Ultrasonic determination of the elastic properties and their pressure and temperature dependences in very dense ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. 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.