Structure and dynamics in yttrium-based molten rare earth alkali fluorides.
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 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-YF3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward one of the yttrium cation when the content in YF3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.
Electrochemical behaviour of thorium(IV) in molten LiF–CaF2 medium on inert and reactive electrodes
Abstract The electrochemical behaviour of the Th(IV)/Th system was examined in molten LiF–CaF 2 medium on inert (molybdenum), reactive (nickel) and liquid (bismuth) electrodes in the 810–920 °C temperature range by several electrochemical techniques. Experimental results showed that (i) thorium fluoride was reduced in a single step exchanging 4 electrons and limited by thorium ions diffusion in the solution, (ii) the oxide ions induce the precipitation of Th(IV) in the form of thorium oxide (ThO 2 ), in a process involving as intermediate compound a soluble oxifluoride (ThOF 2 ), (iii) the reduction of thorium ions on reactive (Ni and liquid Bi) electrodes yields compounds Ni–Th and Bi–Th with a potential shift of around 0.7 V (for Ni and Bi) more anodic than the reduction of Th(IV) on inert substrate.
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