Local structure in gallium‐ and germanium‐substituted silicate glasses investigated by magic angle spinning nuclear magnetic resonance

Abstract

Local structural effects occurring with the substitution of gallium for aluminum and germanium for silicon in synthetic glasses with albite, jadeite, and nepheline analogue compositions are studied by 29Si, 27Al, and 23Na magic angle spinning nuclear magnetic resonance. There are systematic shifts to low field of the single broad resonance peak in each spectrum with progressive substitution by the larger cations. The effect of substitution of the next nearest neighbor tetrahedral cations (and the change in local structure) on chemical shift is calculated using model structures simulated with a computer graphics structure‐modeling program. Chemical shifts calculated for a random arrangement of tetrahedral cations are in closer agreement with observed chemical shifts than those calculated for ordered local structures complying with aluminum/gallium avoidance and having sodium coupled with aluminum/gallium cations. The observed shifts to low field with the substitution of aluminum or gallium for silicon or germanium are explainable only in part by the next nearest neighbor effect. A decrease in mean bond angle at the bridging oxygen atom, attributable to an increase in the number of small‐membered rings of TO4 tetrahedra, could account for further low‐field shifts, thus corroborating the findings of Henderson et al. (1985) from Raman spectroscopy.