Superconductivity above 30 K in [(Bi1-xMox) 0.33Cu0.67]Sr2YCu2Oy:: Appearance of superconductivity by transfer of holes from a block layer to a CuO2 layer

Attention has been paid to a (Bi0.33Cu0.67)Sr2YCu2Oy compound for its uncertain superconductivity in spite of having a large Cu valence. By conducting combined experiments of iodometry, and chlorinemetry, Hall voltage measurement, Rietveld analysis, and a Discrete vibrational (DV)-X alpha calculation for (Bi0.33Cu0.67)Sr2YCu2Oy , evidence of localization of holes in a (Bi0.33Cu0.67)O-y block layer was found. Those experiments suggest that about 90% of holes estimated from the Mott-Hubbard theory are localized. DV-X alpha calculation revealed that the localized holes in (Bi0.33Cu0.67)Sr2YCu2Oy are not located at the apical or planar oxygen of a CuO2 layer but in the block layer. In order to transfer the localized holes from the block layer to the CuO2, layer, substitution of Mo for Bi was performed, resulting in an increase of the itinerant holes from 10 to 60 %. A bond valence sum calculation also suggested the transfer of holes from the block layer to the CuO2 layer with changes in occupancies of the oxygen sites in the block layer. Eventually, in a range of x greater than or equal to 0.55, it showed superconductivity above 30 K. We also discuss the origin of nonsuperconductivity of cuprates with a high Cu valence. [S0163-1829(99)04025-4].