Evidence of complex ionic motion in xLi(2)S+(1-x)B2S3 glassy fast ionic conductors from Li-7 and B-11 NMR and ionic conductivity measurements

Abstract

Ion dynamics in the highly conductive glassy fast ionic conductor, 0.7Li(2)S + 0.3B(2)S(3), were studied with NMR line width and nuclear spin-lattice relaxation, R(1)(omega,T), of both mobile Li-7 and immobile B-11 nuclei, NMR pulsed field gradient diffusivity, and electrical (ionic) conductivity, sigma(omega,T) over wide ranges of temperature and frequency. Some measurements were also done in 0.65Li(2)S + 0.35B(2)S(3). The quadrupolar split NMR spectrum of the B-11 indicated that about 80% of the boron were in BS3 groups and 20% were in BS4 groups, and their relaxations could be resolved. We fitted R(1Li) with a double-peaked distribution, Z(Li), of activation energies, E(a) for the hops, where the peaks are due to the differing environments near BS3 and BS4. The conductivity, sigma(omega,T), was calculated quantitatively from Z(Li)(E(a)) in our model with random walk and percolation over the lowest barriers. The apparent correlation time tau(B)((3)) derived from spin-lattice relaxation of B-11 in trigonal BS3 groups, R(1B)(BS3), were much shorter than tau(B)((4)) from R(1B)(BS4), which again were shorter than tau(Li) from R(1Li), although all relaxations are due to fluctuating electric field gradients from the hopping Li+ ions. The times, tau(Li), are those of single ion hopping, while times tau(B) are due to the fluctuations of the electric field gradient at the BSx groups which was due to the combined effect of several Li+ ions hopping and to the interaction of the Li+ ion with the BSx groups where x = 3 or 4. The Li-7 NMR line narrowing and diffusivity yielded additional information on the correlation time for Li+ hopping motion which are consistent, within our model, with the sigma(omega,T) and R(1)(omega,T) data.