Characterization of the structure of calcium alumino-silicate and calcium fluoro-alumino-silicate glasses by magic angle spinning nuclear magnetic resonance (MAS-NMR)

Abstract

Calcium aluminosilicate and calcium fluoro-aluminosilicate glasses have been characterized by 29Si, 27Al and 19F MAS-NMR. The two calcium aluminosilicate glasses examined were based on the composition 2SiO 2 · Al 2O 3 · 2CaO (ART1) and the mineral anorthite 2SiO 2 · Al 2O 3 · CaO (ART2). The observed chemical shifts for 29Si and 27Al agreed with previous studies. The fluorine containing glasses were based on 2SiO 2 · Al 2O 3 · (2− X)CaO · XCaF 2. The 29Si chemical shift moved in a negative direction with increase fluorine content indicating a progressive reduction in the average number of non-bridging oxygens, NBO, attached to a silicon. The 27Al spectra indicated the presence of four coordinate aluminium in the glasses with X=0.0–0.75, but aluminium was present in Al(IV), Al(V) and Al(VI) coordination states in the highest fluorine content glass with X=1.0. The 19F spectra indicated the presence of F–Ca( n) in low fluorine content glasses and both F–Ca( n) and Al–F–Ca( n) in high fluorine content glasses. We speculate here that the Al–F–Ca( n) species are oxyfluorides [AlO x F y ] n− , where x=1–6, y=1–6 and n is the charge on the total complex when aluminium is in Al(IV), Al(V) and Al(VI) coordinate states. The reduction in the average number of NBO per silicon with increasing fluorine content is explained by fluorine converting Ca 2+ to F–Ca( n).