Complete Description of the LaCl₃–NaCl Melt Structure and the Concept of a Spacer Salt That Causes Structural Heterogeneity

Abstract

Lanthanides are important fission products in molten salt reactors, and understanding their structure and that of their mixtures is relevant to many scientific and technological problems including the recovery and separation of rare earth elements using molten salt electrolysis. The literature on molten salts and specifically on LaCl 3 and LaCl 3 −NaCl mixtures is often fragmented, with different experiments and simulations coinciding in their explanation for certain structural results but contradicting or questioning for others. Given the very practical importance that actinide and lanthanide salts have for energy applications, it is imperative to arrive at a clear unified picture of their local and intermediate-range structure in the neat molten state and when mixed with other salts. This article aims to unequivocally answer a set of specific questions: is it correct to think of long-lived octahedral coordination structures for La 3+ ? What is the nature as a function of temperature of networks and intermediate-range order particularly upon dilution of the trivalent ion salt? Is the so-called scattering first sharp diffraction peak (FSDP) for neat LaCl 3 truly indicative of intermediate-range order? If so, why is there a new lower-q peak when mixed with NaCl? Are X-ray scattering and Raman spectroscopy results fully consistent and easily described by simulation results? We will show that answers to these questions require that we abandon the idea of a most prominent coordination state for M 3+ ions and instead think of multiple competing coordination states in exchange due to significant thermal energy in the molten state.

Do Ionic Liquids Slow Down in Stages?

“…REMD produces smooth S­(q) changes as a function of temperature as opposed to regular MD starting from well equilibrated initial conditions where S­(q) features, particularly those in the region 0.5–0.6 Å –1 , appear to be very sensitive to the actual glass or low temperature liquid being trapped as can be gleaned from Figure S10 ( vide infra ). Figure shows between 1 and 2 Å –1 what we have commonly termed an “adjacency” peak which is associated with all sorts of intramolecular and intermolecular interactions between atoms that are close by (this peak is present for all liquids, but ILs often have two other peaks at lower q value). − Around 0.7–0.8 Å –1 there is what we have called a “charge alternation” peak linked with the spacing between positive moieties separated by an anion or with anions separated by positive charge. − The lowest q-peak below ≈0.5 Å –1 is what is commonly described as a prepeak or a first sharp diffraction peak and is associated with the distance between charge networks spaced by apolar domains. − Notice that at low temperature there is an additional small peak at around 0.5–0.6 Å –1 (blue bar). This peak has been previously observed experimenta...…”

Structural Origins of Viscosity in Imidazolium and Pyrrolidinium Ionic Liquids Coupled with the NTf2– Anion

“…The first thing to notice in Figure is that in the charge alternation regime around 0.8 Å –1 the intensities of the subcomponents, particularly those involving the high X-ray contrast anion ( S A–A ( q ) and S H–A ( q )) are very large compared to that of the overall S ( q ) in the same q -regime. In this regime, S A–A ( q ) and S H–H ( q ) always show as peaks, whereas S H–A ( q ) always shows as an antipeak; for all ionic liquids and molten salts, this behavior is the hallmark of charge alternation. ,− ,,,,− In this q-regime, the overall S ( q ) signal is to a large extent the result of massive cancellations due to the interference of S A–A ( q ), S H–H ( q ), and S H–A ( q ). Because of these cancellations of large numbers, it is not uncommon to find in this q -region that small flaws in force fields give rise to inaccuracies in the overall S ( q ) compared to experiments.…”

“…Figure shows sets of computationally derived subcomponents of S ( q ), specifically the “charge trio” head–head ( S H–H ( q )), anion–anion ( S A–A ( q )), and head–anion ( S H–A ( q )) with species defined in Figure ; for a description of the many ways one can partition the total S ( q ) of ILs into subcomponents, see refs ,− ,,,,− and citations therein. The charge trio subcomponents of S ( q ) are particularly important because at about 0.8 Å –1 they carry information about the structure of the charge network ( and correlations). ,− ,,,,− In the region 1–1.8 Å –1 , only one of these three subcomponents, namely, S H–A ( q ), is most relevant as it provides direct information about the very important adjacency correlations associated with positive and negative interactions along the charge network. S H–H ( q ) and S A–A ( q ) are less important in the q-regime associated with adjacency correlations because, except for intramolecular interactions, species with identical charge are not expected to be adjacent.…”

“…g d ij ( r , t ) is equal to 1 at long times, and at any time it goes to 1 at long distances. Just as we have done in several prior publications, S­( q ) and S ( q , t ) can be partitioned into useful subcomponents. − ,,− ,,,,− These allow us to follow the structural dynamics of liquid structural motifs at specific q values. In the Results and Discussion section, we will plot the time integral of the head–anion subcomponent of S ( q , t ) squared ( S H–A ( q , t ) 2 ).…”

Heterogeneous Structure, Mechanisms of Counterion Exchange, and the Spacer Salt Effect in Complex Molten Salt Mixtures Including LaCl3

“…Molten salt mixtures of the actinides or lanthanides are becoming quite useful for industry and in the energy sector. For example, UCl 3 –NaCl–MgCl 2 and PuCl 3 –NaCl–MgCl 2 are promising molten salt fast reactor fuel candidates, , LaCl 3 –KCl–MgCl 2 mixtures are used to synthesize various LaMg alloys used as tunable mirrors, and various Cl – -based mixture melts have been studied in the context of corrosion. − It is also important to have fast database access to thermodynamic properties of complex binary and ternary salt combinations. − There is a direct link between structural and thermodynamic studies such as ours and the quasi-thermodynamic models that are at the core of such databases since free-energetic properties in those are modeled based on the coordination number of the ions. − Of course, in reality it was found − that, at the temperatures at which salts are in the molten state, cations have an ensemble of interconverting coordination structures that are temperature dependent. For multivalent cations in combination with chloride, a significantly polarizable anion, there are further complexities.…”