ANOMALOUS THERMAL-PROPERTIES OF YBA2CU3O7-DELTA IN THE 200-250 K RANGE

Heat capacity data at normal pressure, C-P(T), measured by a high-reso lution a.c. calorimetric method on single crystals of YBa2Cu3O7-delta, provide evidence for a phase transition at T-t = 220 K in all the 90- K superconducting phases (delta less than or equal to 0.2). The amplit ude of the heat capacity peak is relatively small, less than 3% of the total lattice contribution, and no latent heat was detected through t he transition. The persistence of the Cp(T) anomaly neat T-t in sample s that differ in oxygen-deficit delta indicates that the ordering proc ess associated with this transition takes place in the CuO2 planes.