In a variety of high-temperature superconductors the transition temperature $T_{\mathrm{c}}$ has been found to depend on the sample’s detailed temperature and pressure history. ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+\delta }$ (Tl-2201) exhibits two distinct relaxation channels, one activated below 100 K and the other above 150 K, that have been attributed to oxygen ordering effects. In the present studies to 0.8 GPa we find that the activation energy for the low-temperature process, $E_{\mathrm{A}}^{\mathrm{LT}}\left(1\mathrm{}\hspace{0.5em}\mathrm{bar}\right)\approx 0.25$ eV, is independent of pressure, whereas the activation energy for the high-temperature process, $E_{\mathrm{A}}^{\mathrm{HT}}\left(1\mathrm{}\hspace{0.5em}\mathrm{bar}\right)\approx 0.75$ eV, appears to increase with pressure yielding an activation volume $\sim 1.5\hspace{0.5em}{\mathrm{cm}}^3/\mathrm{mol}.$ The activation volumes for the diffusion of oxygen defects in the high-temperature superconductors Tl-2201, ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (Y-123), and ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ (Hg-1201) are estimated using a simple hard-sphere model and found to be roughly half the experimental values. For Tl-2201 this comparison supports an indirect “interstitialcy” diffusion path for oxygen defects, whereas for Y-123 and Hg-1201 direct “interstitial” diffusion paths are indicated.

• Received 14 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.9155