Effects of Oxygen Stoichiometry on Structure And Properties in Ba//2ycu//3O//X. .

Results of a series of studies on the oxygen stoichiometry in Ba//2YCu//3O//x, 6. 0 less than equivalent to x less than equivalent to 7. 0, are reported. The structure at x equals 7 contains layers of corner-shared, square pyramidal copper and chains of square planar copper. Oxygen is removed from the chains leaving two-fold coordinate copper at x equals 6. Superconductivity is optimal for x approximately equals 7 and for x equals 6 the material is a semiconductor. A valence description ascribes Cu** plus **2 to the Cu in the layers for all x with the Cu in the chains going from Cu** plus **3 at x equals 7 to Cu** plus **1 at x equals 6. A model for twinning based on oxygen defects similar to those in Ba//2YCu//3O//6 is discussed