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

TL;DR

This study investigates the microscopic structure and transport properties of molten LiF-YF3 mixtures using experiments and simulations, revealing that ionic bond lifetime better explains diffusion behavior than coordination number.

Contribution

It introduces the ionic bonds lifetime as a key indicator for diffusion in molten salts, challenging the traditional reliance on coordination numbers.

Findings

Diffusion coefficients and conductivities match experimental data across compositions.

Ionic bonds lifetime correlates better with diffusion than coordination number.

Weak Y-F bonds prevent fluoride diffusion from converging to yttrium cation diffusion.

Abstract

The transport properties of molten LiF-YF3 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 accross 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 (Y3+ 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-YF3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward the one of yttrium cation when the content in YF3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.