Concentration-dependent acoustic and electrical properties of highly conducting Bi_3.5Pb_0.5Sr₃Ca₃Cu₄O_z + xAg₂O-type glassy precursors for high-T_c superconductors

Abstract

We report the concentration-dependent electrical conductivity and acoustic parameters of the (Bi-Pb)₄Sr₃Ca₃Cu₄O_z + xAg₂O (x = 0-10 wt%)-type glassy precursors for high T _c superconductors. Unlike (Bi, Pb)₄Sr₃Ca₃Cu₄O_Z, or many other transition-metal oxide glasses, these Ag-containing glasses are highly conducting. Unlike many other transition-metal oxide glasses, the small-polaron hopping mechanism of conduction is found not to be applicable for these glasses, which might be due to a partial contribution of the Ag ions to the conductivity. The velocities of sound, elastic constants, microhardness, acoustic impedance and Debye temperatures are found to decrease monotonically, as does the density, with increasing Ag concentration. On the other hand, the thermal expansion coefficient, internal friction and Poisson's ratio increase with increasing content of Ag₂O. The concentration-dependent acoustic parameters and the corresponding derived quantities indicate a softening of the glass structure with addition of Ag. All these glasses become high-T _c, superconductors in their respective crystalline phases.