Anisotropy of oxygen tracer diffusion in single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Oxygen tracer diffusion has been measured in single-crystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ from 300 to 650 °C in the c direction, at a few temperatures in the ab plane of twinned crystals, and in the b direction of an untwinned crystal. The diffusion coefficient in the c direction, ${\mathit{D}}_{\mathit{c}}$, is ≊${10}^6$ lower than the diffusion coefficient in polycrystals at 400 °C and diffusion in the b direction is at least 100 times faster than diffusion in the a direction at 300 °C. The oxygen diffusion coefficient in polycrystals is, within an experimental uncertainty of a factor of 2, independent of oxygen partial pressure at 400 °C, in agreement with previous results measured at 600 °C. Some suggestions are presented on the mechanism of diffusion, the relation of diffusion and internal friction in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, and on the meaning of the activation energy.