SILICON SITE DISTRIBUTIONS IN AN ALKALI SILICATE GLASS DERIVED BY 2-DIMENSIONAL SI-29 NUCLEAR-MAGNETIC-RESONANCE

A common approach to quantify Q((n)) species in silicate glasses is to use Si-29 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and assume that the overlapping isotropic chemical shift distribution s of Q((n)) species are Gaussian, We have shown that a two-dimensional isotropic/anisotropic Si-29 NMR experiment can not only determine the distributions of Q((n)) species without any a priori assumptions abou t the distribution, but can also provide over an order of magnitude im provement in the precision of Q((n)) species quantification in silicat e glasses. Using this approach we have investigated an alkali silicate glass of composition 2Na(2)O . 3SiO(2) and have observed a small conc entration of Q((4)) in a sample mainly having Q((2)) and Q((3)). We ha ve found that the distribution oi isotropic chemical shifts for each o f the Q((n)) is approximately Gaussian. The relative populations of Q( (2)), Q((3)), and Q((4)) Obtained from these separated distributions g ive an equilibrium constant of 0.0129 +/- 0.0001 for the disproportion ation reaction 2 Q((3)) reversible arrow Q((2)) + Q((4)). This value i s slightly higher than what is obtained from analyzing the one-dimensi onal MAS spectrum alone, thus revealing a higher degree of disorder in speciation and configurational entropy for the glass.