The structure of ternary fluorozirconate glasses

Abstract

Ternary fluorozirconate glasses in the systems ZrF 4–BaF 2–NaF and ZrF 4–BaF 2–LiF have been prepared by quenching mixtures of pure salts in sealed platinum capsules from 650°C. The glass samples have been examined by IR and Raman spectroscopy. The principal IR and Raman bands assigned to Zr–F stretching vibrations decrease in frequency with increasing F/Zr ratio as reported for fluorozirconate melts. The counter cations (Ba, Na, Li) in the glasses have a minor influence on the Raman bands, but a larger effect on the IR bands which correspond to asymmetric stretching vibrations. There is a systematic shift in stretching frequencies between the glasses and the corresponding liquids. We propose a structural model of these glasses involving a decrease in the coordination number of Zr with increasing temperature. This model, involving Zr coordination changes above T g, associated with breaking of Zr–F–Zr bridging groups, provides an explanation of the observed high degree of fragility of fluorozirconate melts.