Local Coordination about La³⁺ in Molten LaCl₃ and Its Mixtures with Alkali Chlorides

Abstract

The local structure around the La(3+) ions in molten LaCl(3) and its mixtures with alkali and alkaline earth chlorides has been investigated by using extended X-ray absorption fine structure (XAFS) and molecular dynamics (MD) techniques. Such mixtures, which are of current technological interest, are known to be thermodynamically nonideal, and there has been a good deal of work to understand the structural effects factors that contribute to the nonideality. New experimental methods allow observations at the La K-absorption edge at the high temperatures of interest, and the ability of the technique to obtain reliable information even at very low La(3+) concentrations in multicomponent mixtures is demonstrated. Both the mean La(3+)-Cl(-) interionic separation and the mean La(3+) coordination number are found to decrease as the concentration of La(3+) in the mixture decreases. The rate of decrease depends on the identity of the alkali and alkaline earth cations present in the mixtures in a way that parallels the degree of nonideality of the different systems; it is greatest for those alkali cations that coordinate Cl(-) weakly. In dilute mixtures with such cations La(3+) is able to adopt a very stable octahedral coordination geometry but this is inhibited by the presence of more strongly coordinating cations like Li(+) and Mg(2+).

Raman Spectroscopic Study of Rare Earth Chlorides in Alkali Chloride Eutectic Melts

“…This suggests that the O h symmetry of REECl 6 3– for heavy lanthanides remains also in the molten LiCl‐KCl eutectic system. Another EXAFS study shows that the coordination number of La 3+ (5 mol %) in the molten LiCl‐KCl eutectic is 6.27 8. This coordination number might be interpreted as possible coexistence of LaCl n 3– n complexes with coordination number n higher than 7.…”

“…The local structure of La 3+ in alkali chloride melts has been studied by extended X‐ray absorption fine structure (XAFS) analysis8 and computer simulations 8,9. The number of Cl – around La 3+ is six dominantly in the melt with La concentrations ≤ 25 mol %,9 and the number increases from six to eight with the increase of La concentration 8,9. Even in the system with small La concentration, addition of Li + increases the number of Cl – around La 3+ 8.…”

“…The number of Cl – around La 3+ is six dominantly in the melt with La concentrations ≤ 25 mol %,9 and the number increases from six to eight with the increase of La concentration 8,9. Even in the system with small La concentration, addition of Li + increases the number of Cl – around La 3+ 8. This phenomenon is preferably observed for lighter lanthanides.…”

Local structural analyses on molten terbium fluoride in lithium fluoride and lithium–calcium fluoride mixtures

“…In a recent study, it has been reported that the structural disorder around La 3+ in molten chlorides by the mixing with divalent cation, e.g. Mg 2+ , is much larger than by the mixing with alkali cations and these facts are related to the ionic radii of cations, number densities of Cl À and charges of solvent melts [23]. There would be a common feature both in fluoride and chloride systems, thus the solvent effect in molten fluoride systems should be investigated in detail using other techniques.…”

Molecular dynamics study of the transport properties and local structures of molten binary systems (Li, Na)Cl, (Li, K)Cl and (Na, K)Cl

“…Owing to the high temperature, volatility and corrosion difficulties in molten salt experiments and the large number of mixing ratio, molecular dynamics simulations have been proved to be a useful tool in the investigation of mixed molten halides, such as molten AF-ZrF 4 system [10,11], and molten ACl-MCl 3 [12]. Many industrial processes actually adopt molten alkali halide mixtures to improve the properties, and mixing process of molten alkali halide salts have been discussed widely.…”