Probing the local structure of FLiBe melts and solidified salts by in situ high-temperature NMR

Abstract

The 2LiF-BeF₂ (FLiBe) salt melt is considered the primary choice for a coolant and fuel carrier for the generation IV molten salt reactor (MSR). However, the basics of ionic coordination and short-range ordered structures have been rarely reported due to the toxicity and volatility of beryllium fluorides, as well as the lack of suitable high-temperature in situ probe methods. In this work, the local structure of FLiBe melts was investigated in detail using the newly designed HT-NMR method. It was found that the local structure was comprised of a series of tetrahedral coordinated ionic clusters (e.g., BeF₄²⁻, Be₂F₇³⁻, Be₃F₁₀⁴⁻, and polymeric intermediate-range units). Li⁺ ions were coordinated by BeF₄²⁻ ions and the polymeric Be–F network through the analysis of the NMR chemical shifts. Using solid-state NMR, the structure of solid FLiBe solidified mixed salts was confirmed to form a 3D network structure, significantly similar to those of silicates. The above results provide new insights into the local structure of FLiBe salts, which verifies the strong covalent interactions of Be–F coordination and the specific structural transformation to the polymeric ions above 25% BeF₂ concentration.

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Introduction

There has been renewed interest in the research on high-temperature molten salts over the past decades due to the significant advantage of molten salts in areas related to generation IV nuclear energy e.g., molten salt reactors (MSR),^1–5 concentrating solar power plants (CSP),^6,7 and other clean energy storage industries.^8,9 It is known that many multiply charged metal ions can form coordinated local structures in molten salts, which have important effects on the physical and chemical properties of the melt.^10,11 The local structure was also important for alkali halide liquid salt, and different ionic structures can significantly adjust the ionic diffusion and dynamics, which is strongly coupled to the viscosity, density, and other salt thermophysical properties.^12,13 In the 1960s, FLiBe (2LiF-BeF₂) was employed as the coolant and nuclear fuel carrier for the molten salt reactor experiment (MSRE) project at Oak Ridge National Laboratory (ORNL).⁴ Since then, the LiF/BeF₂ binary salt has aroused continuous attention, and the local structure of the FLiBe salt has been widely investigated through theoretical calculation.^14–17 However, relevant experimental studies especially in situ studies on the molecular structure of FLiBe have been rarely reported due to the toxicity,^17–19 volatilization of beryllium fluorides,²⁰ and the lack of suitable high-temperature high-resolution analytical methods.

High-temperature optical absorption spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, Raman spectroscopy, and Nuclear Magnetic Resonance (NMR) spectroscopy have been widely used in the investigation of the structure of molten salts, which are always combined with theoretical calculations such as molecular dynamics (MD) or ab initio molecular dynamics (AIMD).²¹ Recently, these high-temperature methods have been reported in research on the structural studies about the alkaline-earth elements, especially magnesium. In 2020, Fei Wu et al.²² used an X-ray scattering perspective and MD simulations to explain the non-Debye–Waller temperature behavior in the intermediate range order for molten MgCl₂ and its mixtures with KCl. The result reveals that molten MgCl₂ conforms to a network structure rather than an isolated tetrahedral structure. In 2021, Santanu Roy et al.²³ employed X-ray scattering and Raman spectroscopy to study the local structure of molten MgCl₂. Combined with the AIMD simulations, the five-coordinate MgCl₅³⁻ complex ions were proven to exist in the molten pure MgCl₂ and ZnCl₂-MgCl₂. In addition, the structure of Ni (II) ions in molten ZnCl₂-MgCl₂ was also investigated by a multimodal approach combining high-temperature ultraviolet-visible absorption spectra,¹⁰ extended XAFS, AIMD simulations, and rate theory of ion exchange. The research reveals that the coordination states of Ni (II) rely on the temperature and content of molten salts. Despite all the above studies on molten salts, the experimental studies on the local structure of beryllium fluoride melts have rarely been reported. It is known that the X-ray method is quite difficult to apply on the ionic structure studies of beryllium salt melts because of the high X-ray transmittance of Be atoms.²⁴ Only some theoretical calculation studies on the ionic structure of beryllium melts were reported.^14–17 It has been found that the BeF₂-based beryllium fluoride mixed salts adopt an analogue to SiO₂ and silicates, which shows a 3D network structure with corner-sharing tetrahedrally coordinated Be²⁺ cations.²⁵ A series of the tetrahedral coordinated ionic clusters (e.g., BeF₄²⁻, Be₂F₇³⁻, and Be₃F₁₀⁴⁻) and the polymeric intermediate-range units may exist in FLiBe melts.¹⁵ Furthermore, the distribution of the different species and the thermo-physical properties as a function of composition were discussed through MD simulation.^14–17 Accordingly, systematic experimental local structure studies on LiF/BeF₂ binary salts have been particularly desirable in supporting the rapid development of MSR over the past few years.

