LOW-FREQUENCY MOTIONS IN AN ALKALI PHOSPHATE-GLASS STUDIED BY LI-7 AND P-31 NMR

A P-31 and Li-7 NMR study of the phosphate glass Na0.5Li0.5PO3 leads t o the following dynamical picture. Below T-g, only the Li ions diffuse slowly in the glass structure. This motion seems unaffected by the gl ass transition and presents a linear frequency dependent spectral dens ity in the two probed frequency ranges. This behaviour, classical in g lasses, could be explained by soft potential modes occurring with a hi gh density in this disordered material. Above T-g, the phosphate units of the polymer chains begin to move isotropically giving rise to the so-called secondary relaxation in glasses while the primary or structu ral relaxation is only effective on the NMR parameters near 600 K, wel l above T-g.