Bulk modulus and its pressure derivative ofYBa2Cu3O7−x

Pressure dependences of the ultrasonic wave velocities in polycrystalline $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ are reported. Porosity effects are taken into account using wave-scattering theory in a porous medium. The bulk modulus $B_0$ at atmospheric pressure for the nonporous matrix is 65 GPa, much smaller than $B\left(P\right)\mathrm{}$ obtained at high pressures from lattice-parameter measurements. This discrepancy accrues from the large value of ($\frac{\partial B}{\partial P}$). The comparatively small $B_0$ and large ($\frac{\partial B}{\partial P}$) are due to vacant anion sites in this defect perovskite.