NEW-TYPE OF ORTHORHOMBICITY AND CELL-VOLUME EXPANSIONS NEAR HALF FILLING OF O INTERCALATION IN ERBA2CU3O6-CENTER-DOT-5

Novel orthorhombics are observed on special processing near half filli ng of the O intercalation in ErBa2Cu3Oy. They are based on local confi guration (m)n = (1)3, where rn is the Cu site and n its O coordination and designated O(1)3. Specially sequenced getter annealing with slow cooling produces O(1)3 in a region where conventionally prepared mater ials are based on O(1)4(1)2, corresponding to a mix of four-and twofol d O coordination. These O(1)3 can exist as cell volume neutral (V-0) m aterials resembling the volumes of the tetragonal semiconductors. In a narrow region near y = 6.46, 0(1)3 can undergo strong, nearly isotrop ic cell volume expansions (V+). These materials are larger than their cell volume contracted (V-) O(1)4 counterparts and even larger than th e semiconductors at y = 6. These observations enlarge the phase diagra m and pose new questions concerning the general origin of redox proces ses in superconducting O intercalation cuprates. These questions are d iscussed based on bond valency calculations. For the cell volume expan ded materials a charge rebalancing between Cu and 0 leading to massive self-doping is suggested. (C) 1998 Academic Press.