Structural properties of sodium phosphate glasses from 23Na → 31P cross-polarization NMR

Abstract

One-dimensional heteronuclear correlation solid-state nuclear magnetic resonance (NMR) experiments are utilized to determine local structural changes in sodium phosphate glass compositions with ratios of Na/P of 0.25, 0.78, 1 and 1.3. Glasses containing only Q 2 (metaphosphate composition), Q 2 and Q 1 (between meta- and pyrophosphate compositions), and Q 3 and Q 2 (ultraphosphate region) have been investigated. 23Na → 31P cross-polarization magic-angle-spinning (CPMAS) NMR detects direct interactions between distinct phosphorus sites and sodium nuclei in the second coordination sphere. Variable-contact CPMAS experiments provide additional information about association of 23Na nuclei with specific phosphate sites. Time constants describing the transfer of magnetization from sodium to phosphorus nuclei are found to be affected by sodium ion concentration in ultraphosphate glasses, and an important finding of these investigations is the degree of correlation of sodium ions with Q 3 sites in ultraphosphate glasses.