6 Li and 7 Li NMRs are used to investigate the lithium ion dynamics in LiPO 3 glass. In particular, 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 ␤ = 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 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 time scale. A detailed analysis of the temperature dependence of these line shapes provides information about the distribution of correlation times.

The relation between structure and ion dynamics in the three polymorphs of Ag(5)Te(2)Cl has been investigated using (109)Ag, (125)Te, and (35)Cl NMR spectroscopies. Specifically, the influence of the structural phase transitions observed near 240 K (P2(1)/c<-->P2(1)/n) and near 332 K (P2(1)/n<-->I4/mcm) upon silver ion mobilities has been studied by temperature dependent (109)Ag NMR lineshapes and spin-lattice relaxation times. While the superionic high temperature phase alpha-Ag(5)Te(2)Cl is characterized by a molten cationic sublattice, fast ion dynamics in the medium-temperature phase beta-Ag(5)Te(2)Cl occurs in spatially restricted regions comprising all the crystallographically distinct silver sites. Temperature dependent magic-angle-spinning linewidths yield an activation energy of 0.38 eV, consistent with 0.44 eV measured from dc electric conductivities. For the low-temperature gamma-modification, results of two- and three-time (109)Ag correlation spectroscopies provide a detailed insight into the nature of the silver ionic hopping motion. Temperature dependent jump rates measured by two-time correlation functions yield an activation energy E(a)= 0.48 eV. (109)Ag NMR three-time correlation functions indicate that the non-exponential relaxation behavior of the silver ions can be attributed to a broad distribution of jump rates rather than correlated forward-backward jumps. Nevertheless, all the silver ions are mobile down to temperatures of about 185 K.

We show that NMR multi-time correlation functions provide interesting new insights into the nature of lithium and silver ion dynamics in solids. For solid ion conductors, they usually probe the elementary jumps of the long-range charge transport. NMR two-time correlation functions yield rates and activation energies of the ionic hopping motion. Moreover, they reveal that the ionic relaxation exhibits a strong nonexponentiality in the studied crystals and glasses. NMR three-time correlation functions enable quantitative determination of the origin of the nonexponentiality. For various solid-state electrolytes, the analysis shows that pronounced dynamical heterogeneities govern the ionic hopping motion. If dynamical heterogeneities exist, NMR four-time correlation functions allow one to measure the time scale of exchange processes within the rate distributions. For silver ion dynamics in a crystalline and a glassy material, the results indicate that initially slow ions exhibit random new rates from broad rate distributions after very few jump events.

We show that NMR stimulated-echo experiments provide detailed information about the jump dynamics of each of the ionic species in mixed mobile ion glasses. The potential of this technique is exploited to measure two-time correlation functions of the lithium and silver ionic hopping motions in Li x Ag 1−x PO 3 glasses. Comparison of stimulated-echo decays from 6 Li or 7 Li NMR with that from 109 Ag NMR shows that the residence times at the ionic sites are significantly longer for the respective minority component than for the majority component at both ends of the composition range, while lithium and silver ions exhibit similar jump rates for x = 0.5. Substitution of silver by lithium results in a strong and continuous slowdown of the silver ionic jumps, whereas the lithium ionic jumps show a weaker dependence on the glass composition. In the vicinity of the conductivity minimum, the activation energies obtained from the stimulated-echo studies for both lithium and silver ionic jumps are significantly smaller than that obtained from the dc conductivity. This suggests that mixing of cation species promotes differences between short-range and long-range ionic motions. For all studied glass compositions and for both lithium and silver, we can rule out the existence of a significant fraction of truly immobile cations. However, broad distributions of jump rates lead to strongly nonexponential correlation functions of the ionic hopping motion. Interestingly, the correlation functions become more and more exponential when the observed cation species is successively replaced by the unobserved cation species. The present results suggest that dynamical heterogeneities and correlations of ionic motions, which involve like and unlike ions and length scales of several interatomic distances, are important aspects of ion dynamics in mixed mobile ion glasses.

BRINKMANN, C.; FASKE, S.; VOGEL, M.; NILGES, T.; HEUER, A.; ECKERT*, H.; Phys. Chem. Chem. Phys. 8 (2006) 3, 369-378; Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-48149 Muenster, Germany; Eng.) -Schramke 13-014