Structure and transport properties of LiF–BeF2 mixtures: Comparison of rigid and polarizable ion potentials#

Abstract

Molecular dynamics simulations are performed to study the structure and dynamics of the LiF–BeF2 system over a range of compositions using the transferable rigid-ion model (TRIM). The densities obtained with the TRIM potential are approximately 17–20% lower than the experimental values while polarizable ion models (PIM) give densities within 5% of the experimental value. The TRIM and PIM potentials give essentially identical radial distribution functions (RDFs) for Li–F and Be–F ion pairs though the Be–Be pair correlation differ significantly and reflect the corresponding density differences. The variation in the radial distribution functions with concentration, particularly the anion–anion pair correlation function, reflects the reorganization of the fluoride ions as the addition of BeF2 in the mixture promotes the formation of the tetrahedral fluoroberyllate network. Along the 67 mole% LiF isopleth, diffusivities and Nernst–Einstein ionic conductivities from simulations using the PIM and TRIM potentials are in good agreement for temperatures up to 925 K. The viscosity data using the PIM model is also found to be in good agreement with the TRIM results presented here along the 873 K isotherm for compositions ranging from 0 to 50 mole% BeF2. The main conclusion from this study is that the non-polarizable, TRIM provides reasonable results for the structural correlations and transport properties of the LiF–BeF2 system in comparison with first-principles-based, PIM. Behaviour of ionic conductivity as a function of temperature, calculated using a rigid ion model is compared with experiment and with a polarizable ion model for the LiF–BeF2 system at 33 mole% BeF2. Simulation snapshots of the LiF–BeF2 melt at concentrations of 10 and 70 mole% BeF2 along the 1500K isotherm are shown on the right. The green, blue and red coloured spheres represent the lithium, beryllium and fluorine, respectively.