Analysis of NMR and conductivity-relaxation measurements in glassyLi2S-SiS2fast-ion conductors

Abstract

${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ nuclear spin-lattice relaxation (NSR) and Li-ion electrical-conductivity relaxation (ECR) data of (${\mathrm{Li}}_2$S${)}_{0.56}$(${\mathrm{SiS}}_2$ ${)}_{0.44}$, a glassy fast-ionic conductor, have been reanalyzed. Both NSR and ECR data are fitted by using a stretched exponential, exp[-(t/${\mathrm{\tau }}^{\mathrm{*}}$ ${)}^{\mathrm{\beta }}$], for the correlation function in each relaxation. It is found that ${\mathrm{\tau }}_{\mathit{s}}^{\mathrm{*}}$ for NSR can be several orders of magnitude longer than ${\mathrm{\tau }}_{\mathrm{\sigma }}^{\mathrm{*}}$ for ECR at the same temperature, ${\mathrm{\beta }}_{\mathit{s}}$ and ${\mathrm{\beta }}_{\mathrm{\sigma }}$ are not the same with ${\mathrm{\beta }}_{\mathit{s}}$<${\mathrm{\beta }}_{\mathrm{\sigma }}$, and the activation energy ${\mathit{E}}_{\mathit{s}}^{\mathrm{*}}$ of ${\mathrm{\tau }}_{\mathit{s}}^{\mathrm{*}}$ is significantly larger than ${\mathit{E}}_{\mathrm{\sigma }}^{\mathrm{*}}$ of ${\mathrm{\tau }}_{\mathrm{\sigma }}^{\mathrm{*}}$. In this contribution we show how these pronounced differences of the parameters in the correlation functions of NSR and ECR are explained in the framework of the coupling model. An additional predicted relation: ${\mathrm{\beta }}_{\mathit{s}}$ ${\mathit{E}}_{\mathit{s}}^{\mathrm{*}}$=${\mathrm{\beta }}_{\mathrm{\sigma }}$ ${\mathit{E}}_{\mathrm{\sigma }}^{\mathrm{*}}$ is found to be consistent with the experimental data and the common value of the two products is identified naturally with the true single ion activation energy. The constraints that these experimental data impose on any viable theory of the dynamics of carriers in glassy ionic conductors are discussed.