Mobility and aggregation of oxygen in YBa2Cu3O6+x in the low-concentration limit

Elastic energy-loss experiments were made on a YBa2Cu3O6+x sample, in order to investigate the O mobility and reordering in the CuOx planes at x close to zero. The height of a peak with an activation energy of 0.11 eV, previously attributed to jumps of isolated oxygens (Snoek effect), has been shown to decrease both after severe degassing treatments in vacuum and after several months of room-temperature aging. The discussion is focused on the apparent paradox that the extremely high mobility of the isolated oxygens seems to be in contrast both with the slow reordering kinetics and with the relatively high fraction of isolated oxygens estimated from the height of the dissipation peak below 100 K. An explanation is proposed to solve the above inconsistencies, based on the Coulomb repulsion between the isolated O ions in the semiconducting environment. It is also estimated that the binding energy of an O atom to a chain should be about 0.25 eV in the very diluted tetragonal phase. A brief discussion is made of an alternative interpretation in terms of polaron hopping of the same 0.11-eV relaxation process, recently observed also by NQR measurements.