Relating composition, structural order, entropy and transport in multi-component molten salts

Abstract

Molecular dynamics simulations of the LiF-BeF(2) molten salt mixture are used to establish relationships between composition, structural order, entropy, and transport properties of multi-component ionic liquids. A sharp rise in tetrahedral order associated with formation of the fluoroberyllate network occurs for compositions with BeF(2) concentrations greater than that of the Li(2)BeF(4)-BeF(2) eutectic. The excess entropy of the liquid in this regime, within the pair correlation approximation, is strongly correlated with the local tetrahedral order. The different degree of participation of beryllium, fluorine, and lithium ions in the cooperative dynamics of the fluoroberyllate network can be related to the degree of deviation from Rosenfeld-type excess entropy scaling, with the lithium ions remaining essentially unaffected by the liquid state network. We demonstrate that the deviations from Nernst-Einstein and Stokes-Einstein behaviour emerge only in temperature-composition regimes where tetrahedral order strongly correlates with the pair entropy. Implications for understanding structure-property relationships in other ionic liquids, such as molten salts, oxide melts, and RTILs are considered.

Stokes‐Einstein relation in liquid iron‐nickel alloy up to 300 GPa

“…According to the entropy scaling laws proposed by Rosenfeld and Dzugutov, the transport coefficient of liquid can be expressed as single‐valued functions of the excess entropy S ex , which can be calculated from experimental data. While the entropy scaling laws were initially formulated for simple liquids, recent simulations show that the scaling laws are applicable to a much wider variety of liquids than originally assumed, including water, core‐softened fluids, model polymeric melts, molecular fluids, and multicomponent ionic melts [e.g., Abramson , ; Abramson and West‐Foyle , ; Agarwal et al , ; Errington et al , ; Goel et al , ; Jabes and Chakravarty , ; Krekelberg et al , ]. More recently, the entropy‐scaling laws have been examined extensively and found to be valid for monatomic liquid metals over a wide range of pressure [e.g., Cao et al , , , ].…”

Water and water-like liquids: relationships between structure, entropy and mobility

“…The LiF-BeF 2 system is a convenient example of this class of systems, with applications as an amorphous electrolyte and a solvent in molten salt reactors. 51,[122][123][124] Fig. 10 shows the phase diagram of the system.…”

Excess-entropy scaling in supercooled binary mixtures

“…Equation ( 1 ) enables prediction of unknown transport coefficients for a given system if its excess entropy is known. Later studies revealed that the exponential behavior of the excess entropy does not apply for supercooled liquids whereas excess-entropy scaling in the form may still apply 25 , 31 , 34 , 40 . Furthermore, the quasiuniversal relation of single-component atomic liquids was found to break down for, e.g., molecules which in general do not show quasiuniversal behavior 25 , 56 , 57 .…”