Multiple frequency spin-lattice relaxation time and ionic conductivity measurements of Li 2S + SiS 2 glasses from 1 Hz to 40 MHz

Abstract

Many measurements have been made of the temperature dependence of the spin-lattice relaxation time T 1 for ionically conducting glasses and the ubiquitous observation is that the activation energy for T 1 ranges from a factor 2–5 lower than the dc ionic conductivity activation energy for the same glass. This has prompted much deserved discussion and yet a definitive solution to the discrepancy has not yet been advanced. In this study, wide frequency range measurements of both T 1 and σ ac have been made on glasses in the highly conducting series Li 2S + SiS 2 to test the hypothesis that the source of the discrepancy lies not in two fundamentally different phenomena being measured by the two different techniques, but rather in the simpler fact that the timescales that they use to measure the same phenomena are vastly different. In NMR, the frequencies used are always above 1 MHz and always correspond to a frequency-dependent regime of T 1, whereas the conductivity is measured as a frequency-independent quantity. These highly conducting glasses provide the rare opportunity to compare activation energies for both NMR and conductivity under conditions of frequency independence and it is found that the two activation energies thus determined agree within experimental error. The source of the discrepancy reported until now is suggested to be due to the fact that the conductivities of glasses have not been high enough to show frequency-independent behavior at the NMR frequencies below T g.