Structural analysis of alkali cations in mixed alkali silicate glasses by 23Na and 133Cs MAS NMR

We report the structural analysis of Na+ and Cs+ in sodium cesium silicate glasses by using 23Na and 133Cs magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. In the NMR spectra of cesium silicate crystals, the peak position shifted to higher magnetic field for structures with larger Cs+ coordination numbers and to lower magnetic field for smaller Cs+ coordination numbers. The MAS NMR spectra of xNa2O-yCs2O-2SiO2 (x = 0, 0.2, 0.33, 0.5, 0.66, 0.8, 1.0; x + y = 1) glass reveal that the average coordination number of both the alkali cations decreases with increasing Cs+/(Na+ + Cs+) ratio. In addition, the coordination number of Na+ in xNa2O-yCs2O-2SiO2 glass is smaller than that of Cs+. This difference between the average coordination numbers of the alkali cations is considered to be one structural reason of the mixed alkali effect.