STOICHIOMETRY DEPENDENCE OF OXYGEN ANELASTIC RELAXATION IN YBA2CU3O7-X AND THE ORTHORHOMBIC-TO-TETRAGONAL PHASE-TRANSITION

Measurements of the elastic energy dissipation coefficient and of the dynamic Young modulus were performed on polycrystalline YBCO samples b y the vibrating-reed technique and torsional pendulum, in the frequenc y range 5-5000 Hz. Two relaxational damping peaks with activation ener gy H-o1=(1.5+/-0.1) eV and H-o2=(1.3+/-0.1) eV have been observed abov e room temperature and their origin is discussed in terms of the short -range oxygen dynamics. The dynamic modulus shows a relaxation in corr espondence to the two peaks and a softening attributed to the orthorho mbic-to-tetragonal phase transition.