Molecular dynamics simulations were made of fast ion-conducting phases which were based upon the present material. An inter-ionic potential model was developed which provided a good fit to experimentally measured cation diffusion coefficients. The static distribution of mobile cations was explained in terms of the occupancy of interstitial sites, and anion-cation coordination. It was shown that the cations were usually bonded to one, and often two, O atoms of neighboring sulfate groups. It was concluded that the latter played an essential role in the cation diffusion mechanism. The diffusion coefficients were partitioned in a manner which made it possible to identify separately the contributions which arose from center-of-mass displacements and rotations of the sulfate groups. It was proposed that the diffusivity of Li+ was increased by up to 50% by anion reorientation. The equivalent increase in the case of Na+ was some 30%. The similarity of Li+ and Na+ diffusion coefficients in the pure and Na-doped materials was attributed to a balance between mass and size effects.

M.Ferrario, M.L.Klein, I.R.McDonald: Molecular Physics, 1995, 86[4], 923-38