Anionic Species Determination in CaSiO₃ Glass Using Two-Dimensional ²⁹Si NMR

Abstract

The structure of the alkaline earth silicate glass CaSiO3 has been investigated using a two-dimensional 29Si NMR experiment that correlates the isotropic magic-angle-spinning (MAS) spectrum with an anisotropic off-magic-angle-spinning spectrum. Although the one-dimensional magic-angle-spinning (MAS) spectrum is completely unresolved, all five types of SiO4 tetrahedra (represented by the notation Q(n), where n = 0-4 representing the number of bridging oxygen) can be resolved and quantified on the basis of the separated anisotropic line shapes in the 2D spectrum. The distribution of isotropic chemical shifts derived from the 2D spectrum suggests that in the case of CaSiO3 glass the conventional approach of fitting the one-dimensional MAS spectrum with overlapping Gaussian line shapes would lead to significant errors in Q(n) quantification. The equilibrium constants for the disproportionation reaction Q(n) ⇌ Q(n-1) + Q(n+1) with n = 1, 2, and 3 were determined from the 2D spectrum to be 0.105 ± 0.019, 0.156 ± 0.005, and 0.106 ± 0.022, respectively. These results clearly indicate a significantly greater deviation from a binary model of Q(n) species disproportionation in alkaline earth silicate melts when compared to alkali silicate melts and thus suggest a relatively more disordered structure.