Nuclear magnetic and conductivity relaxations by Li diffusion in glassy and crystalline LiAlSi4O10

The dependence of lithium diffusion on the degree of structural order was investigated in LiAlSi4O10 in its glassy and polycrystalline states by nuclear magnetic resonance (NMR) and ionic conductivity relaxation techniques. The NMR measurements were performed in the temperature range from 150 K to 1000 K, which covers the diffusion induced spin-lattice relaxation rate (T-1(-1)) maximum for each of the modifications. The dependence of T-1(-1) on the Larmor frequency was observed in the range from 11.3 MHz to 38.9 MHz. Impedance spectroscopy was carried out on the glassy sample in the frequency interval from 100 Hz to 10 MHz and for temperatures between ambient temperature and 670 K. The results obtained from the two methods reflect significant deviations of the relaxation processes from their respective standard models (BPP and Debye) with regard to anomalous frequency dependences of T-1(-1) and of the conductivity, and a difference between the low and high temperature activation energies. These anomalies are discussed in terms of the dynamic structure model and other theories for anomalous relaxation in ionic conductors. When the results are analyzed in the framework of the dynamic structure model there are indications for clustering of the mobile species which results in correlated motion of Li ions. In a crystalline sample, the activation energy is larger and the jump frequency is smaller than in the amorphous material.