Mixed-cation LixAg1-xPO3 glasses studied by 6Li, 7Li, and 109Ag stimulated-echo NMR spectroscopy

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 LixAg1-xPO3 glasses. Comparison of stimulated-echo decays from Li-6 or Li-7 NMR with that from Ag-109 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.