Formation and mobility of oxygen vacancies in RuSr2GdCu2O8

Oxygen vacancies are introduced in the RuO2 and possibly CuO2 planes of RuSr2GdCu2O8 by annealing in vacuum above 600 K. The diffusive jumps of the O vacancies are accompanied by a reorientation of the local distortion, and are probed by measuring the elastic energy loss and modulus versus temperature at 1-10 kHz. An intense acoustic absorption peak develops near 670 K at 1 kHz and finally stabilizes after heating up to 920 K in vacuum. The analysis of the peak shows a barrier for the O diffusion of ~1.4 eV, and a slowing down of Curie-Weiss type, with TC =400-470 K, due to the interaction among the O vacancies. A secondary peak is attributed to O vacancies trapped at defects in the RuO2 planes, or to vacancies in the CuO2 planes. No sign of structural phase transformation is found up to 920 K.