Identification and structural implications of the 90 K superconducting phase

Abstract

The phase relations of Cu-containing compounds in the system Y/sub 2/O/sub 3/-BaO-CuO were determined at a temperature of about 950/sup 0/C and a fixed partial pressure of 0.21 atm of oxygen. Mixtures of Y/sub 2/O/sub 3/, BaCO/sub 3/, and CuO were heated and the products identified by powder X-ray diffraction. The stable ternary phases containing Cu are Y/sub 2/Cu/sub 2/O/sub 5/, Y/sub 2/BaCuO/sub 5/, YBa/sub 2/Cu/sub 3/O/sub 8-y/, and BaCuO/sub 2/. The 90 K superconducting material is YBa/sub 2/Cu/sub 3/O/sub 8-y/. It is an ordered phase and does not display detectable solid solution between Y and Ba. It is orthorhombic a = 3.824 (1) A, b = 3.891 (1) A, c = 11.685 (2) A. A pseudotetragonal structure based on a approx. = b approx. = 3.87 A, c = 11.68 A, P4/m mm has Y situated between square coplanar CuO/sub 2/ layers; there are no oxygen atoms at this level. In the stoichiometry YBa/sub 2/Cu/sub 3/O/sub 8/ the copper between Ba layers occupies a tetragonally distorted octahedron (c/a = 0.88). It is most likely that the oxygen vacancies are associated with this Cu-O octahedral layer in YBa/sub 2/Cu/sub 3/O/sub 8-y/. The dramatic increase in superconducting transition temperature on goingmore » from the K/sub 2/NiF/sub 4/ structure of La/sub 2-x/Ba/sub x/CuO/sub 4-y/ to the YBa/sub 2/CU/sub 3/O/sub 8-y/ structure is considered with reference to large bipolarons, the position of the d/sub z/sup 2///sup 2/ energy level relative to the Fermi energy, and the overlap of the Cu:3d and O:3d and O:2p bands.« less