A First-Principles Description of Liquid BeF₂ and Its Mixtures with LiF:  2. Network Formation in LiF−BeF₂

Abstract

A polarizable ionic interaction potential, constructed from first-principles calculations, is used to examine the structure, vibrational spectra, and transport properties of molten mixtures of LiF and BeF2 across a range of compositions. The simulations reproduce the experimentally measured vibrational frequencies of the fluoroberyllate (BeF4(2-)) ions, which form in the melt, as well as conductivity and viscosity values across the composition range. Examination of the structures of the melts reveals the emergence of a slowly relaxing network of BeF4 units as the concentration of BeF2 is increased. The relationship between the appearance of the network and the composition dependence of the transport properties is explored.

Including many-body effects in models for ionic liquids

“…37 We could show that our model performed very well in reproducing the polymeric fluoroberyllate speciation and the vibrational properties in these melts. 87 Calculated dynamic properties (electrical conductivity, viscosity) also agree quantitatively with the experimental data: 90 To illustrate this, we show a comparison of the calculated 87 and experimental 88 viscosities of LiF-BeF 2 mixtures with varying compositions at T = 873 K on figure 5. The interfacial properties were also tested by calculating the surface tension from a simulation of the liquid-vapor interface.…”

Structure and dynamics in yttrium-based molten rare earth alkali fluorides

“…These quantities seem to show that the first solvation shell of Y 3+ is more weakly bound than the one of Zr 4+ in molten fluorides. In the case of the Li + -F − pair, the RDF has a shape very similar to the case of other molten fluoride mixture 32 . The extraction of precise coordination numbers is a difficult experimental task, for example by extended X-ray absorption fine structure (EXAFS) spectroscopy, that leads to errors of the order of 20 % for liquid state systems.…”

Fluctuating ionic polarizabilities in the condensed phase: first-principles calculations of the Raman spectra of ionic melts

“…In figure 5, we show the experimental polarized and depolarized spectra for the room temperature glass measured by Galeener et al 34 , together with the corresponding spectra calculated from the simulations of the liquid. The Raman spectrum of BeF 2 is much more structured than that of LiF due to the persistence of quasi-molecular ionic complexes, like BeF 2− 4 , on much longer timescales than the Raman observation 32,40 . We were unable to find an experimental spectrum for the liquid phase but, to the extent that there is no major shift in the equilibria governing the formation of these species due to the change of temperature, we can expect the liquid and glassy spectra to be comparable.…”

“…Firstly, we create several sets of ionic configurations for the materials of interest. They are generated by molecular dynamics simulations using high quality interaction potentials obtained by ab initio force-fitting as described previously 31,32,39 .…”

“…Alongside cryolite 5,24 , the electrolyte involved in aluminium extraction, it is by far the best-characterized fluoride melt. We have already determined interaction potentials for these mixtures and compared simulation results with a wide range of observable properties 32,39,40 in the liquid and solid state.…”