CORRELATION-FUNCTIONS FOR IONIC MOTION FROM NMR RELAXATION AND ELECTRICAL-CONDUCTIVITY IN THE GLASSY FAST-ION CONDUCTOR (LI2S)0.56(SIS2)0.44

The Li-7 NMR spin-lattice relaxation and the electrical conductivity i n the typical glassy fast-ion conductor (Li2S)0.56(SiS2)0.44 are discu ssed from models of Li+ ionic motion with distributions of activation energies, as well as from stretched-exponential time-correlation funct ions. The measured correlation times from the two effects differ by tw o orders of magnitude, and the derived distributions are shifted great ly relative to each other. We relate the great differences to percolat ion around the high barriers in the distribution. We present a phenome nological theory that yields good quantitative fits to the observed NM R relaxation with a Gaussian distribution, and to the conductivity and related dielectric properties with the continuous-time random-walk mo del and the same Gaussian truncated at the percolation limit. This cor relates the two effects in a simple and effective way; both time-corre lation functions can be calculated approximately from the distribution s, and even the dc conductivity can be calculated from the NMR results . The present approach is discussed and compared with previously propo sed models to explain the anomalies in ac electrical-conductivity and NMR relaxation rates in glassy fast-ion conductors.