Variation of oxygen content and crystal chemistry of YBa₄Cu₃O_{85 + δ}

All phases discovered so far in the Y-Ba-Cu-O system have layered (2-dimensional) structures with the exception of YBa₄Cu₃O_{8.5 + δ} (Y143), which has a cubic (3-dimensional) structure. In an attempt to understand the crystal chemistry of the Y143 phase and to make it superconductive, the Y143 compound has been treated at 450°C in oxygen and subsequently annealed at several different temperatures in a nitrogen atmosphere. The samples have been characterized by X-ray diffraction, thermogravimetric analysis (TGA), iodometric titration, and SQUID magnetometer measurements. The Y143 phase was found to be nonsuperconductive for all annealing conditions. The oxygen content was found to decrease with increasing annealing temperature in two steps, corresponding to the loss of oxygen from two different lattice sites. The lattice parameter decreased during the first step and increased during the second step with increasing annealing temperature or with decreasing oxygen content. Rietveld refinement of X-ray powder diffraction data showed that the O3 and O4 sites, which are geometrically identical within the crystal lattice, have an equal occupancy of 51.5% in samples not annealed in N₂. However, when samples were annealed in N₂, an oxygen redistribution from the O4 site to the O3 site occurred during the low temperature ( < 500°C) oxygen loss step. The variation in oxygen content and crystal chemistry has been explained on the basis of the unique crystal structure of Y143 and the internal stresses on the metal-oxygen bonds. © 1998 Elsevier Science B.V.