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

Abstract

The transport properties of molten LiF-YF 3 mixtures have been studied by pulsed field gradient nuclear magnetic resonance spectroscopy, potentiometric experiments, and molecular dynamics simulations. The calculated diffusion coefficients and electric conductivities compare very well with the measurements across a wide composition range. We then extract static (radial distribution functions, coordination numbers distributions) and dynamic (cage correlation functions) quantities from the simulations. Then, we discuss the interplay between the microscopic structure of the molten salts and their dynamic properties. It is often considered that variations in the diffusion coefficient of the anions are mainly driven by the evolution of its coordination with the metallic ion (Y 3+ here). We compare this system with fluorozirconate melts and demonstrate that the coordination number is a poor indicator of the evolution of the diffusion coefficient. Instead, we propose to use the ionic bonds lifetime. We show that the weak Y-F ionic bonds in LiF-YF 3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward the one of yttrium cation when the content in YF 3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.

Pierre Turq, an inspirational scientist in charge and at interfaces

“…Mixtures of LiF-NaF-ZrF 4 and LiF-YF 3 with varying compositions were investigated [87][88][89]. In all cases, a very good agreement between experiments and simulations was obtained.…”

“…This property was chosen because of its collective nature. If the self-diffusion coefficients are also known [88], it allows to determine the importance of the cross-correlations between the ionic species. It therefore provides an excellent test of the interaction potentials used in the simulations.…”

Evidence of dynamical local scale distribution heterogeneities in CsF-AF (A=Li, Na, K and Rb) molten salts

“…58,32,19 The residence times of the fluorine anions in the coordination shell of the alkaline cations is much shorter than in the A c c e p t e d M a n u s c r i p t 9 one of rare earth, actinide and transition metal cations. 67,68 Therefore, in the case of alkali fluorides mixtures, only free fluorines are considered. However, the character "free" of fluorine in molten alkali has to be balanced according the alkaline.…”

An in situ spectroscopic study of the local structure of oxyfluoride melts: NMR insights into the speciation in molten LiF–LaF3–Li2O systems

“…[1][2][3] In molten fluorides, such information has been experimentally brought by Raman, 4-6 NMR 7-10 and EXAFS [11][12][13][14] spectroscopies. [1][2][3] In molten fluorides, such information has been experimentally brought by Raman, 4-6 NMR 7-10 and EXAFS [11][12][13][14] spectroscopies.…”

“…Molten fluorides are particularly interesting systems because their liquid structure may switch from a simple bath of charged hard spheres to a loose ionic network or a liquid with free 'complexes'. 2 In molten oxides, Raman [17][18][19] and NMR 20,21 spectroscopies have also been employed but mainly X-ray 22,23 and neutron scattering. 16 It turned out that the lifetime of these configurations is crucial for the physical and chemical properties of the melt.…”