  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.
