The effects of oxygen stoichiometry on structure and properties in Ba sub 2 YCu sub 3 O sub x.

A review of recent studies on the oxygen stoichiometry in Ba sub 2 YCu sub 3 O sub x, 6.0 = x = 7.0, is presented. The article will be published in the proceedings of the American Chemical Society Meeting. The structure at x=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=6. Superconductivity is optimal for x~7 and for x=6 the materials is a semiconductor. A valence description ascribes Cu sup (+2) to the Cu in the layers for all x with the Cu in the chains going from Cu sup (+3) at x=7 to Cu sup (+1) at x=6. A model for twinning based on oxygen defects similar to those in Ba sub 2 YCu sub 3 O sub 6 is discussed.