Interesting note on the coordination of ZrFx molten salts in the presence of Li, Na, K, Rb and Cs

[NNadir]

I've been going over a recent paper on the oxidation states of uranium in molten chloride salts (which in general I do not favor) and got to thinking about my favorite molten salts, those involving ZrF4. (Used nuclear fuel contains a lot of zirconium, both structural zirconium and fission product zirconium.)

I stumbled across this paper: In Situ Determination of Speciation and Local Structure of NaCl–SrCl2 and LiF–ZrF4 Molten Salts Timothy J. Lynch, Nancy R. Birkner, Matthew S. Christian, Jacob A. Wrubel, Juliano Schorne-Pinto, Arjen Van Veelen, John R. Bargar, Theodore M. Besmann, Kyle S. Brinkman, and Wilson K. S. Chiu The Journal of Physical Chemistry B 2022 126 (7), 1539-1550

The introduction gives why this system should be of interest:

The local structure and dynamics of molten salts is important for understanding and modeling their properties and predicting their behavior. This is helpful in enabling the development of technologies such as molten salt reactors, nuclear fuel processing, solar-thermal power systems and new approaches to recycling rare-earth metals. The local structure reflects solution properties such as composition/speciation, oxidation state, precipitating solid phases (melting points), phase formation, chemical potentials/redox for controlling corrosion and transport, viscosity, and vapor pressures.

The structure and dynamics of molten salts have been investigated over the past five decades using NMR spectroscopy, (1) Raman spectroscopy, (2) ultraviolet–visible–infrared (UV–vis–IR) spectroscopy, (3) and X-ray absorption spectroscopy (XAS) (4?6) to determine physical and chemical properties. (7?10) The spectroscopic and scattering methods have the specificity and sensitivity to measure the change of the actinide oxidation state, e.g., the reduction of U(IV) to U(III) in high-temperature alkali chloride melts, (3) as well as the local structure and bonding of species in solution such as interatomic distances and coordination numbers. (11,12) Additionally, computer-based simulations have previously been shown to be robust in predicting thermal properties. (8,9,13,14) Combining instrumental and computational approaches allows the study of molten salts to high temperature to determine how local structure impacts physical and chemical properties...

What struck me as of interest is this remark:

For larger alkali atoms (i.e., KF, RbF, and CsF), which are considered as glass progenitors, the coordination of zirconium decreases such that N = 8 is not observed in the KF–ZrF4 system. (20) Lower N values result in a phase diagram with a deeper eutectic (21) than that of LiF or NaF systems. In contrast, SrF4 with alkali fluorides is known to exhibit a constant N = 6 value. A benchmark in situ approach is needed to measure coordination numbers in salt melts.

Rb and Cs are fission products.

Reference 21 is an obscure Russian Journal from 1961: Novoselova, A. V.; Korenev, Y. M.; Simanov, Y. P. Research on the System LiF-ZrF4. Dokl. Akad. Nauk SSSR 1961, 139, 892– 894.

I should ask my son if he can get it, if it's in English. (He reads some Russian, but not all that much.)

This is very obscure, I know, but nonetheless it strikes me as something potentially of great importance.