ORDERING AND DIFFUSION OF OXYGEN AT LOW TEMPERATURE IN Y-Ba-Cu-O BY MEASUREMENTS OF ELASTIC ENERGY DISSIPATION AND MODULUS

The anelastic relaxation spectrum between 300 and 50 K has been investigated in YBa 2 Cu 3 O 7−x in the super- and semiconducting state. In the superconductor a phase transformation has been detected at T t ≃ 240 K . The 2nd phase forming on cooling, likely has the same orthorombic symmetry (with higher distorsion) of that of the lattice and consists of a different oxygen vacancy ordering in the chains of the basal planes. The necessity of a phase diagram where the two orthorombic phases coexist in an extended region down to low temperature is pointed out. The 2nd phase is considered to contribute to the observed softening and hysteresis of frequency (elastic constants). The raise of elastic energy dissipation around T t with thermal cycling, which is not completely recoverable by the room temperature ageing, indicates the occurrence of progressive lattice damaging with cycling number, and is attributed to the non-elastic accommodation of the 2nd-phase misfitting domains. These lattice defects are identified with the additional twins, which have been reported to form and dissolve on cycling below 240 K. Other reproducible features observed in the superconducting state are dissipation maxima at 160 and 110 K, just above the critical temperature and a drastic decrease in dissipation below T c . When oxygen has been removed from the basal planes (6.2 ± 0.10 atoms per formula unit) all processes illustrated above practically disappear and a new intense thermally activated relaxation process manifests itself around 70 K (in the kHz range), attributed to the stress-induced hopping of oxygen in the basal planes. The derived diffusion coefficient is extremely high: D(T) = 4 × 104 exp (− 0.10 eV/kT ).