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 constant Ba/Na (Li) ratio = 1 or a constant F/Zr ratio = 5 (5.5). The high-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 ratio is related to structural changes involving either a decreasing Zr coordination 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 stretching 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 Raman band assigned to symmetric stretching. The correlation of vibrational frequencies with the F/Zr ratio also demonstrated that the high-frequency IR and Raman bands increase significantly upon going from the glass to the melt. We propose that this is due to a decreasing coordination number of Zr with increasing temperature, which reflects the known fragility of fluorozirconate melts.