Lithium Ion Motion in LiZr2(PO4)3

Abstract

An interionic potential to investigate Li ion motion in the rigid framework of PO4 tetrahedra and ZrO6 octahedra of LiZr2(PO4)3a Nasicon-type superionic conductorhas been developed by fitting to the low-temperature X-ray structure and conductivity data at 700 K. A molecular dynamics simulation employing this potential function has been carried out. A detailed analysis of the molecular dynamics trajectories suggests that the proposed interionic potential can predict properties of the LiZr2(PO4)3 in good agreement with known X-ray, NMR, calorimetric, conductivity, and other data. The transition from normal to superionic conductor takes place between 550 and 600 K and is accompanied by a peak in the constant volume specific heat, suggestive of a higher order transition as well as migration of Li ions from crystallographic 6b (site 1) to 18e (site 2) position. The activation energy and path of migration of Li ions from site 1 to 2 are in good agreement with experiment. Density contours surrounding sites 1 and 2, which reveal the exact geometry of the void space around these sites, are in excellent qualitative agreement with thermal ellipsoid parameters obtained from X-ray diffraction. The frequency of vibration of Li ion in the two sites are found to depend strongly on the geometry of the void space at these sites dictated by the potential energy surface.