ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Structure of Fluorozirconate Glasses and Melt

S. Aasland and T. Grande

Department of Inorganic Chemistry, Norwegian University of Science and Technology, N-7034 Trondheim, Norway

Abstract: We present a thermodynamic model for fluorozirconate melts which takes into consideration the strong negative deviation from ideality manifested by deep eutectic wells in phase diagrams and exothermic heats of mixing. The model is based on chemical equilibria between complexes with 6-8-fold coordination of Zr. Only 6-coordinated complexes are present in the glass-forming regions. The results of the thermodynamic model are related to structural data obtained by infrared and Raman spectroscopy. Frequencies of Zr-F stretching vibrations have been found to decrease with increasing n(F)/n(Zr) ratio for both melts and glasses, which indicates either an increasing Zr coordination number or decreased connectivity between ZrF_n, polyhedra. At constant n(F)/n(Zr) ratio the stretching frequencies increase significantly going from the glass to the melt. We propose that the coordination number of Zr in fluorozirconate glasses is decreasing significantly when heating above the glass transition temperature. The fragility of fluorozirconate glasses shown by the non-Arrhenius behaviour of the viscosity is thereby explained by a decreasing coordination number of Zr causing the connectivity between ZrF_n polyhedra to decrease. The viscosity of fluorozirconate melts demonstrates that the fragility is not very sensitive to changes in composition. However, both the glass transition temperature and the viscosity increase with increasing charge of the counter-cations.

Full paper in Portable Document Format: 521a21.pdf

Chemical Papers 52 (1) 21–28 (1998)