Ultrasonic determination of the elastic moduli and their pressure dependences in very dense YBa2Cu3O7-x

The effects of hydrostatic pressure and temperature have been measured on the velocities of longitudinal and shear ultrasonic waves propagated in a very dense (96% of theoretical density) ceramic specimen of YBa2Cu3O7-x. In YBa2Cu3O7-x ceramics with such a high density the effects of porosity on the elastic properties should be much reduced. Nevertheless the bulk modulus of this dense material has the same small magnitude (∼ 55GPa) as that measured ultrasonically in much less dense YBa2Cu3O7-x ceramics. The temperature dependences of the velocities of longitudinal and shear ultrasonic waves, which have been measured between 10 K and 300 K, show the step-like increase at 200 K on cooling and a similar decrease at 225 K during warming with hysteresis in the range 190 K to 235 K that has previously been observed in less dense ceramics and tentatively attributed to a phase transformation. The pressure dependences of both mode velocities for dense YBa2Cu3O7-x ceramic show a pronounced change of slope at a pressure Pc. For pressures below and above Pc the pressure dependence of ultrasonic velocity is essentially linear. Above the knee, the enormous pressure dependences of the longitudinal mode velocity and hence of the bulk modulus persist. The temperature dependences of pressure derivatives of elastic stiffnesses and bulk modulus have been measured between 250 K and 295 K. The pressure Pc at which the kink occurs decreases almost linearly with decreasing temperature and extrapolates to atmospheric pressure at about 220 K