Elastic behaviour under pressure of high-T c superconductors RBa2Cu3O7-x (R = Y, Gd and Eu)

The effects of hydrostatic pressure on the velocities of longitudinal and shear ultrasonic waves have been measured in YBa2Cu3O7-x, GdBa2Cu3O7-x and EuBa2Cu7O7-x. Measurements have been made for samples having different porosities to determine the effects of porosity on elastic behaviour under pressure. Results have been analysed on the basis of multiple scattering theory to provide the elastic properties of the non-porous matrix. The effects of hydrostatic pressure on the ultrasonic wave velocities are markedly nonlinear and have been used to obtain combinations of the effective third- and fourth-order elastic constants appropriate to a porous small-grained ceramic material treated as an isotropic body. The hydrostatic pressure derivatives (∂CIJ/∂P)r = o of the isotropic elastic constants and the acoustic mode Grüneisen parameter in the long-wavelength limit are very large, features which can be attributed to the open structure of this defect perovskite, although a valence change of copper could in part be responsible.