REORDERING STAGES OF OXYGEN AROUND 500-K IN REBA2CU3O6+X BY ANELASTIC RELAXATION MEASUREMENTS

We report new effects on the elastic energy dissipation and modulus of ReBa2Cu3O6+x (Re = Eu, Y) occurring around 500 K for x less-than-or-equal-to 0.3. The observed phenomenology is discussed in relation with the kinetics of the O ordering and with the phase diagram of ReBa2Cu3O6+x. The achievement of equilibrium is thought to pass through metastable states, each associated to the activation energy characteristic for dissociation of the particular species, like O pairs, isolated chain fragments or ordered domains.