6Li and 7Li NMR line-shape and stimulated-echo studies of lithium ionic hopping in LiPO3 glass

Li-6 and Li-7 NMRs are used to investigate the lithium ion dynamics in LiPO3 glass. In particular, Li-6 NMR stimulated-echo experiments are used to provide straightforward access to two-time correlation functions characterizing the lithium ionic hopping motion in the millisecond regime in a glassy ion conductor. Temperature-dependent measurements serve to separate the spin diffusion contribution and the dynamic contribution to the stimulated-echo decays. The Li-6 NMR correlation functions of LiPO3 glass describing the lithium ionic motion show pronounced nonexponential decays, which can be well described by a stretched exponential function with a temperature-independent small stretching parameter beta=0.27, indicating the complex nature of the lithium dynamics. The temperature dependence of the mean correlation times resulting from these stimulated-echo experiments is described by an activation energy E-a=0.66 eV. The values of are in good agreement with time constants from previous electrical and mechanical relaxation studies. At appropriate temperatures, the Li-6 and Li-7 NMR spectra are superpositions of a broad and a narrow spectral component, which result from slow and fast lithium ions, respectively, on the NMR time scale. A detailed analysis of the temperature dependence of these line shapes provides information about the distribution of correlation times.