Instability of Y–Ba–Cu–O structure in the vicinity of the superconducting transition

Anomalies in the thermal expansion and specific heat of YBa2Cu3O6+δ in the temperature range preceding the superconducting transition are observed by x-ray crystallographic methods and low-temperature adiabatic calorimetry. The anomaly in thermal expansion is observed at temperatures which are 10−15 K above Tc, while the anomaly in specific heat takes place at Ti = 97 K in a sample with Tc = 92.2 K. A broadening of x-ray lines is discovered at temperatures below 200 K. This broadening is found to be larger in samples with a smaller value of oxygen index δ. These anomalies can be explained by proceeding from the assumption about the existence in HTS of superstoichiometric weakly bound oxygen and lattice instability due to its migration.

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