Nuclear magnetic resonance (NMR) spectroscopy is a high-resolution and powerful method to examine the microstructure and dynamics of complicated multiple salt systems.²⁶ With the development of the high-temperature NMR (HT-NMR) method, the research on the ionic structure of molten salts has deepened. In this work, the local structure of FLiBe melts was investigated in detail using a newly designed HT-NMR method. A laser beam coupled with an optical fiber was used to construct the salt heating system for HT-NMR, which was systematically presented in our previous work.²⁷ The structure and the evolution of different coordinated cluster anions were discussed through the analysis of ¹⁹F, ⁷Li, and ⁹Be NMR chemical shifts. The strong covalent interactions of Be²⁺ ions were also discussed based on the above results. Furthermore, the structure of solid FLiBe solidified mixed salts at ambient temperature was also investigated by 1D and 2D solid-state NMR (ss-NMR). The network structure of the FLiBe glass state was certified, which should have close relations with the formation and the amount of the polymeric Be–F network during the solidification of FLiBe melts. Our work deepens the understanding of the local structure of FLiBe salts and demonstrates the significant potential of the NMR method on the structural studies of fluoride salts, which is closely related to their applications in the field of the development of new energy production technology.

Experimental section

Sample preparation

The LiF and BeF₂ were purchased from Aladdin. LiF-BeF₂ salts were synthesized as follows: 10 g of salts were weighed in proportion and mixed thoroughly in a nickel crucible sealed with a screw cap; then it was heated to 850 °C by 5 °C min⁻¹ and kept for 3 hours in an electrothermal furnace. After that, it was cooled down to room temperature at 5 °C min⁻¹ in the furnace. All the operations were carried out in an argon-atmosphere glovebox.

X-ray diffraction

X-ray diffraction measurements were performed on a Bruker D8 ADVANCE using Cu-Kα (1.5406 Å) radiation (40 kV, 20 mA). All samples were mounted on the same sample holder and scanned from 2θ = 10° to 70° at a speed of 10° min⁻¹. To avoid the deliquescence of the salt samples, a Kapton membrane was covered on the sample holder and the background signals are shown in Fig. S3 (ESI†). The experiments were performed at room temperature.

DFT calculation

The quantum chemical calculations of Be_(1+n)F_(4+3n)⁽²⁺ⁿ⁾⁻ (n = 0–3) were carried out using the Gaussian 09 program. The geometries of the anions were fully optimized at the B3LYP/AVTZ level, in which the B3LYP hybrid density functional with the aug-cc-pVTZ (AVTZ) basis set was employed. Frequency analyses were performed at the same level of theory to confirm that the optimized geometries were true minimum points. The ¹⁹F, ⁹Be NMR chemical shifts of F and Be atoms in each anionic species were also calculated at the B3LYP/AVTZ level using the optimized structures.

