THE BASAL-PLANE OXYGEN DEPENDENCE OF YBA2CU3O7-X ANELASTIC RELAXATIONAND ORTHORHOMBIC TO TETRAGONAL PHASE-TRANSITION

Measurements of the elastic energy dissipation coefficient and of the dynamic Young's modulus were performed on polycrystalline YBa2Cu3O7-x by the vibrating reed technique and torsional pendulum, in the frequen cy range 1-5000 Hz. The stoichiometry of the samples was varied in the range x = 0.02 - 0.7. From the isothermal modulus measurements a stro ng softening stage characterizing the onset of the ortho-tetragonal tr ansformation was observed. The long time isotherms give an activation energy of 0.9 = 0.1 eV. Two relaxational damping peaks have been obser ved and attributed to the short range oxygen dynamics in the well oxyg enated and oxygen deficient superconducting phases. On the basis of th ese results models for the structural transformation linked to oxygen out diffusion and for the oxygen dynamics in different stoichiometric conditions are discussed.