EXCESS OXYGEN DIFFUSION IN LA2CUO4+DELTA INVESTIGATED BY MECHANICAL SPECTROSCOPY

The diffusion of excess oxygen atoms in La2CuO4+delta (0<delta<0.035) was studied by mechanical spectroscopy with a vibrating reed method (2 20 Hz<f<1.3 kHz; 4 K<T<380 K). For samples with delta=0.013(2) we obse rve a very large internal friction peak (T-max=275 K, f=0.22kHz. It sh ifts to higher temperatures with increasing frequencies indicating a t hermally activated relaxation behavior with an activation enthalpy of 0.31 (3) eV. The relaxation peak vanishes after vacuum annealing the s ample (delta approximate to 0) leading to the conclusion that the rela xation peak is due to the diffusion of excess oxygen atoms. A relaxati on peak of similar strength as in the sample with delta=0.013 is also detected in an ozygen enriched sample (delta=0.035). This unexpected f inding of the relaxation strength is discussed in terms of the propose d phase diagrams for La2CuO4+delta. In addition, a shoulder at the hig h temperature wing of the strong relaxation peak is seen the position of which does not vary with the vibration frequencies.