Resonant echo study on ion dynamics in superionic conductors

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Two-pulse phonon echo measurements were performed in the micro particles of polycrystalline and single crystalline LiNbO_3 from room temperature to 900 K to investigate the relationship between lithium ionic diffusion and decay time, T2, in the phonon echo. The decay time was dependent on temperature, showing a rapid decrease above 800 K in the polycrystalline sample. The origin of this anomaly is interpreted in terms of a simple Debye-type relaxation due to the lithium ionic diffusion. The activation energy of the lithium ionic diffusion was evaluated to be 0.93 eV in the polycrystalline samples, which is comparable with reported values from ionic conductivity and NMR results. On the other hand, single crystalline LiNbO_3 showed no sign of lithium ionic diffusion up to 900 K. The contribution to T_2 from the lithium ionic diffusion would be dominant at higher temperatures than 900 K because of much higher activation energy in the single crystalline sample. These facts show that the decay time in the phonon echo is strongly connected with ionic diffusion in the particles.Li^+ ionic diffusion was observed in lithium transition-metal oxides, LiCuO_2 and Li_xV_2O_5 using NMR technique. The activation energy of LiCuO2 is comparable with that of in LiCoO_2. In Li_xV_2O_5, the large activation energy beyond 0.5 eV, which might arose from the phase transition among of ε, δ and γ-phase at high temperatures, was observed.