Abstract: | Damping of ultrasonic waves with frequencies in the 100 kHz region was measured in the temperature region 300 … 1000 K. Two peaks could be observed (near 400 K or near 700 K, respectively). Occurrence of these peaks depends on oxygen content of the sample. The higher temperature peak is attributed to phase transition Pmmm 〈– – –〉 P 4/mmm of YBa2Cu3O7 – x. |