ISOTHERMAL PHASE-TRANSITION IN YBA2CU3O7-X

We present the results of neutron diffraction measurements on YBCO in isothermal conditions. These measurements were correlated with those o btained by internal friction and dynamic modulus. We have evidences th at the orthorhombic to tetragonal phase transition has the same kineti cs of the disappearing of an internal friction peak due to the oxygen motion in the basal plane. Furthermore, the oxygen disordering occurrs before the phase transition and corresponds to a modulus softening.