Local structure of alkalis in mixed alkali borate glass to elucidate the origin of mixed-alkali effect

Abstract

We report the structural analysis of Na⁺ and Cs⁺ in sodium cesium borate crystals and glasses using ²³Na and ¹³³Cs magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The composition dependence of NMR spectra of the borate was similar to that of the silicate: (1) the peak position of cesium borate crystals shifted to upfield for structures with larger Cs⁺ coordination numbers, (2) the MAS NMR spectra of xNa₂O-yCs₂O-3B₂O₃ (x = 0, 0.25, 0.5, 0.75, 1.0, x + y = 1) glass showed that the average coordination number (CN) of both the alkali cations decreases with increasing Cs⁺/(Na⁺ + Cs⁺) ratio. However, the degree of decrement in borates is much smaller than that in silicates. We have considered that the small difference in CN is due to 4-coordinated B, because it is electrically compensated by the alkali metal ions resulting in the restriction of having various coordinations of O to alkali metal.