EVIDENCE OF COMPLEX IONIC MOTION IN XLI(2)S-X)B2S3 GLASSY FAST IONIC CONDUCTORS FROM LI-7 AND B-11 NMR AND IONIC-CONDUCTIVITY MEASUREMENTS((1)

Ion dynamics in the highly conductive glassy fast ionic conductor, 0.7 Li(2)S + 0.3B(2)S(3), were studied with NMR line width and nuclear spi n-lattice relaxation, R(1)(omega,T), of both mobile Li-7 and immobile B-11 nuclei, NMR pulsed field gradient diffusivity, and electrical (io nic) 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 abou t 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-pe aked distribution, Z(Li), of activation energies, E(a) for the hops, w here 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 low est barriers. The apparent correlation time tau(B)((3)) derived from s pin-lattice relaxation of B-11 in trigonal BS3 groups, R(1B)(BS3), wer e much shorter than tau(B)((4)) from R(1B)(BS4), which again were shor ter than tau(Li) from R(1Li), although all relaxations are due to fluc tuating 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 w hich 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 informa tion on the correlation time for Li+ hopping motion which are consiste nt, within our model, with the sigma(omega,T) and R(1)(omega,T) data.