NMR experiments

All of the ¹⁹F, ⁷Li, and ⁹Be NMR experiments were performed on a Bruker AVANCE NEO 400 WB spectrometer operated at 376.61 MHz, 155.55 MHz, and 56.24 MHz for ¹⁹F, ⁷Li, and ⁹Be, respectively. For all the high-temp NMR experiments, a 7.0 mm double-resonance laser-heating HT probe was used. Homemade sample containers were used during the experiments, which were reported in our previous work.²⁷ The experiment temperature was set to 50 °C above the melting point of each LiF/BeF₂ binary molten salt with different BeF₂ concentrations. The melting points of the samples were obtained from the calculated LiF-BeF₂ phase diagram in Fig. S5 in the ESI.†²⁵ The experiment temperatures were set to 810 °C (15%), 795 °C (17%), 750 °C (22%), 680 °C (25%), 654 °C (27%), 554 °C (32%), 506 °C (35%), and 499 °C (37%) for LiF/BeF₂ binary salt samples with different BeF₂ concentrations respectively. The experiment temperatures were calculated by using the KBr external standard method. The recycle delay was set to 2 s for all of the ¹⁹F, ⁷Li and ⁹Be HT-NMR experiments. For the solid-state NMR experiments at room temperature, a 3.2 mm double-resonance magic angle spinning (MAS) probe was used. For single pulse excitation (SP) NMR experiments, the recycle delay was set to 60 s, 5 s, and 30s for ¹⁹F, ⁷Li, and ⁹Be, respectively. The spinning rate was set to 20 kHz. For ¹⁹F-⁷Li and ¹⁹F-⁹Be HETCOR NMR experiments, the cross polarization time was set to 200 us. The MAS rate was set to 10 kHz. To suppress the spin diffusion, FSLG decoupling was applied during t₁ (the ¹⁹F dimension) and the ¹⁹F RF field was set to 100 kHz. The ¹⁹F, ⁷Li and ⁹Be chemical shifts were calibrated using C₂H₄O₂F₃N (δ = −74.5 ppm), the LiCl aqueous solution (1 mol L⁻¹, δ = 0 ppm) and BeF₂ (solid, δ = 0 ppm), respectively.

Results and discussion

To investigate the ionic structure of the FLiBe melts, ¹⁹F HT-NMR (Fig. 1a) was firstly performed on LiF/BeF₂ binary salts with different BeF₂ concentrations. The experiment temperature was set to 50 °C above the melting point of each LiF/BeF₂ binary molten salt. Only one signal was observed in each ¹⁹F spectrum because of the rapid and dynamic exchange between the different species involving the observed nucleus. The peak position was the average between the chemical shifts of the different units present in the melt weighted by their relative proportions. It was observed that the ¹⁹F signal shifted to the low field with the increase of the BeF₂ concentration. To exhibit the changes of the signals clearly, the chemical shift evolution of LiF-BeF₂ molten salts with different BeF₂ concentrations is illustrated in Fig. 1b. To obtain the averaged chemical shifts accurately, signal simulation was carried out and the detailed expatiation is given in the ESI† (Fig. S7 and Tables S1, S2). It is clearly seen that ¹⁹F chemical shifts follow an almost linear change when the BeF₂ concentration is below 25%. As reported and described above, the linear change always implies a dynamic exchange of two species due to the average between the two specific chemical shifts. However, when the BeF₂ concentration was above 25%, a non-linear monotonous evolution of the ¹⁹F chemical shifts was observed. The deviation thus demonstrated the existence of, at least, a third type of fluorine anion complexes in FLiBe melts.

Fig. 1 ¹⁹F HT-NMR spectra and chemical shift evolution of LiF-BeF₂ molten salts with different BeF₂ concentrations.

⁹Be HT-NMR was also performed on the above salt samples to investigate the coordinated structure of Be²⁺ ions and the chemical shift evolution plot is illustrated in Fig. 2a. Unlike the monotonous evolution of ¹⁹F NMR, the plot of ⁹Be NMR showed quite a different trend. First, with the increase of BeF₂ concentration, the ⁹Be NMR signal stayed almost unchanged when the BeF₂ concentration was below 25%. This indicates that the chemical structure of Be²⁺ ions also remained unchanged. When the BeF₂ concentration reached 25%, the ⁹Be NMR signal started to go into high field, suggesting the formation of the new structure species of Be–F complexes. Such a change is also consistent with the ¹⁹F NMR results.

