Quasichemical short-range order in high-entropy reciprocal melts: Li, Na, K‖F, Cl.

Abstract

We present a combined molecular dynamics and analytical study of quasichemical short-range order and entropy decomposition in the high-entropy reciprocal quaternary melt LiF-NaF-KF-LiCl-NaCl-KCl (equimolar composition Li1/6Na1/6K1/6F1/4Cl1/4) at 900 K. Using a machine-learning interatomic potential trained on ab initio data, we compute all fifteen partial radial distribution functions and the corresponding coordination numbers. The simulations reveal pronounced chemical short-range order: Li+ is strongly preferentially coordinated by F- (NLi-F ≈ 2.88), whilst K+ is predominantly surrounded by Cl- (NK-Cl ≈ 4.86), with Na+ occupying a mixed environment (NNa-Cl/NNa-F ≈ 1.56). This selective pairing produces local LiF-like structural motifs, manifested as enhanced F-F correlations at short distances. To clarify the thermodynamic origin of the observed order, the entropy is decomposed within the charged hard-sphere model into ideal translational (Sackur-Tetrode), configurational, hard-sphere (MCSL) and coulombic (Blum MSA) contributions. The large negative excess entropy arising from steric and electrostatic interactions (together ≈-9kB per ion) is largely compensated by the ideal translational term. The results demonstrate that high configurational entropy and quasichemical short-range order can coexist in a dense multicomponent ionic liquid, and provide a quantitative link between reciprocal thermodynamics and atomistic structure.

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