Dynamics of oxygen in the YBa2Cu3O7-x basal planes by elastic-energy-loss measurements

We report elastic-energy-dissipation measurements in YBa2Cu3O7-x from 50 to 300 K at frequencies between 0.48 and 6.3 kHz. It is shown unambiguously that the two peaks around Tc are thermally activated with activation energies of 0.16 and 0.19 eV. When oxygen is reduced to about 6.5 atoms per formula unit, the higher-temperature process nearly disappears, while the other one increases. The peaks are interpreted in terms of jumps of the O atoms in the Cu-O basal planes, and two mechanisms are proposed and discussed: short jumps between the off-center positions in the chains and jumps between O(4) and O(5) positions of isolated atoms in hypothetical oxygen-depleted islands of the orthorhombic basal planes