7Li nuclear spin-lattice relaxation and ionic conductivity measurements of LiI-doped Li2S+GeS2+B2S3 glasses (figure 3) were performed in order to investigate the ion hopping dynamics and the non-Arrhenius conductivity behavior that had also been observed in some Ag fast-ion-conducting glasses. The nuclear magnetic resonance nuclear spin-lattice relaxation experiments were performed at 4 and 8MHz and at 70kHz in the rotating frame at 183 to 523K. Conductivity measurements of these glasses were performed over the same temperature range to determine whether the commonly observed non-Arrhenius ionic conductivity in Ag fast-ion-conducting glasses was also observed in Li fast-ion-conducting glasses. A previously developed distribution of activation energies model was used to fit both the nuclear spin-lattice relaxation and the conductivity results. It was found that a bimodal distribution of activation energies was required in order to fit the broad nuclear spin-lattice relaxation maximum. One distribution of activation energies was associated with Li ions residing in anion sites created by tetrahedral B units in the thioborate structural regions of the glass, and the other was associated with Li ions residing in the anion sites created by the non-bridging S units in the thiogermanate regions of the glass. The average activation energy for Li ions residing in the thioborate and thiogermanate sites in the ternary glasses agreed very well with the average activation energies for Li ions in pure binary thioborate and thiogermanate glasses, respectively, with the thiogermanate energies being significantly larger (~45 instead of ~30kJ/mol) than those for the thioborate sites. This trend was in agreement with the fact that the thiogermanate structures possessed non-bridging S units whereas the thioborate structures did not. It was found that some of the non-Arrhenius conductivity behavior could be associated with the bimodal distribution of activation energies and the conductivity could be largely fitted to the distribution of activation energies model. However, the strong deviation from Arrhenius behavior at high temperatures could not be accounted for and an extension of the distribution of activation energies model was therefore proposed. The effect of a small fraction of mobile ions which were thermally excited above the barriers was considered. They were assumed to conduct via many (temperature-dependent) filled sites before reaching a second unoccupied site. This ion-trapping model explained well the high-temperature deviation of the conductivity from Arrhenius behavior.

NMR Spin-Lattice Relaxation and Ionic Conductivity in Lithium Thioborogermanate Fast-Ion-Conducting Glasses. B.Meyer, F.Borsa, D.M.Martin, S.W.Martin: Physical Review B, 2005, 72[14], 144301 (13pp)

Figure 3

Ionic Conductivity of zLiI-(1-z)[xLi2S+(1-x)(0.5GeS2+0.5B2S3)] Glasses

(a: z = 0.3 and x = 0.55, b: z = 0 and x = 0.45)