Lithium Ion Disorder and Conduction Mechanism in LiCe(BH4)(3)Cl

Abstract

We investigate the diffusion mechanism of Li ions in LiCe(BH4)(3)Cl, which exhibits fast Li ion conduction. It was previously shown that eight Li ions partially occupy the 12d Wyckoff sites in the I (4) over bar 3m structure and the Li ion diffusion takes place via jumping through the three-dimensional network of the 12d sites. In this study, we employ first-principles-nudged elastic band simulation to elucidate the diffusion mechanism and discover that the Li ion does not directly jump to the neighboring 12d site, but instead passes through the closest 6b site. Moreover, the 6b site turns out to be another stable Li ion site, not just a transient point during a jump event. The occupation of the 6b site and the Li ion diffusion mechanism were assured by first-principles molecular dynamics simulations. The partial occupancy of the 12d site and 6b site at 500 K is approximately 1/2 and 1/3, respectively. The experimental diffraction data can be consistently interpreted. The peculiar crystal structure of LiCe(BH4)(3)Cl allowing efficient and fast Li ion diffusion is again highlighted together with the role of [BH4](-) ion in thermodynamically stabilizing LiCe(BH4)(3)Cl.