Abstract
Quasielastic neutron scattering and high-frequency (10 MHz to 60 GHz) conductivity measurements have been performed on the high-temperature phase of LiNaSO4, a typical representative of a group of simple inorganic salts exhibiting both fast-cation conduction and anion rotational disorder.
The quasielastic neutron spectra are composed of two Lorentzian contributions which are consistently attributed to cation diffusion and anion rotational motion. The narrower component is dominated by incoherent sodium scattering; it shows the characteristics of jump diffusion with a jump distance of 3.8 Å and residence times between 14 ps and 22 ps.
The broader quasielastic component reflects the anion dynamics with typical reorientation times of 2 ps. The reduced Q-dependent intensities are in good agreement with the model of isotropic diffusion. The quasielastic intensity in the low Q regime (Q < 1 Å−1) indicates the involvement of cations in the reorientational motion of the translationally fixed anions.
Analysis of the conductivity data in terms of diffusivities points to an unusual cation conduction mechanism: the Haven ratio, H
R = D
*/D
σ, turns out to be considerably larger than one. This behavior, rarely observed in a typical fast ion conductor, can be traced back to a charge correlation factor which is clearly smaller than unity, indicating that charge transport is less effective than tracer transport in this material.
摘要
对 LiNaSO4 的高温相进行了准弹性中子散射和高频(10 MHz 至 60 GHz)电导率测量,LiNaSO4 是一组同时表现出快速阳离子传导和阴离子旋转无序的简单无机盐的典型代表。
准弹性中子谱图由两个洛伦兹分量组成,这两个分量始终归因于阳离子扩散和阴离子旋转运动。较窄的分量由非相干钠散射主导;它显示了跃迁扩散的特征,跃迁距离为 3.8 Å,停留时间介于 14 ps 和 22 ps 之间。
较宽的类弹性成分反映了阴离子动力学,其典型的重新定向时间为 2 ps。随 Q 值降低的强度与各向同性扩散模型十分吻合。低 Q 值(Q < 1 Å-1)下的类弹性强度表明阳离子参与了平移固定阴离子的重新定向运动。
根据扩散率对电导率数据的分析表明了一种不同寻常的阳离子传导机制:哈文比(H
R = D
*/D
σ,结果远远大于 1。在典型的快速离子导体中很少观察到这种行为,它可以追溯到电荷相关因子,该因子明显小于 1,表明在这种材料中,电荷传输不如示踪传输有效。