A low temperature solution-chemical process for fluoride salt (NaF) waste immobilization into metal halide perovskite structured Na₂SnF₆

Abstract

Complex salt waste streams generated from chemical reprocessing of spent nuclear fuels and advanced molten salt reactor technologies require innovative materials and processes for effective immobilization and management. In this study, we report a simple, low-temperature, solution-based approach for immobilizing fluoride salts, specifically alkali halides, into metal halide perovskite (MHP) waste forms. Using NaF as a representative system, systematic experiments were conducted to immobilize alkali-halides into Na₂SnF₆, achieving a high fluorine content of 40.9 wt% and a fluoride loading capacity of 57.4 wt%. By tailoring key parameters such as the use of surfactants, salt grain size, and reaction temperature, the low temperature solution chemistry process achieves consistently high immobilization efficiencies above 95%. X-Ray diffraction (XRD) analysis coupled with Rietveld refinement confirmed that Na₂SnF₆ predominantly crystallizes in the tetragonal P4₂/mnm structure as the primary phase. A hexagonal metastable phase was also observed, attributed to rapid precipitation during synthesis. Microstructural characterization using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) confirms uniform elemental distribution and microchemical compositions consistent with the Na₂SnF₆ crystal structure. Thermogravimetric analysis (TGA) demonstrates thermal stability of the synthesized waste forms up to 400 °C. This work establishes a foundational, cost-effective pathway for immobilizing alkali fluoride salt waste into MHP-type structures with high waste loading using novel low-temperature solution chemistry for effective management of salt waste.

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1. Introduction

Molten salt reactors (MSRs) have gained considerable interest as Generation IV reactors in which molten salts, such as fluorides or chlorides, are used as either heat carriers or both coolant and fuel carriers in reactors.¹ Fluoride- and chloride-based MSRs generate complex salt waste streams that require immobilization for long-term storage. Molten salt waste is known for strong corrosivity, poor chemical durability, low melting temperature, highly volatile and a highly hygroscopic nature. Therefore, it is essential to effectively immobilize salt waste into a chemically durable waste form with high waste loading. Similarly, the reprocessing of spent nuclear fuels also generates salt waste, highlighting the need for their effective immobilization strategies to ensure safe disposal.

Direct immobilization of unseparated salts is not an attractive option for salt management due to the lack of recovery of expensive elements like ⁷Li and ³⁷Cl. However, the high volatility associated with the alkalis and halides, and their low solubility poses a significant challenge for immobilization of alkali-halides in borosilicate glass, which is commonly used for vitrifying high level radioactive waste.^2,3 It is not feasible to immobilize the salt waste into a single waste form with high waste loading levels. Separating salt waste is highly desirable to enable more efficient waste management and to facilitate the design of multiple waste forms that optimize chemical durability and enhance waste loading. Various ceramic waste forms namely, apatite, phosphate, sodalite, nesosilicate, cancrinite, and pyrochlore have been studied for immobilization of salt waste constituents.^4–7 The selection of waste forms for immobilizing salt-based nuclear waste has expanded beyond glasses and ceramics to include composites like glass-bonded and metal-bonded ceramics.^8–10

There are a few reports on immobilization of fluoride waste into phosphate glass due to lower processing temperature and higher halide solubility as compared to borosilicate glass. Sun et al. developed a sodium aluminophosphate (NaAlP) glassy waste form to immobilize 20 mol% of MF_n (where M = Cs, Sr, Zr, Ce, and Sm) and further addition of iron oxide to increase the chemical durability, resulting in low waste loading (6.4 mol%).¹¹ Liu et al. reported that 30 wt% SrF₂ or 10 wt% CeF₃ was immobilized in a glassy iron phosphate waste form by vitrification at 1050–1200 °C.^12,13 They further pushed the investigation by immobilizing 22 wt% of CeF₃ in a phosphate glass matrix composed of 26 wt% Fe₂O₃–64 wt% P₂O₅–10 wt% B₂O₃ without any precipitation.¹⁴ Jena et al. utilized borosilicate glass to immobilize Sr-chlorapatite up to 10–13 wt% loading without phase segregation.¹⁵ However, detailed information regarding fluorine retention within the vitrified glass was not provided. Fluorozirconate glasses were also developed with ZBLAN composition (ZrF₄–BaF₂–LaF₃–AlF₃–NaF) for their optical properties, but poor chemical durability limits their application for halide waste immobilization.^16,17 Furthermore, lead tellurite glasses were developed to immobilize chloride salts such as alkali chlorides, alkaline earth chlorides, rare earth chlorides, and rare earth oxychlorides.^18–21 Riley et al. reported no perturbation in a glass network of 78% TeO₂–22% PbO on loading 5–17.5 wt% SrCl₂ or loading of chlorine up to 7.8%.¹⁸ Due to their high halide solubility and lower processing temperatures, tellurite glasses are currently among the most effective materials for halide waste immobilization. However, the natural scarcity and high cost of TeO₂ limit their scalability for large-scale immobilization of halide-containing waste. In addition, secondary wastes, such as corrosive gases like HCl and HF, were also generated during the vitrification process, making high temperature glass melting – typically at several hundred degrees Celsius – an unacceptable process for the immobilization of salt waste.

