Structure−Property Correlations in Lithium Phosphate Glasses: New Insights from 31P ↔ 7Li Double-Resonance NMR

The macroscopic properties of binary alkali phosphate glasses are characterized by the presence of unexpected minima in the glass transition temperatures at alkali concentrations near 20−25 mol %. In the present study, the structural origin of this behavior is explored on the basis of multinuclear single- and double-resonance NMR experiments. 31 P magic-angle spinning NMR spectra track the successive network depolymerization caused by Li 2 O addition, while 6 Li chemical shifts indicate a lithium coordination number of four to five. The monotonic evolution of these spectra with increasing alkali content reveals that the nonlinear macroscopic behavior does not reflect any abrupt structural changes at the local level. In contrast, 31 P{ 7 Li} rotational echo double-resonance spectroscopy indicates substantial restructuring on longer distance scales, relating to the spatial arrangement of the lithium ions with respect to the phosphorus sites in the network. The data suggest that the average number of lithium atoms surrounding a phosphorus site increases abruptly near 20−25 mol % Li 2 O, as the phosphate chains become cross-linked by electrostatic Li + −oxygen interactions.