An internal friction and frequency study in YBa 2Cu 3O 7-x

Measurements of anelastic relaxation in the kHz range have been conducted in YBa ₂Cu ₃O _7-x between 300 and 60 K. Two peaks are observed around the superconducting transition temperature, at 110 K and just below T _c, and an additional maximum at - 160 K in the sample prepared with BaCO ₃. The previously reported phase transformation at T _t- 240 K is shown to give rise to hysteretic effects which are absent when thermal cycling is conducted above T _t. The possible relation of the presently studied phase transformation with the non-reproducible enhancement of T _c above 100 K is discussed.