusters remained, with some increase in size in the z=0.00 sample, and the superconductive onset temperature T0 increased from its value after [O], but its superconductive transition width DELTA-T(c) also increased. A Fe-57(4+) daughter is observed for Co3+ in a fivefold-coordinated Cu(1) site. These observations are discussed in comparison with results of similar experiments by others.