New materials and advanced processes are required to immobilize salt waste with increased waste loading and minimized loss of highly volatile alkali and halides during waste incorporation and waste form processing. Alternative approaches have been sought, including a solid-state process^22,23 and hot isostatic pressing (HIPing).²⁴ Donald et al. employed a solid-state process to immobilize chloride waste in sodium aluminosilicate mineral, Na₈(AlSiO₄)₆Cl₂ and calcium phosphate, Ca₅(PO₄)₃Cl. Fluorapatite was developed to immobilize fluoride waste through coupled cation–anion substitutions. Similarly, Auxilia L. et al. studied Ca₁₀(PO₄)₆F₂ to immobilize fluoride containing waste.²⁵ Cao et al. employed wet chemical synthesis to immobilize radioactive iodine in the apatite-based waste form.²⁶ However, fluoride content in these crystalline mineral structures is relatively low, especially when compared to the higher fluoride loading of 17–21 wt% achieved in the HIP glass-ceramic composite waste form. Therefore, there is a need for advanced materials or mineral structures to immobilize alkali and alkaline earth halides with high waste loadings, while utilizing low-temperature processes to avoid the production of secondary waste.

A new concept of inorganic metal halide perovskites (MHPs) has been recently introduced to immobilize halide salt waste. MHPs are widely known for their application in the optoelectronic industry, recently emerging as promising candidates for nuclear waste forms owing to their compositional flexibility and diverse chemistry for immobilizing alkali and alkaline earth halides. Yang et al. reported successful immobilization of Cs and iodine in Cs₃Bi₂I₉.²⁷ Similarly, Cs and Cl were encapsulated into Cs₂SnCl₆, which has been reported to achieve high waste loadings of 44.5 wt% Cs and 35.6 wt% Cl.²⁸

The perovskite-type structures, such as CaTiO₃, are generally represented by the formula ABX₃, in which ‘A’ is typically a larger monovalent cation, ‘B’ is a smaller divalent cation, and ‘X’ is a monovalent anion. The structure features two distinct cationic sites: the B-site cation is coordinated by six anions forming a BX₆ octahedron, while the A-site cation occupies the larger cuboctahedral interstices. In an ideal cubic perovskite structure, the X anions are located at face-centered positions, coordinating with the B-site cations located at the center of the cube. The A-site cation is situated at the corners of the cube and is 12-fold coordinated with surrounding anions. Considering the wide range of cation valence states, cations with higher oxidation states can also be incorporated into the perovskite lattice, leading to the formation of more complex perovskite-type compounds such as A₃BX₆, A₂BX₆, A₃B₂X₉, and A₄BX₆, all derived from the ABX₃ structure. This highlights the compositional versatility of MHPs, allowing for a broad selection of A- and B-site cations from across the periodic table.²⁹

In this work, we report a novel low-temperature, solution-based approach to immobilize sodium fluoride (NaF) into an MHP compound, namely Na₂SnF₆. The synthesis of Na₂SnF₆ was systematically investigated as a function of key reaction parameters to elucidate their influence on the product yield, phase formation, microstructure and product stability. The chemical process offers high reaction yield, cost-effectiveness, and scalability. Moreover, the synthesized MHP (Na₂SnF₆) offers a high fluorine loading of 40.9 wt% and a fluoride loading capacity of 57.4 wt%, assuming near a 100% reaction yield. We have systematically explored the reaction conditions to maximize the reaction yield (>95%) and investigated how different synthesis conditions influence the structural, microstructural, and thermal decomposition behaviors of the fluoride waste containing MHPs. Finally, a monolithic sample of wasted loaded MHP (Na₂SnF₆) synthesized at room temperature was densified via an innovative cold sintering process (CSP) using spark plasma sintering (SPS). DMSO was used as an additive to enhance diffusion and particle bonding during the CSP to demonstrate the consolidation of Na₂SnF₆ MHP waste forms at 200 °C to mitigate potential loss of volatile wastes.

