STRUCTURAL DISORDER AND CHARGE-TRANSFER IN THE SUPERCONDUCTOR (PB0.5CU0.5)(SR0.5LA0.5)(2)CUO5+DELTA

Two samples of (Pb0.5Cu0.5)(Sr0.5La0.5)(2)CuO5+delta with different ox ygen contents (delta = 0.03 and 0.13) showed superconducting transitio n temperatures somewhat different from each other: 24 K for the fully oxidized sample and 35 K for the deoxidized sample. The structural par ameters of these samples were refined by the Rietveld analysis of neut ron powder diffraction data to clarify the structure/property relation ship in this nonstoichiometric oxide. In the oxidized sample, excess o xygen partially occupies an interstitial site at (0.151, 1/2, 0) in a (Pb0.5Cu0.5)O-1+delta layer, whereas this site proved to be vacant in the deoxidized sample. All the atoms on this layer are displaced from ideal rock-salt-type positions to achieve more favorable bonding envir onments. The length of a Cu-O bond on the CuO2 conduction sheet is dec reased from 1.8985 to 1.8915 Angstrom upon oxygen removal. This findin g provides unambiguous evidence for the idea that the hole concentrati on on the CuO2 sheet increases with releasing excess oxygen from the ( Pb0.5Cu0.5)O-1+delta layer.