Preferential Binding of Fluorine to Aluminum in High Peralkaline Aluminosilicate Glasses

Abstract

For two series of fluoride-containing aluminosilicate glasses of high peralkaline type, we apply 27Al, 19F, 29Si, and 23Na NMR spectroscopy to understand the structural changes introduced by the addition of alkali fluorides. Adding fluoride in concentrations above the solubility limit causes crystallization of different phases in sodium and potassium glasses despite identical composition. However, the NMR spectra reveal that the structural evolution of the precrystallized states is similar in both series. In particular, fluorine coordinates exclusively to alkaline cations and aluminum. No indication of direct bonding with silicon was found from 19F --> 29Si cross-polarization experiments. In contrast to other glass systems, double resonance experiments in these peralkaline systems show that halide addition produces at most a minor fraction of tetrahedral aluminum containing fluorine in its coordination sphere. Instead, the fluorine addition prior to crystallization converts up to about 20% of the initial tetrahedral aluminum (1 mol % in absolute units) to 5- and 6-fold coordinated aluminum. A minor portion of five-coordinated aluminum groups is considered as the intermediate to the growing fraction of octahedral aluminum in the silicate matrix. The initialization of the crystallization process is correlated with the saturation of the silicate matrix by octahedral aluminum clusters segregating out under further doping by fluoride. It is suggested that the formation of the nonframework Al-F bonds is responsible for structural relaxation, reflected by the reduction of the glass transition temperature.

MAS-NMR studies of glasses and glass-ceramics based on a clinopyroxene–fluorapatite system

“…As the Al:Si ratio is increased, the Al is incorporated in the network as a Q 4 (4Si) species, which increases the crosslinking of the glass structure. In contrast to previous reports of similar fluoro-aluminosilicate glasses 36,37 , for the However, in contrast to these studies, in our glasses, though the ratio of Al:F (Table 3) is close to the previous compositions, we have much higher fraction of Group II Na NMR data indicates that the distribution of sodium sites is invariant within the series.…”

Influence of Gd2O3 on thermal stability of oxyfluoride glasses

“…(3) indicates that the activation energy for aluminosilicate glass (FGA_0) is slightly higher than Si-O bond dissociation energy (360-430 kJ mol -1 ) in silicate glasses [18]. [20]. That is why it can be assumed that obtained in the study values apply to aluminosilica oxide network crystallization.…”

Effect of fluoride ion incorporation on the structural aspects of barium–sodium borosilicate glasses

“…Remaining 15% is the contribution from À52, À9, and 116 ppm peaks. Based on the previous 19 F MAS NMR studies on different types of borosilicate glasses containing F À ions [7][8][9][10][11]13], the peaks around 50, 30, and À9 ppm have been attributed to FNa(x)Ba(y) type linkages having unknown values of x and y. Structural units having highest value of x appears around À9 ppm and that having highest value of y appears around 50 ppm.…”

“…In a related study [6] we have also reported that simultaneous incorporation of both thorium and uranium oxides in barium-sodium borosilicate glass results in the phase separation of ThO 2 when uranium oxide incorporation exceeds 5 wt% along with 15.87 wt% of ThO 2 in the glass. There are numerous investigations on the structural aspects of fluoride ion containing silicate/alumino-silicate glasses investigated by 19 F NMR and XPS techniques [7][8][9][10][11]. Based on these studies it has been concluded that most of the F À ions in the glass exist as F-M(n) (fluorine with unknown number of network modifying cations like Na + , Ca 2+ , Ba 2+ etc.)…”