Ultrasonic study of YBa2Cu3O7-x superconductors

We investigated the mechanical properties of the sintered YBa2Cu3O7-x superconductors from the view point of the porosity and elastic anomalies. In the highly textured sample (5.93 g/cm3, porosity of 7%), the Debye temperature was 440 K determined from the longitudinal and transverse velocities (Vl = 4.9 km/s, and Vt = 3.2 km/s), which agreed very well with those measured by the specific heat and Mossbauer experiments ever reported. The background attenuation due to the grain-boundary scattering was 7 dB/cm at 9.7 MHz -longitudinal ultrasonic mode. On the other hand, in the porous sample (4.7 g/cm3, porosity of 25%), the Debye temperature was 320 K determined from the velocities of Vl = 3.5 km/s and Vt = 2.3 km/s, and the background attenuation was 10 dB/cm at 9.7 MHz - longitudinal mode. Some kinds of elastic anomalies were observed. In the sample (x = 0.53), Tc the step-like attenuation was detected just above Tc (2 dB/cm), which was the ultrasonic absorption due to the lattice instability, and essentially related to the superconducting transition. The relaxation-type sharp peak was observed (9 dB/cm at 62 K) in the sample of YBa2Cu3O7-x (x = 0, Tc = 90 K), with a narrow temperature width of 15 K. It was due to the ultrasonic absorption by jump motion of oxygen atoms between two stable sites. (orig.)