The effect of hydrostatic pressure on the elastic behaviour of orthorhombic and tetragonal GdBa2Cu3O7-x

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Abstract
The pressure dependences of ultrasonic longitudinal and shear wave velocities have been measured in the orthorhombic form (which becomes superconducting) and the tetragonal form (non-superconducting) of GdBa2Cu3O7-x. The effects of porosity have been taken into account by using wave scattering theory in a porous medium. The pressure derivative ( delta B/ delta P)P=O of the bulk modulus of the non-porous matrix at room temperature is large for both forms. Since twins are present only in the orthorhombic form it is concluded that twins cannot be the cause of this large ( delta B/ delta P)P=O. For the orthorhombic form the adiabatic bulk modulus BO measured ultrasonically at atmospheric pressure is much smaller than that BT(P) determined from X-ray measurements of lattice parameters at high pressure; this discrepancy can be accounted for by the large pressure dependence of the bulk modulus. It is suggested that the rather small bulk modulus and its large pressure dependence found for both forms could be due to a combination of fluctuating copper valence and to the open nature of the defect crystal structure.