Internal friction of superconducting YBa2Cu3O6+x wires at 200 kHz under in situ heat treatment

Using piezoelectric LiNbO₃ crystal oscillator method at 200 kHz, we have measured internal friction and modulus of YBa₂Cu₃O_6+x short wires with 1 mm-diameter and about 5mm length under in situ cyclic heating-cooling treatment at temperatures of 300–1020K. Polycrystalline YBa₂Cu₃O_6+x wires show anelastic behavior as well as orthorhombic-to-tetragonal (O-T) phase transition within present temperature range. There appears a strong internal friction peak below the O-T transition temperature, corresponding to the atomic site relaxation between oxygen and a vacancy in the Cu-O plane of oxygen defective YBa₂Cu₃O_6+x. A novel peak is observed after the cyclic heat treatments at around 700K. This relaxation is attributed to the hopping movement of oxygen in the defective local structure of YBa₂Cu₃O_6+x.