Defect chemistry and transport in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$

A defect model for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ (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 ${\mathit{P}}_{\mathrm{O}2\mathrm{}}$ 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 ${\mathit{P}}_{\mathrm{O}2\mathrm{}}$) are derived within the framework of the model.