$^6$Li and $^7$Li NMR line-shape and stimulated-echo studies of lithium ionic hopping in LiPO$_3$ glass
Sandra Faske, Hellmut Eckert, and Michael Vogel

$^6$Li and $^7$Li NMR are used to investigate the lithium ion dynamics in LiPO$_3$ glass. In particular, for the first time, $^6$Li 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 $^6$Li NMR correlation functions of LiPO$_3$ 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 $\langle \tau \rangle$ resulting from these stimulated-echo experiments is described by an activation energy $E_a\!=\!0.66~\mathrm{\,eV}$. The values of $\langle \tau \rangle$ are in good agreement with time constants from previous electrical and mechanical relaxation studies. At appropriate temperatures, the $^6$Li and $^7$Li NMR spectra are superpositions of a broad and a narrow spectral component, which result from slow and fast lithium ions, respectively, on the NMR timescale. A detailed analysis of the temperature dependence of these lineshapes provides information about the distribution of correlation times.

© 2008 The American Physical Society.