Fig. 2 (a) ⁹Be HT-NMR chemical shift evolution of LiF-BeF₂ molten salts with different BeF₂ concentrations. (b) Ionic structure of the BeF₄²⁻ ion and the Be–F polymeric ions.

As revealed by the ¹⁹F and ⁹Be NMR results, when the BeF₂ concentration is lower than or higher than 25%, the ionic coordinated structure of Be–F ions should be significantly different. The theoretical studies have suggested that BeF₄²⁻ ions will form first and gradually become the main species with the addition of BeF₂ into the LiF salt.¹⁵ At a low BeF₂ concentration, the melt is essentially well-dissociated, and it comprises Li⁺, BeF₄²⁻, and F⁻ species. With the increase of BeF₂ concentration, BeF₄²⁻ ions continuously increase, and free F⁻ anions decrease. As a result, the fast exchange between BeF₄²⁻ anions and F⁻ anions makes the ¹⁹F NMR chemical shifts follow a linear variation. Meanwhile, all the Be²⁺ ions form BeF₄²⁻ complexed ions and thus ⁹Be signal keep almost unchanged. When the BeF₂ concentration reaches 25%, the evolution of ¹⁹F and ⁹Be NMR chemical shifts tend to be particularly different from that lower than 25%, suggesting the formation of the new coordinated ionic structure.

According to the theoretical calculation results by Smith et al.,¹⁵ polymeric species (e.g., Be₂F₇³⁻, Be₃F₁₀⁴⁻, and Be₄F₁₃⁵⁻) were formed progressively when the BeF₂ concentration increased, until a fully connected network was constructed for pure BeF₂. When BeF₂ concentration was higher than 25%, the polymeric Be–F network tended to be the main species in the FLiBe melt. The formation of the polymeric network should account for the changes in ¹⁹F and ⁹Be evolution. The ionic structure of the BeF₄²⁻ ions and the Be–F polymeric ions are presented in Fig. 2b. The polymeric Be–F network is made of tetrahedral corner-sharing Be²⁺ cations linked by F⁻ anions. The formation of the network will lead to the increase of corner-sharing Be²⁺ ions and the linked F⁻, thus resulting in the different ¹⁹F and ⁹Be NMR chemical shifts.

DFT calculations were carried out to further investigate the relations between ionic structure and the chemical shift variation trend. The calculated average chemical shifts of the NMR signals are listed in Table 1. The detailed calculation results and the data processing method are provided in the ESI† in Fig. S9 and S10. It could be observed that ¹⁹F chemical shifts would go into low field when the polymeric ions (Be₃F₁₀⁴⁻, Be₄F₁₃⁵⁻) formed while for ⁹Be HT-NMR signals, it would go into high field with the formation of the polymeric ions. The variation tendency is consistent with the experimental ¹⁹F and ⁹Be HT-NMR results, although the values of the calculated chemical shifts are a little different. Based on the above results, we conclude that at low BeF₂ concentration, the melt comprises the isolated species (F⁻, BeF₄²⁻ and Be₂F₇³⁻). With the increase of the concentration, especially after 25%, polymeric Be–F network (Be₃F₁₀⁴⁻, Be₄F₁₃⁵⁻ and even polymer chains) is formed and gradually becomes the main species in the FLiBe melt.

