VARIATION OF OXYGEN-CONTENT AND CRYSTAL-CHEMISTRY OF YBA4CU3O8.5+DELTA

All phases discovered so far in the Y-Ba-Cu-O system have layered (2-d imensional) structures with the exception of YBa4Cu3O8.5+delta (Y143), which has a cubic (3-dimensional) structure, In an attempt to underst and the crystal chemistry of the Y143 phase and to make it superconduc tive, the Y143 compound has been treated at 450 degrees C in oxygen an d subsequently annealed at several different temperatures in a nitroge n atmosphere. The samples have been characterized by X-ray diffraction , thermogravimetric analysis (TGA), iodometric titration, and SQUID ma gnetometer measurements. The Y143 phase was found to be nonsuperconduc tive for all annealing conditions, The oxygen content was found to dec rease with increasing annealing temperature in two steps, correspondin g to the loss of oxygen from two different lattice sites. The lattice parameter decreased during the first step and increased during the sec ond step with increasing annealing temperature or with decreasing oxyg en content. Rietveld refinement of X-ray powder diffraction data showe d that the O3 and O4 sites, which are geometrically identical within t he crystal lattice, have an equal occupancy of 51.5% in samples not an nealed in N-2. However, when samples were annealed in N-2, an oxygen r edistribution from the O4 site to the O3 site occurred during the low temperature (< 500 degrees C) oxygen loss step. The variation in oxyge n content and crystal chemistry has been explained on the basis of the unique crystal structure of Y143 and the internal stresses on the met al-oxygen bonds. (C) 1998 Elsevier Science B.V.