DEFECT CHEMISTRY AND TRANSPORT IN YBA2CU3O6+X

A defect model for YBa2Cu3O6+x (YBCO) is proposed that explicitly takes into account the high defect concentrations existing in this system. The high intrinsic ionic disorder of YBCO as well as the strong ionic-electronic defect interactions are emphasized. The model explains the partial-pressure dependence of the electronic conductivity particularly well at high temperature and small x without the need to neglect nonidealities. The free-hole density under these conditions is believed to be significantly lower than expected from simple analysis of the oxidation states. The increase in x with increasing Po2 is chiefly ascribed to the incorporation of neutral-oxygen defects and can be fitted in this way. The temperature dependence of the partial conductivities (at constant x and constant Po2) are derived within the framework of the model.