Oxygen stoichiometry in Ba sub 2 YCu sub 3 O sub x.

Thermogravimetry (TG), evolved gas analysis (EGA), x-ray powder diffraction and dilatometry have been used to characterize the oxygen stoichiometry in the 93K superconductor, Ba sub 2 YCu sub 3 O sub x (7.0>x>6.0). Above 400C equilibration with ambients containing oxygen occurs rapidly. A reversible structural transformation occurs with changing oxygen stoichiometry going from orthorhombic at x=7.0 to tetragonal at x=6.0. The upper limit of x, which is achieved by annealing in oxygen at 400-500C, shows the sharpest superconducting T sub c 's. The tetragonal phase is not superconducting. Structural models are proposed for the tetragonal structure.