Abstract
The pressure dependences of ultrasonic longitudinal and shear waves have been measured in the high-Tc superconductor GdBa2Cu3O7-x. The effects of porosity have been taken into account by using wave scattering theory in a porous medium. The pressure dependence ( delta B/ delta P)P=0 of the bulk modulus of the nonporous matrix at room temperature has the considerable value of 55. The adiabatic bulk modulus B0s measured ultrasonically at atmospheric pressure is much smaller than that BT(P) determined by X-ray measurements of lattice parameters at high pressure; this discrepancy is due to the large pressure dependence of the bulk modulus. It is suggested that the rather small bulk modulus and its large pressure dependence could be due to a combination of fluctuating copper valence and to the vacant sites in the crystal structure.
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