In Situ Experimental Evidence for a Nonmonotonous Structural Evolution with Composition in the Molten LiF−ZrF4 System

Transferable Deep Learning Potential Reveals Intermediate-Range Ordering Effects in LiF-NaF-ZrF4 Molten Salt

LiF-NaF-ZrF₄ multicomponent molten salts are promising candidate coolants for advanced clean energy systems owing to their desirable thermophysical and transport properties. However, the complex structures enabling these properties, and their dependence on composition, is scarcely quantified due to limitations in simulating and interpreting experimental spectra of highly disordered, intermediate-ranged structures. Specifically, size-limited ab initio simulations and accuracy-limited classical models used in the past are unable to capture a wide range of fluctuating motifs found in the extended heterogeneous structures of liquid salt. This greatly inhibits our ability to design tailored compositions and materials. Here, accurate, efficient, and transferable machine learning potentials are used to predict structures far beyond the first coordination shell in LiF-NaF-ZrF₄. Neural networks trained at only eutectic compositions with 29% and 37% ZrF₄ are shown to accurately simulate a wide range of compositions (11-40% ZrF₄) with dramatically different coordination chemistries, while showing a remarkable agreement with theoretical and experimental Raman spectra. The theoretical Raman calculations further uncovered the previously unseen shift and flattening of bending band at ∼250 cm^-1 which validated the simulated extended-range structures as observed in compositions with higher than 29% ZrF₄ content. In such cases, machine learning-based simulations capable of accessing larger time and length scales (beyond 17 Å) were critical for accurately predicting both structure and ionic diffusivities.

Computational methods to simulate molten salt thermophysical properties

Molten salts are important thermal conductors used in molten salt reactors and solar applications. To use molten salts safely, accurate knowledge of their thermophysical properties is necessary. However, it is experimentally challenging to measure these properties and a comprehensive evaluation of the full chemical space is unfeasible. Computational methods provide an alternative route to access these properties. Here, we summarize the developments in methods over the last 70 years and cluster them into three relevant eras. We review the main advances and limitations of each era and conclude with an optimistic perspective for the next decade, which will likely be dominated by emerging machine learning techniques. This article is aimed to help researchers in peripheral scientific domains understand the current challenges of molten salt simulation and identify opportunities to contribute.

In Situ Determination of Speciation and Local Structure of NaCl-SrCl2 and LiF-ZrF4 Molten Salts

Understanding the local environment of the metal atoms in salt melts is important for modeling the properties of melts and predicting their behavior and thus helping enable the development of technologies such as molten salt reactors and solar-thermal power systems and new approaches to recycling rare-earth metals. Toward that end, we have developed an in situ approach for measuring the coordination of metals in molten salt coupling X-ray absorption spectroscopy (XAS) and Raman spectroscopy. Our approach was demonstrated for two salt mixtures (1.9 and 5 mol % SrCl₂ in NaCl, 0.8 and 5 mol % ZrF₄ in LiF) at up to 1100 °C. Near-edge (X-ray absorption near-edge structure, XANES) and extended X-ray absorption fine structure (EXAFS) spectra were measured. The EXAFS response was modeled using ab initio FEFF calculations. Strontium's first shell is observed to be coordinated with chlorine (Sr²⁺-Cl^-) and zirconium's first shell is coordinated by fluorine (Zr⁴⁺-F^-), both having coordination numbers that decrease with increasing temperature. Multiple zirconium complexes are believed to be present in the melt, which may interfere and distort the EXAFS spectra and result in an anomalously low zirconium first shell coordination number. The use of boron nitride (BN) powder as a salt diluent for XAFS measurements was found to not interfere with measurements and thus can be used for investigations of such systems.

Probing the ionic structure of FLiNaK–ZrF4 salt mixtures by solid-state NMR

In this study, by applying ¹⁹F, ²³Na and ⁷Li high-resolution NMR methods, the evolution of the [Zr_xF_y]^4x−y local ionic structures in FLiNaK–ZrF₄ salt mixtures were elucidated. K₃ZrF₇, Na₃ZrF₇ and Na₇Zr₆F₃₁ crystal phases were identified when the melt salts were being solidified. The distribution of these [Zr_xF_y]^4x−y species was dependent on the content of ZrF₄ in FLiNaK eutectic salts. Moreover, K₃ZrF₇ phase transition from an orthorhombic lattice into a disordered cubic lattice was clarified, thereby causing dynamics of the coordinated F⁻ ions to be reduced and the well-ordered crystal lattices to be destroyed. These mentioned results provide a further insight into the Zr–F based ionic structure and the formation of the disordered Zr–F structure in ZrF₄-based eutectic salts.

The evolution of the [Zr_xF_y]^4x−y ionic structures in FLiNaK–ZrF₄ salt mixtures was elucidated through solid-state NMR techniques when the melt salts were being solidified.

