No abstract

The velocity and attenuation of longitudinal and shear ultrasonic waves propagated in ceramic specimens of single phase Bi(Pb)2223, Bi(Pb)2212 and lead-free Bi2212 have been measured between 10 and 295 K. The anomalous elastic effects and hysteresis in the range about 190-240 K reported by other workers have been found in samples of Bi(Pb)2223 and lead-free Bi2212. However, for the Bi(Pb)2212 phase, in which some Bi is substituted by Pb, these anomalies and hysteresis are suppressed. Quenching a sample of the Bi2212 phase from 800 degrees C into liquid nitrogen removes these anomalies, which have therefore been shown to be strongly dependent on chemical composition: either oxygen content or lead substitution for bismuth. The elastic stiffnesses of these bismuth cuprates are unusually small due largely to (i) their open microstructure on the macroscopic level but also in part as a result of (ii) their rather open layer-like structure with weak interlayer binding forces and (iii) the intermediate valence state of the copper ions. It is known that quenching the Bi2212 phase from just below the melting temperature (about 880 degrees C) into liquid nitrogen results in the reduction in oxygen content that is necessary to optimize the superconducting properties of this phase. The authors have now established that quenching also results in substantial longitudinal acoustic mode softening whilst enhancing Tc.

Pressure dependences of the ultrasonic wave velocities in polycrystalline YBa2Cu307are reported. Porosity effects are taken into account using wave-scattering theory in a porous medium. The bulk modulus Bo at atmospheric pressure for the nonporous matrix is 65 GPa, much smaller than B(P) obtained at high pressures from lattice-parameter measurements. This discrepancy accrues from the large value of (BB/8P). The comparatively small Bp and large (BB/8P) are due to vacant anion sites in this defect perovskite.

No 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.