Phase Transformation at 240 K in YBa 2 Cu 3 O 7- x by Measurements of Elastic Energy Dissipation and Modulus and its Possible Relation with the Enhancement of T c Above 100 K

The nature of the phase transformation occurring in YBa 2 Cu 3 O 7- x at 240 K has been investigated by internal friction and frequency measurements. The hysteresis in the frequency (elastic constants) reveals a slow kinetics, and the nonrecoverable raise of dissipation around 240 K generated by thermal cycling is attributed to the nonelastic accommodation of the 2nd-phase misfitting domains. A model is proposed to interpret the observed phenomenology and the unstable enhancements of T c above 100 K reported in the literature. According to this model, local T c enhancements are caused by the strains generated by the 2nd-phase domains, and this explains the strong dependence of the T c increase on the thermal hystory of the sample.