Compressibility and thermal expansion of YBa2Cu3Ox

The linear compressibilities and the thermal expansion of an untwinned, nea rly optimal doped YBa2Cu3O6.94 single crystal were measured along the three crystallographic axes with a high-resolution dilatometer mounted in a high -pressure cell. The measurements were performed in a temperature range from 50 K to 320 K under hydrostatic gas pressure of maximal 0.65 GPa. At a tem perature of 300 K the measured bulk modulus is reduced and shows a very str ong enhancement under increasing pressure due to pressure-induced oxygen or dering. This ordering process can also be related to a glass transition see n in the thermal expansion above 225 K. At lower temperatures, when oxygen ordering is frozen, the bulk modulus is increased and shows a significantly reduced enhancement under pressure. This pressure dependence, however, is still anomalous high, probably caused by bonds of extremely different compr essibilities within the unit cell. The widely spread literature data of the bulk modulus can be explained by this high pressure dependence.