ORTHORHOMBIC TO TETRAGONAL PHASE-TRANSITION IN YBA2CU3O7-X OBSERVED BY DYNAMIC YOUNGS MODULUS MEASUREMENTS

Dynamic Young's modulus measurements, have been performed by the vibrating reed technique (10(2)-10(4) Hz) on polycrystalline YBa2Cu3O7-x samples in the temperature range 300-900 K. A strong modulus softening, indicative of a phase transition, is measured at about x = 0.45. DTG measurements versus temperature show in correspondence a maximum in the oxygen desorption rate. Long time modulus isotherms display an exponential trend linked to a thermally activated structural process with activation energy H = 0.9 +/- 0.1 eV. On the basis of the experimental results a model for the structural transformation linked to the twinned interfaces mobility and to oxygen diffusion is presented and discussed.