IMPLICATIONS OF CU VALENCE ON THE STRUCTURE OF BI-BASED SUPERCONDUCTING PRECURSOR GLASSES

The implication of copper valence state on the structure of glasses in the Bi-Sr-Ca-Cu-O system (Bi-based system) has been investigated. It was found that the density of Bi2Sr2CaCu2Ox glass decreases with incre asing the ratio of Cu+ to total copper, R(Cu+) = Cu+/Sigma(Cu+ + Cu2+) , and goes from 6.25 +/- 1% for the glass with R(Cu+) = 0.82 to 6.10 /- 1% g cm(-3) for that with R(Cu+) = 0.98. The activation energy for viscous flow also decreased with increasing R(Cu+) and went from 987 /- 5 kJ mol(-1) for the glass with R(Cu+) = 0.82 to 637 +/- 5 kJ mol(- 1) for that with R(Cu+) = 0.98. The decrease in both the density and t he viscosity revealed that the structure of the glass becomes more loo se with increasing Cu+ content. The study of the kinetics of the cryst allization of Bi2Sr2CaCu2Ox glass showed a decrease in the activation energy for crystal growth with the increase in Cu+ content. Based on t he analyses of the change in the properties of the glass, the model st ructure of [BiO6]-Cu-[BiO6] for the glass, which is composed of BiO6 o ctahedra connected to each other through two coordinated Cu+, is propo sed. (C) 1997 Elsevier Science B.V.