OXYGEN-CONTENT EFFECT IN Y-BA-CU-O SUPERCONDUCTORS

The dependence of electronic structure of superconducting YBa2Cu3Oy on the oxygen content is investigated using the standard CNDO/2 and quas i-relativistic CNDO/1 molecular orbital method. Electronic structures of model clusters [Cu3On]q with total charges q = 7 - 2n and n = 11 to 14 in various geometries are compared. Possible Cu(III) in real syste ms may be ascribed to penta-coordinate Cu-2 in CuO2 planes for y < 7 o r to Cu-1 in Cu-O chains for y > 7. The apical oxygen position within Cu-1-O-2-Cu-2 frame is influenced by actual Cu-1 coordination implied by the y value.