CATION SITES IN MIXED-ALKALI PHOSPHATE-GLASSES

Two lithium/sodium mixed-alkali phosphate glasses were investigated wi th solid-state two-dimensional heteronuclear correlation NMR. The syst ems represent two distinct families of mixed-alkali glasses, as they c ontain a fixed sodium-to-lithium ratio of 1:3, but different overall c oncentrations of cations. In both glasses, direct NMR evidence is foun d for the presence of multiple sodium cation environments using freque ncy-selective editing of two-dimensional correlation spectra. Frequenc y-specific subspectra were obtained by projection of Na-23 resonances correlated to distinct phosphate tetrahedral environments. Dynamic-ang le-spinning (DAS) NMR provides for high-resolution subspectra of quadr upolar Na-23 nuclei allowing spectral separation of multiple distinct sodium environments. Identification of distinct cation sites is import ant for understanding the physical origin of the 'mixed-alkali effect' , where non-linear behavior of physical properties is observed in glas ses as a function of relative cation populations. (C) 1998 Elsevier Sc ience B.V. All rights reserved.