POINT-DEFECT MODELING OF THE OXYGEN STOICHIOMETRY IN THE YBA2(CU1-YZNY)3O7-X ORTHORHOMBIC DOMAIN (Y = 0.00, 0.02, 0.04) - CONSEQUENCES ON TC AND THE CHARGE-TRANSFER

Point defect models of the oxygen content isotherms in the orthorhombi c domain of YBa2(Cu1-yZny)3O7-x (y=0.00, 0.02, 0.04) show that Zn has a 1:1 mixed Cu site occupancy for y=0.02, 0.04, inducing oxygen vacanc ies on the apical-oxygen site and enhancing the localized character of the carriers. This observation can be linked to the observed T(c) ver sus doping curves which show a plateau-like behavior for 0.02 < y < 0. 06 in the fully oxygenated samples, similar to the T(c) versus oxygen content curve in the undoped compound. This result has been interprete d as a consequence of the presence of zinc in the chains which gives r ise to a nonlinear charge-transfer mechanism in the same way as occurs in undoped samples when oxygen is removed from the structure.