2. Experimental details

2.1. Synthesis

The MHP (Na₂SnF₆) was synthesized through a cost-effective solution-based chemical process at low-temperatures of less than 100 °C. Sodium fluoride (NaF) was used as a representative salt to simulate separated salt waste of alkali halide, and NaF is also a major component in the FLiNaK salt coolant. In addition to NaF, tin(IV) chloride pentahydrate (SnCl₄·5H₂O) and oleic acid were utilized as additives to facilitate the formation of Na₂SnF₆. Analytical reagent (AR) grade powders of sodium fluoride with two different grain sizes were purchased from Sigma Aldrich (≥99%) and Alfa Aesar (≥99.99%). SnCl₄·5H₂O and oleic acid also were purchased from Sigma Aldrich and used as-purchased.

The chemical reaction between NaF and SnCl₄·5H₂O was performed in methanol (organic solvent) to avoid the hydrolysis of Sn⁴⁺ in DI water. Due to the relatively low solubility of NaF in methanol (3.28 g L⁻¹ at 20 °C (ref. 30)), oleic acid was added to increase the dispersion or solubility of NaF in methanol. Serving as a surfactant, oleic acid is expected to increase the number of effective nucleation centers for the formation of Na₂SnF₆. Henceforth, in the present manuscript, the chemical reactions will be referred to as an organic solvent (OS) route, in which methanol was used as the solvent to dissolve NaF, and a surfactant-assisted or coordination-ligand-assisted or (OS-ligand) route, in which methanol and oleic acid were used as the solvent system to dissolve NaF. In the OS route, NaF was added into methanol and stirred to dissolve or disperse salt and 10–15 minutes later, SnCl₄·5H₂O was added to the solution. Once SnCl₄·5H₂O was added to the solution, a white precipitate formed immediately according to the following reaction (1),

6NaF + SnCl₄·5H₂O → Na₂SnF₆↓ + 4NaCl + 5H₂O (1)

After stirring the mixed solution for around 30 minutes, the white precipitate was separated out from the solution through centrifugation at 9000 rpm for 5 min. The precipitated solid was rinsed two times with 10–10 mL methanol and dried in oven at 70 °C overnight. The surfactant-assisted OS route followed a similar procedure, with the exception of adding oleic acid to the methanol prior to dissolving the salts (NaF). The volume of oleic acid used was 1/16 of the methanol volume. First, the reaction conditions were optimized to completely immobilize NaF into Na₂SnF₆ and then, evaluate the effect of reaction temperature on reaction yield and reaction temperature as follows.

2.1.1. Solution-based immobilization of NaF into MHP. The methodology underlying the total immobilization of salt waste (NaF) into a MHP involves converting the entire amount of pretreated alkali fluoride salts into a crystalline waste form through a chemical reaction with an appropriate reagent. A small-scale reaction was performed by dissolving ∼0.904 g (21.53 mmol) NaF with a grain size of 5–8 µm in 200 mL of methanol and reacting it with a stoichiometric amount of ∼1.258 g (3.588 mmol) SnCl₄·5H₂O at 60 °C in the OS-route. Similarly, the same amount of NaF was dissolved in 100 mL methanol along with 6.25 mL oleic acid (1/16th of the methanol volume) and reacted with a stoichiometric amount of SnCl₄·5H₂O at 60 °C in the OS-ligand route. The phase confirmation in Fig. S1(a) and (b) (see SI Data) shows a minor contribution from unreacted NaF along with Na₂SnF₆ diffraction peaks. The presence of unreacted NaF shows incomplete immobilization. To achieve complete salt immobilization, the precipitate was further added in the remaining supernatant and reacted with additional 20% SnCl₄·5H₂O. After further reaction with additional SnCl₄·5H₂O for 30 minutes, the obtained precipitate was filtered and dried followed by further structural characterization. Fig. S1(c) and (d) show that the diffraction maxima align with the PDF reference pattern (01-080-1155) of Na₂SnF₆. These results indicate that a 20% excess of SnCl₄·5H₂O is sufficient to achieve full immobilization of NaF into MHP as Na₂SnF₆.

