Lithium ion conduction mechanism in LiInI4 studied by single crystal 7Li NMR

Conduction mechanism of Li+ cation in a stoichiometric LiInI4 was studied experimentally. In general, the interpretations of the 7Li NMR powder spectrum and its temperature dependence are not straightforward because of a broadening due to not only dipole–dipole interactions but quadrupole effect (7Li, I = 3/2). In this NMR experiment on LiInI4, the quadrupole effect and the dipole–dipole interactions could be discussed separately using a single crystal. Since LiInI4 belongs to a monoclinic crystal, two pairs of satellite transitions (Δm = 3/2 ↔ 1/2 and − 3/2 ↔ − 1/2 ) as well as a central transition (Δm = − 1/2 ↔ 1/2) were observed for 7Li NMR due to the first order quadrupole effect. Above 380 K, however, these two satellite pairs merged into one pair associated with the motional narrowing phenomenon. This finding directly suggests a site exchange of the Li+ between two crystallographically equivalent sites. The exchange rate (kex) was estimated using a spectrum simulation of the coalescence phenomena and, furthermore, the bulk ionic conductivities were calculated as a function of temperature which agreed well to those from the complex impedance method. The site exchange phenomenon especially appeared on the single crystal NMR spectra was proved to be a good technique to evaluate ionic conductivity and its mechanism in solid.