Investigation of the local structure of molten ThF4-LiF and ThF4-LiF-BeF2 mixtures by high-temperature X-ray absorption spectroscopy and molecular-dynamics simulation

The microscopic structures of ThF₄-LiF and ThF₄-LiF-BeF₂ molten salts have been systematically investigated by in situ high-temperature X-ray absorption fine-structure (XAFS) spectroscopy combined with molecular-dynamics (MD) simulations. The results reveal that the local structure of thorium ions was much more disordered in the molten state of the ThF₄-LiF-BeF₂ salt than that in ThF₄-LiF, implying that the Th and F ions were exchanged more frequently in the presence of Be ions. The structures of medium-range-ordered coordination shells (such as Th-F_2nd and Th-Th) have been emphasized by experimental and theoretical XAFS analysis, and they play a significant role in transport properties. Using MD simulations, the bonding properties in the molten ThF₄-LiF and ThF₄-LiF-BeF₂ mixtures were evaluated, confirming the above conclusion. This research is, to the best of our knowledge, the first systematic study on the ThF₄-LiF-BeF₂ molten salt via quantitative in situ XAFS analysis and MD simulations.

In situ high-temperature EXAFS measurements on radioactive and air-sensitive molten salt materials

The development at the Delft University of Technology (TU Delft, The Netherlands) of an experimental set-up dedicated to high-temperature in situ EXAFS measurements of radioactive, air-sensitive and corrosive fluoride salts is reported. A detailed description of the sample containment cell, of the furnace design, and of the measurement geometry allowing simultaneous transmission and fluorescence measurements is given herein. The performance of the equipment is tested with the room-temperature measurement of thorium tetrafluoride, and the Th-F and Th-Th bond distances obtained by fitting of the EXAFS data are compared with the ones extracted from a refinement of neutron diffraction data collected at the PEARL beamline at TU Delft. The adequacy of the sample confinement is checked with a mapping of the thorium concentration profile of molten salt material. Finally, a few selected salt mixtures (LiF:ThF₄) = (0.9:0.1), (0.75:0.25), (0.5:0.5) and (NaF:ThF₄) = (0.67:0.33), (0.5:0.5) are measured in the molten state. Qualitative trends along the series are discussed, and the experimental data for the (LiF:ThF₄) = (0.5:0.5) composition are compared with the EXAFS spectrum generated from molecular dynamics simulations.

Design and characterization of a 2-(2′-hydroxyphenyl)benzimidazole-based Sr2+-selective fluorescent probe in organic and micellar solution systems

Fluorescence detection of Sr(ii) by Sr(ii)-selective fluorescent probe BIC and its complex formed in solution.

An in situ spectroscopic study of the local structure of oxyfluoride melts: NMR insights into the speciation in molten LiF-LaF3-Li2O systems

The local structure of molten LaF3-LiF-Li2O has been investigated by high temperature NMR spectroscopy. The (139)La and (19)F chemical shifts have been measured as a function of temperature and composition. The NMR spectra show that Li2O reacts completely with LaF3 to form a LaOF compound in the solid state below the melting temperature of the sample. LaOF is not completely dissolved in the fluoride melt and solid LaOF is observed in the (19)F spectra for Li2O concentrations above 10 mol%. We discuss the local environment of lanthanum ions in molten LaF3-LiF-Li2O and compare the results to those with the LaF3-LiF-CaO system. The analysis of the temperature and Li2O concentration dependences of the (139)La and (19)F chemical shifts suggests that several kinds of lanthanum oxyfluoride long-lived LaOxFy(3-x-y) units are present in the melt.

Chemical interactions between zirconium and free oxide in molten fluorides

The chemical interactions between zirconium and free oxide in FLiNaK melts at different zirconium to oxide (n_Zr/n_O) molar ratios were studied by means of a carbothermal-reduction technique (LECO oxide analyzer) and Raman spectroscopy.

The PAW/GIPAW approach for computing NMR parameters: a new dimension added to NMR study of solids

In 2001, Mauri and Pickard introduced the gauge including projected augmented wave (GIPAW) method that enabled for the first time the calculation of all-electron NMR parameters in solids, i.e. accounting for periodic boundary conditions. The GIPAW method roots in the plane wave pseudopotential formalism of the density functional theory (DFT), and avoids the use of the cluster approximation. This method has undoubtedly revitalized the interest in quantum chemical calculations in the solid-state NMR community. It has quickly evolved and improved so that the calculation of the key components of NMR interactions, namely the shielding and electric field gradient tensors, has now become a routine for most of the common nuclei studied in NMR. Availability of reliable implementations in several software packages (CASTEP, Quantum Espresso, PARATEC) make its usage more and more increasingly popular, maybe indispensable in near future for all material NMR studies. The majority of nuclei of the periodic table have already been investigated by GIPAW, and because of its high accuracy it is quickly becoming an essential tool for interpreting and understanding experimental NMR spectra, providing reliable assignments of the observed resonances to crystallographic sites or enabling a priori prediction of NMR data. The continuous increase of computing power makes ever larger (and thus more realistic) systems amenable to first-principles analysis. In the near future perspectives, as the incorporation of dynamical effects and/or disorder are still at their early developments, these areas will certainly be the prime target.