Ultrasonic evidence of strong vibrational anharmonicity in high-Tcsuperconductors and its effect on determination of their elastic properties

To shed further light on outstanding problems associated with the magnitude of the bulk modulus and its pressure dependence in high-${\mathit{T}}_{\mathit{c}}$ superconductors, a comparative study has been made of the effects of hydrostatic pressure on the elastic properties of ceramic specimens of the electron-doped ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ and the hole-doped ${\mathrm{La}}_{1.8}$ ${\mathrm{Sr}}_{0.2}$ ${\mathrm{CuO}}_4$ superconductors and their nonsuperconducting parent compounds ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ and ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. The effects of porosity on the elastic properties have been taken into account by use of wave-scattering theory in a porous medium. The ultrasonic-wave velocities and bulk modulus ${\mathit{B}}_0$ measured in the polycrystalline ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ are in reasonable agreement with those determined from elastic-constant measurements in single-crystal ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. This observation lends strong support to the suggestion that for these mixed oxide ceramics the true bulk modulus at atmospheric pressure is the one ${\mathit{B}}_0$ derived from ultrasonic measurements rather than that ${\mathit{B}}^{\mathit{T}}$(P) obtained from x-ray experiments at very high pressures.