Low-frequency motions in an alkali phosphate glass studied by 7Li and 31P NMR Original Research Article

A 31P and 7Li NMR study of the phosphate glass Na0.5Li0.5PO3 leads to the following dynamical picture. Below Tg, only the Li ions diffuse slowly in the glass structure. This motion seems unaffected by the glass transition and presents a linear frequency dependent spectral density in the two probed frequency ranges. This behaviour, classical in glasses, could be explained by soft potential modes occuring with a high density in this disordered material. Above Tg, 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 structural relaxation is only effective on the NMR parameters near 600 K, well above Tg.