Resonant echo study on ion dynamics in superionic conductors

在多晶和单晶LiNbO_3的微粒子中进行了从室温到900 K的双脉冲声子回波测量，研究了声子回波中锂离子扩散与衰减时间T2的关系。的衰减时间是依赖于温度，在多晶样品中显示出快速下降800 K以上。这种异常的起源解释在一个简单的德拜型松弛由于锂离子扩散。锂离子扩散的活化能被评估为0.93 eV的多晶样品中，这是可比的从离子电导率和NMR结果的报道值。而LiNbO_3单晶在900 K以下没有锂离子扩散的迹象。在900 K以上的温度下，由于单晶样品中锂离子扩散的激活能较高，锂离子扩散对T_2的贡献占主导地位。用核磁共振技术在锂过渡金属氧化物LiCuO_2和Li_xV_2O_5中观察到Li^+离子的扩散。LiCuO_2的活化能与LiCoO_2相当。在Li_xV_2O_5中观察到大于0.5eV的激活能，这可能是由于高温下ε、δ和γ相之间的相变引起的。

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.