L. Van Wullen et al., Structure-property correlations in lithium phosphate glasses: New insightsfrom P-31 <-> Li-7 double-resonance NMR, CHEM MATER, 12(7), 2000, pp. 1840-1846
The macroscopic properties of binary alkali phosphate glasses are character
ized by the presence of unexpected minima in the glass transition temperatu
res at alkali concentrations near 20-25 mol %. In the present study, the st
ructural origin of this behavior is explored on the basis of multinuclear s
ingle- and double-resonance NMR experiments. P-31 magic-angle spinning NMR
spectra track the successive network depolymerization caused by Li2O additi
on, while Li-6 chemical shifts indicate a lithium coordination number of fo
ur to five, The monotonic evolution of these spectra with increasing alkali
content reveals that the nonlinear macroscopic behavior does not reflect a
ny abrupt structural changes at the local level. In contrast, P-31{Li-7} ro
tational echo double-resonance spectroscopy indicates substantial restructu
ring 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 s
ite increases abruptly near 20-25 mol % Li2O, as the phosphate chains becom
e cross-linked by electrostatic Li+-oxygen interactions.