Fast Cation Conduction and Anion Rotational Disorder in the High-Temperature Phase of Lithium Sodium Sulfate

Abstract

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.