SPECTROSCOPIC INVESTIGATIONS OF FLUOROZIRCONATE GLASSES IN THE TERNARY-SYSTEMS ZRF4-BAF2-AF (A=NA, LI)

Ternary fluorozirconate glasses in the systems ZrF4-BaF2-NaF and ZrF4- BaF2-LiF have been examined by infrared and Raman spectroscopies. The glass compositions were constrained along compositional joins with a c onstant Ba/Na (Li) ratio = 1 or a constant F/Zr ratio = 5 (5.5). The h igh-frequency Raman and IR bands, assigned to symmetric and asymmetric Zr-F stretching vibrations of fluorozirconate groups, show systematic changes with the F/Zr ratio for both melts and glasses. The observed increase in the frequency of these two bands with decreasing F/Zr rati o is related to structural changes involving either a decreasing Zr co ordination number or an increased connectivity between ZrFn polyhedra caused by increasing the number of bridging fluorines. The Ba/Na (Li) ratio only affects the IR active band assigned to asymmetric stretchin g of Zr-F bonds, which is proposed to be more sensitive to the effect of the counter cations on the symmetry of ZrFn polyhedra than is the R aman band assigned to symmetric stretching. The correlation of vibrati onal frequencies with the F/Zr ratio also demonstrated that the high-f requency IR and Raman bands increase significantly upon going from the glass to the melt. We propose that this is due to a decreasing coordi nation number of Zr with increasing temperature, which reflects the kn own fragility of fluorozirconate melts.