High-Resolution Multinuclear NMR Structural Study of Binary Aluminosilicate and Other Related Glasses

Abstract

The structure of homogeneous binary SiO 2 −Al 2 O 3 glasses with 0.4 to up to 12.0 wt % Al 2 O 3 have been studied using high-resolution 27 Al, 17 O, and 29 Si NMR spectroscopic techniques. All glasses are found to contain a mixture of 4-, 5-, and 6-fold coordinated Al sites (Al IV, Al V, and Al VI ). The relative proportions of these sites are strongly dependent on composition with Al IV (Al V ) being most dominant in glasses with <1 wt % (≥7 wt %) Al 2 O 3 . On the other hand, no significant dependence of Al speciation on fictive temperature is observed. The coordination polyhedra of a significant fraction of the Al VI sites in these glasses are found to be unusually distorted, similar to that in the case of crystalline Al 2 SiO 5 polymorphs. The 17 O NMR spectra show the presence of three types of oxygen sites, Si−O−Al V, Si−O−Al IV, and Si−O−Si, in these glasses. The 29 Si MAS NMR spectra corroborate these results and reveal the presence of Q 4 sites with and without Al next-nearest neighbors. It is hypothesized that the tetrahedral [AlO 4/2 ] -1 units in glasses with <1 wt % Al 2 O 3 are predominantly charge balanced by the formation of oxygen triclusters, whereas the Al V and Al VI sites play the role of charge compensators in glasses with higher Al content. Addition of either low field strength alkali ions such as K + or high field strength rare earth ions such as La 3+ to these glasses results in charge balance and stabilization of Al IV sites.