The diffusion of excess O atoms in La2CuO4+, where  was between 0 and 0.035, was studied by means of mechanical spectroscopy using a vibrating-reed at frequencies of between 209Hz and 1.3kHz and temperatures of between 4 and 980K. In samples with  = 0.013, very high internal friction was observed at a temperature of 275K and a frequency of 0.22kHz. This shifted to higher temperatures with increasing frequency; thus indicating athermally activated relaxation, with an activation enthalpy of 0.31eV. The relaxation peak vanished after vacuum annealing, and it was therefore concluded that this peak was due to the diffusion of excess O atoms. A relaxation peak, of similar magnitude to that observed in samples with  = 0.013, was also detected in an O-enriched sample with  = 0.035.

B.Kappesser, H.Wipf, R.K.Kremer: Journal of Low Temperature Physics, 1996, 105[5-6], 1481-6