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 ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. In ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{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 (=55 GPa) as that measured ultrasonically in much less dense ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ ceramics. The temperature dependences of the velocities of longitudinal and shear ultrasonic waves, which have been measured between 10 and 300 K, show a steplike increase at about 200 K on cooling and a similar decrease at about 225 K during warming with hysteresis in the range 190–235 K, features previously observed in less dense ceramics and tentatively attributed to a phase transformation.

The pressure dependences of both mode velocities for dense ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ ceramic show a pronounced change of slope at a pressure ${\mathit{P}}_{\mathit{c}}$. For pressures below and above ${\mathit{P}}_{\mathit{c}}$, 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 and 295 K. The pressure ${\mathit{P}}_{\mathit{c}}$ at which the kink occurs decreases almost linearly with decreasing temperature and extrapolates to atmospheric pressure at about 220 K. This is within the temperature range where the steep changes in the temperature dependences of the ultrasonic velocities occur, suggesting that the anomalous elastic behavior under pressure at ${\mathit{P}}_{\mathit{c}}$ correlates with that observed at about 200–235 K at atmospheric pressure. The mean long-wavelength acoustic-mode Grüneisen parameter ${\gamma }^{\mathrm{el}}$ has a large value of 11, implying that the vibrational anharmonicity of the long-wavelength acoustic modes is considerable.

• Received 14 January 1992

DOI:https://doi.org/10.1103/PhysRevB.46.1157