Table 1 DFT calculated average chemical shifts of ¹⁹F and ⁹Be HT-NMR signals of different Be–F coordinated species

	^19F c.s./ppm	^9Be c.s./ppm
BeF4^2−	−197.44	−1.92
Be2F7^3−	−192.09	−2.28
Be3F10^4−	−189.75	−2.37
Be4F13^5−	−187.69	−2.38

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Formation of the polymeric Be–F network should be probably derived from the strong covalent interactions between Be and F atoms. Similar to silica SiO₂ and some silicates,^28,29 the strong interactions can facilitate the formation of tetrahedral network. The similarity between FLiBe and silicates thus illustrates that Be–F bonds should have some covalent properties. As reported in the literature,^13–15 the Be–F distances in FLiBe melts with high BeF₂ concentration is about 1.57 Å, which is calculated via MD and AIMD simulation. The relatively short ionic distance thus indicates that the Be–F bonds exhibit the particular covalency. Moreover, compared to Mg, the Be–F interactions are much higher than Mg–F interactions although they are the same alkali earth group atoms. According to the literature,³⁰ the calculated bond dissociation energy of BeF⁻ ions is 88.7 kcal/mol, which is also much higher than that of MgF⁻ ions (61.9 kcal/mol). This could also indicate the strong covalent interactions of Be–F bonds. The strong covalent interactions of Be²⁺ ions thus can facilitate the coordination between Be²⁺ ions and F⁻ ions, leading to the formation of the network structure in FLiBe melts with high BeF₂ concentration.

Previous work has suggested that Li⁺ ions do not form any complex with Be²⁺ or F⁻ ions.²⁶ To examine the coordinated structure of Li⁺ ions, ⁷Li HT-NMR was also performed on these FLiBe salt samples and the spectra are presented in Fig. 3a. The evolution of ⁷Li chemical shifts is also primarily similar to ¹⁹F and ⁹Be results, indicating the formation of the polymeric Be–F networks. Unexpectedly, two broad signals could be distinguished in the spectra of FLiBe melts with 22% and 25% BeF₂ concentration. In these two samples, the melt consists of both BeF₄²⁻ ions and the polymeric Be–F network. The two ⁷Li signals should be assigned to the Li⁺ ions coordinated with the dissociated BeF₄²⁻ ions and the polymeric Be–F network. The separation of the two ⁷Li signals may be due to the reduced exchange rate of these two species, which is caused by the formation of the polymeric Be–F network. This indicates that Li⁺ ions don’t simply follow the dissociative state which doesn’t form any complex with F⁻ ions. The two separable ⁷Li signals thus verified that Li⁺ ions should be also coordinated by the dissociated F⁻, BeF₄²⁻ ions, and the polymeric Be–F network, respectively. Furthermore, considering the reduced exchange behavior between the two species, we should also observe two signals in the ¹⁹F HT-NMR spectra. However, such individual signals were hardly observed due to the relatively low resolution of ¹⁹F signals.

Fig. 3 ⁷Li HT-NMR spectra and chemical shift evolution of LiF-BeF₂ molten salts with different BeF₂ concentrations.

Fluoride crystals have wide applications in the optical industry such as luminescence materials and laser crystals. In general, BeF₂ is believed to adopt a structure analogue to SiO₂, with a 3D network of corner-sharing tetrahedrally coordinated structure.²⁸ The crystal behavior and the glass state of the beryllium fluorides can significantly influence their applications. As discussed above, the addition of alkali-metal elements, LiF, can adjust the ionic structure of FLiBe melts. Formation of the polymeric Be–F network would reduce the crystallization and thus lead to the glass state during the solidification of FLiBe melts.

To investigate the local structure of solidified FLiBe mixed salts, high-resolution solid-state¹⁹F MAS NMR spectroscopy was performed on LiF/BeF₂ salts with different BeF₂ concentrations and the spectra are presented in Fig. 4. Four signals could be observed in the above spectra, which are named F-1 (−205.6 ppm), F-2 (−202.0 ppm), F-3 (−195.8 ppm), and F-4 (−187.3 ppm) respectively. To exhibit the differences of the signals, the software “Dmfit” was employed for signal decomposition and the calculated intensities of the signals are presented in Fig. S2 (ESI†). With the increase of BeF₂ concentration, the intensity of F-1 and F-2 signals tended to decrease gradually and the intensity of the F-3 signal increased. When BeF₂ concentration reached 60%, F-1 and F-2 signals disappeared, and a new F-4 signal appeared in the spectrum. Signal assignment should be conducted to further analyze the local structure of FLiBe mixed salts.

