Temperature Dependence of Short and Intermediate Range Order in Molten MgCl₂ and Its Mixture with KCl

Abstract

This article addresses the non-Debye−Waller temperature behavior in the intermediate range order for molten MgCl 2 and its mixtures with KCl from a theory, Molecular Dynamics, and experimental X-ray scattering perspective and puts these findings in the context of discussions and controversies extending at least four decades. We find that these liquids are defined by two structural motifs. The first motif is associated with chains of positive−negative charge alternation; the second motif, which results in a prepeak in the structure function S(q), is associated with the interaction of Mg 2+ and Cl − ions that do not belong to the same charge alternation chain or aggregate. Our complementary X-ray scattering and computational results provide a quantitative explanation for the increase in intensity of the prepeak with temperature as opposed to the behavior of other peaks following normal Debye−Waller behavior. Temperature has opposite effects on the prevalence of each of the two structural motifs, and the enhancement of one pattern appears to be at the detriment of the other. Whereas the intensity in S(q) associated with the charge alternation motif is diminished at higher temperature, the opposite is true for the prepeak associated with intermediate range order due to the second structural motif.

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium

“…What may at rst seem counterintuitive, the observed shrinking of the cation-anion closecontact distances is quite typical for inorganic salts and is associated with a decrease in the coordination numbers on melting, oen leading to shorter nearest-neighbour bond lengths than those observed in the solid state. 25,[28][29][30] The characteristically broad peaks in D(r) at the highest temperature associated with the liquid state make the interpretation of scattering results difficult, requiring more sophisticated procedures in the data analysis.…”

“…The MgCl 2 (anhydrous, >98%) was purchased from Sigma-Aldrich and further puried by fractional distillation described previously. 25,29 The KCl (anhydrous, 99.999%) and CrCl 3 (anhydrous, 99.9%) were both purchased from Sigma-Aldrich and used for experiments as received. Great care was taken so that the salts were maintained under a vacuum or inert atmosphere for the duration of sample preparation.…”

A Brief Guide to the Structure of High-Temperature Molten Salts and Key Aspects Making Them Different from Their Low-Temperature Relatives, the Ionic Liquids

“… (a) Inorganic molten salt mixture (MgCl 2 /KCl) at 1073 K. (b) Experimental setup typically used for high-energy X-ray scattering measurements: 1, quartz tube; 2, thermocouple; 3, heater; 4, salt sample. Reprinted with permission from ref ( 61 ). Copyright 2020 American Chemical Society.…”

“… 187 Our group has studied the case of MgCl 2 and its mixtures with KCl and what we found is that MgCl 2 is a networked liquid with chains of Cl – -decorated Mg 2+ ions. 58 , 61 , 84 These in-network Mg 2+ ions share Cl – counterions, but there are nearby Mg 2+ ions that are either part of a different network or simply form a cluster and do not share Cl – counterions with the first network. The distance between Mg 2+ ions sharing counterions is different from that of those not sharing counterions.…”

Developing Practical Models of Complex Salts for Molten Salt Reactors

“…The effect can be seen in in situ studies of molten salts, such as EXAFS, and the results from molecular dynamics calculations. [9][10][11][12] Means for addressing these issues have been developed as they are common to systems beyond salts. Pelton and Blander [13] noted that SRO is observed in alloy liquids that are combinations of relatively electropositive and electronegative metals (e.g., Cu-Au, Mg-Bi, and Li-Pb), and to a great extent in silicate-containing systems via the formation of orthosilicate ions, as well as in salt systems.…”