Pressure-dependent oxygen diffusion in superconducting Tl2Ba2CuO6+delta, YBa2Cu3O7-delta, and HgBa2CuO4+delta: Measurement and model calculation

In a variety of high-temperature superconductors the transition temperature T-c has been found to depend on the sample's detailed temperature and pres sure history. TI2Ba2CuO6+delta (TI-2201) exhibits two distinct relaxation c hannels, one activated below 100 K and the other above 150 K, that have bee n 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-A(LT) (1 bar)approximate to 0.25 eV, is independent of pressure, whereas the acti vation energy for the high-temperature process, E-A(LT)(1 bar)approximate t o 0.75 eV, appears to increase with pressure yielding an activation volume similar to 1.5 cm(3)/mol. The activation volumes for the diffusion of oxyge n defects in the high-temperature superconductors TI-2201, YBa2Cu3O7-delta (Y-123), and HgBa2CuO4+delta (Hg-1201) are estimated using a simple hard sp here model and found to be roughly half the experimental values. For TI-220 1 this comparison supports an indirect "interstitialcy" diffusion path for oxygen defects, whereas for Y-123 and Hg-1201 direct "interstitial" diffusi on paths are indicated.