Fig. 4 Solid-state ¹⁹F MAS NMR spectra of FLiBe mixed salts with different BeF₂ concentrations. The software “Dmfit” was used for signal decomposition.

To assign the ¹⁹F signals, 2D ¹⁹F-⁷Li and ¹⁹F-⁹Be Heteronuclear Correlation (HETCOR) NMR spectroscopy was performed on the LiF/BeF₂ salts (40% BeF₂) and the spectra are presented in Fig. 5. It could be observed that F-1 (¹⁹F) signal has cross peaks with ⁷Li signal only. No cross peaks could be observed between F-1 and ⁹Be signals, suggesting the above F⁻ ions are only bonded to Li⁺ ions. Thus, we assign F-1 signal to the F⁻ ions of LiF crystals. For F-2 and F-3 signals, they have cross peaks with both ⁷Li signal and ⁹Be signal, indicating that these F⁻ ions are bonded to Li⁺ ions and Be²⁺ ions. It is known that Li₂BeF₄ crystals can be formed when the molar ratio of LiF:BeF₂ is about 2 : 1. To verify if Li₂BeF₄ crystals exist in the mixed salts, XRD was performed on the samples and the XRD patterns are presented in Fig. S3 (ESI†). As shown, the characteristic diffraction peaks of Li₂BeF₄ crystals could be clearly observed, illustrating the existence of Li₂BeF₄ crystal lattices. Thus F-2 signal is assigned to the F⁻ ions of Li₂BeF₄ crystals.

Fig. 5 ¹⁹F-⁷Li and ¹⁹F-⁹Be HETCOR NMR spectra of LiF-BeF₂ salts (40% BeF₂).

It was observed that the intensity of F-1 and F-2 signals continuously decreased with increasing the concentration of BeF₂. After the concentration of BeF₂ reached 60%, F-1 and F-2 signals almost disappeared, demonstrating the absence of LiF and Li₂BeF₄ crystals in the salt samples with high BeF₂ concentration. Moreover, the intensity of F-3 signal increased gradually and a new signal F-4 appeared at high BeF₂ concentration. As expected, FLiBe should be also in a glassy state similar to silicate glasses with a 3D network structure (such as Na₂SiO₃). The above results reveal that F-3 and F-4 signals should be assigned to the F⁻ ions of FLiBe glasses.

In comparison with silicate glasses, Be–F glasses could be also distinguished by these different structure models, which were named Q1, Q2, Q3, and Q4 respectively. The structure models are shown in Fig. 6b. F-4 signal existed only in the ¹⁹F ss-NMR spectrum of FLiBe salts with a significantly high BeF₂ concentration and we assign F-4 signal to the F⁻ ions of Q3 or Q4 model with Be–F 3D networks. For F-3 signal, the signal intensity increased with the increase of BeF₂ concentration. Thus F-3 signal was assigned to the F⁻ ions of Q1 and Q2 models with Be–F dimers or chains. Changes of the ⁹Be NMR spectra of LiF-BeF₂ solidified mixed salts with different BeF₂ concentrations also follow the above signal assignment. ⁹Be ss-NMR spectra and signal simulation of LiF-BeF₂ mixed salts are presented in Fig. 6a. Two signals were observed in the spectra, named Be-1 and Be-2. Be-1 should be the Be²⁺ ions of Li₂BeF₄ crystals and Q1 model with Be–F dimers. Be-2 should be the Be²⁺ ions of Q2, Q3, or Q4 models. The abundant corner-sharing tetrahedrally coordinated Be²⁺ cations of Q2 and Q3 should account for the distinguished chemical shifts of Be-1 and Be-2.

Fig. 6 (a) ⁹Be NMR spectra and signal simulation of LiF-BeF₂ mixed salts with different BeF₂ concentrations. (b) Illustration of the structure models of Be–F glasses.

