OBSERVATION OF A NEW CELL-VOLUME EXPANDED SUPERCONDUCTING STATE IN YBA2(CUNI)(3)O-Y NEAR Y=6.5

Strong cell volume expansions (V+) are observed near y = 6.5 on reoxid ation of reduced YBa2(Cu1-xNix)(3)O-y, x = 0.05 and y similar to 6.3. These materials develop bulk superconductivity with T-c similar to 72 K. This superconductivity disappears upon further reoxidation and reap pears in fully reoxygenated materials with T-c = 68 K in conventional volume contracted (V-_) p doped materials. Bond valence calculations m ake plausible that the local configuration (1)3, where (1) denotes the Cu(1) site and 3 the O coordination is involved in V+ which is corrob orated by TEM. Accordingly the nominally 2 + Cu at y = 6.5 is reduced and O oxidized. Bond valence calculation and the magnitude of T-c (com parable to the fully oxygenated material) suggest that Cu is in a stoi chiometric valence v(s) = 1.67 + creating one hole on O. This represen ts a new mechanism of self-doping by rebalancing according to YBa2Cu1Cu22+O5.52- with charge equilibration implied (written for the idealiz ed formula). V+ effects are also found in related systems and by relat ed preparations and are discussed as a generic new feature of reductiv e Cu chemistry.