A NEUTRON POWDER DIFFRACTION STUDY OF BA2YCU3O

The structure of an orthorhombic sample of Ba//2YCu//3O//7 with a superconducting transition at 89 K has been determined by Rietveld profile analysis of neutron powder diffraction data. The space group chosen was Pmmm. The structure may be viewed as a heavily-distorted oxygen-defective ordered derivative of a perovskite-type structure. Two-thirds of the Cu atoms have four near-neighbor oxygens and one more distant neighbor in a tetragonal pyramidal arrangement, and form a two-dimensional network in the ab planes linked through corner shared oxygens about 0. 3 Angstrom out of the planes. The remaining one-third of the Cu atoms have fourfold planar coordination in the bc planes and form chains linked through corner-shared oxygens along the b axis.