The structure transition of FLiBe solidified mixed salts can be investigated through the above signal assignment of ¹⁹F and ⁹Be ss-NMR. At a low BeF₂ concentration, the LiF-BeF₂ mixed salts consisted of LiF crystals and Li₂BeF₄ crystals. Moreover, there was also a small amount of the oligomer (e.g., Q1 Be–F tetrahedral ions). With the increase of BeF₂ concentration, LiF crystals and Li₂BeF₄ crystals became gradually disordered. Instead, numerous corner-shared Be–F tetrahedra formed and the number of Q1/Q2 structures increased, thus becoming the main species. This should be caused by the strong Be–F covalent interactions, which is quite similar to some silicates. When the BeF₂ concentration was extremely high (>60%), the Be–F network (Q3 and Q4) was also formed, and most the LiF crystals and Li₂BeF₄ crystals disappeared. Obvious signal broadening could be observed of the XRD patterns, indicating a decreased degree of crystallinity. This also illustrates that the main part of the samples transforms into the glass state. Upon further increasing the BeF₂ concentration, the mixed salt would transform completely into the glass state (similar to BeF₂). In addition, the formation of the FLiBe glass state should also have close relations with a large amount of the polymeric Be–F structures. The strong Be–F covalent interactions of the Be–F network of FLiBe melts would reduce the crystallization and thus lead to the formation of FLiBe glass. Upon adding LiF into the system, the alkali metal ions will break the long chains or networks, which is beneficial to the crystallization during the solidification of FLiBe melts. This should be the transition process of LiF-BeF₂ mixed salts from crystals into glasses.

Conclusions

In summary, the local structures of LiF-BeF₂ molten salts and the solidified mixed salts are systematically interpreted in detail using the HT-NMR and solid-state MAS NMR methods. In this study, it was suggested that the strong covalent interactions of Be²⁺ ions can facilitate the coordination between Be²⁺ ions and F⁻ ions. As a result, polymeric Be–F chains and networks can be formed, which will be closely related to the ionic dynamics. ¹⁹F, ⁷Li, and ⁹Be NMR experiments were used to understand the local structure and the structural transition of the investigated materials. As revealed from the NMR results, the BeF₄²⁻, Be₂F₇³⁻, and Be₃F₁₀⁴⁻ coordinated species as well as the polymeric Be–F chains/networks exist in FLiBe melts, the amounts of which can be adjusted via the BeF₂ concentration. Moreover, the structure of solid FLiBe mixed salts at ambient temperatures was also investigated by 1D and 2D solid-state NMR (ss-NMR). The network structure of FLiBe was verified to be similar to those of silicate glasses. According to the results of the observations, the local structure of FLiBe salts was elucidated using experimental methods, and more insights were provided into the transition of the structure from the isolated ionic systems into the polymeric networks and were found to be significantly related to the thermo-physical properties and their applications in new energy production technology. This work also demonstrates that HT-NMR methods are promising in the investigation of the micro-structure and dynamics of fluoride salts. Our work shows that HT-NMR is also applicable to research on other fluorine-containing materials.

Data availability

The authors declare that data relating to the characterization of materials and products, general methods, experimental procedures, mechanistic studies, XRD data, and NMR spectra are available within the article and the ESI† or from the corresponding author upon request.

Author contributions

Y. Q. and H. L. conceived and designed the experiments. X. F. and Y. L. performed the experiments and analyzed the data. H. H., H. W. and J. S. helped synthesize the BeF₂-LiF salts. L. H. helped draw the structure diagram. X. F. wrote the original manuscript. M. G. helped revise it. All authors proofread the paper, made comments, and approved the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation of China (22103094), the K. C. Wong Education Foundation (Grant No. GJTD-2018-10), and the Young Potential Program of Shanghai Institute of Applied Physics, Chinese Academy of Sciences. The authors also express their gratitude to Professor Ian Farnan from the University of Cambridge for his advice on manuscript modification.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp01096a

‡ These authors contributed equally.

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