2.1.2. Solution-based immobilization of NaF with varying grain sizes into MHP. The overall process of precipitation or co-precipitation involves the dissolution of reactants, nucleation of the solid phase from solution, and subsequent crystal growth. However, the dissolution rate of salt waste such as NaF in methanol is notably limited. Therefore, it is essential to investigate the various factors that may significantly influence the dissolution behavior of salt waste, as this could reduce methanol consumption and enhance the cost-effectiveness of the immobilization process. In particular, understanding the effect of salt grain size on its dissolution rate, nucleation behavior, and reaction yield would offer valuable insights for applications in the nuclear industry. Therefore, in addition to using NaF with grain sizes of a few microns, larger-grained NaF (of the order of a few hundred microns), and NaF with grain sizes of a few hundred nanometers has also been employed for salt waste immobilization. The nano-sized NaF powders were obtained by high energy ball milling of as-purchased micron-sized NaF to refine the particle size. To immobilize coarser powders of sodium fluoride (NaF) into Na₂SnF₆ in the OS-route, coarser powders of ∼0.181 g NaF (4.31 mmol) were dissolved in 200 mL methanol and reacted at 60 °C with a non-stoichiometric amount of ∼0.302 g SnCl₄·5H₂O (0.718 mmol). Similarly, the same amount of NaF was dissolved in 100 mL methanol along with 6.25 mL oleic acid (1/16th of the methanol volume) and reacted with the same amount of SnCl₄·5H₂O in the OS-ligand route. The white precipitates obtained through the OS route and OS-ligand route weighed approximately 0.171 g and 0.187 g, respectively. Accordingly, the reaction yield for immobilizing NaF into Na₂SnF₆ was estimated to be 85.5% and 93.5% in OS and OS-ligand routes, respectively, relative to the targeted product mass of 0.2 g. Similarly, 0.904 g of NaF powder (i.e., five times that used for the coarser NaF powder) with a grain size of 5–8 µm and 100–200 nm were dissolved in 200 mL and 150 mL of methanol. The reactions were performed at 60 °C with a non-stoichiometric amount of SnCl₄·5H₂O. The corresponding reaction times were 30 and 45 minutes. The methanol volume was reduced for the finer powder, and the reaction duration was increased to offset the decreased solvent quantity. In the OS-ligand route, the same amount of powder was dissolved in 100 mL and 75 mL volume of methanol and the reaction was performed under the same conditions with the help of oleic acid. In the OS-route, the reaction yield for 5–8 µm and 100–200 nm sized NaF powders was measured to be 85.4% and 87%, respectively. In the OS-ligand route, the reaction yield increased to 94.03% and 97.2%, respectively.

2.1.3. Solution-based immobilization reaction at different temperatures. The reaction was conducted at different temperatures such as 35 °C and room temperature (RT) to assess the impact of temperature, as room-temperature processing can significantly enhance cost-effectiveness. The immobilization of 0.904 g of NaF with 5–8 µm grain size was performed at 35 °C with a non-stoichiometric amount of SnCl₄·5H₂O. The reaction time and amount of methanol were similar to those in the previous reaction at 60 °C. The reaction yield was found to be 84.2% and 93.3% in OS and OS-ligand routes, respectively. Similarly, 0.226 g of NaF with 5–8 µm grain size was added into 50 mL methanol and reacted with excess SnCl₄·5H₂O (0.377 g ∼120%). The reaction was performed at RT for 1 h with continuous mixing through a magnetic stirrer. In the OS-ligand approach, 50 mL of methanol was mixed with 3.2 mL of oleic acid at a 16 : 1 ratio, followed by the addition of 1.13 g of NaF (26.9 mmol) into the methanol–oleic acid solution. Excess SnCl₄·5H₂O (1.887 g ∼120%) was added into the solution and further reaction was performed at RT for 2 h. The reaction yield of NaF immobilization into Na₂SnF₆ at RT was found to be 96.8% and 91.2% in OS and OS-ligand routes, respectively.

2.2. Phase and microstructure characterization

The crystal structure of dried powders was analyzed through X-ray diffraction (XRD) using a Malvern Panalytical Empyrean system. The diffraction patterns were collected over a 2θ range of 15° to 80°, with a step size of 0.013°, using a conventional copper-K_α radiation source operated at 40 kV and 40 mA. The crystallographic orientation of synthesized powders was analyzed through Rietveld refinement of diffraction patterns using Fullprof-2005 software. Initially, the background was selected through a linear interpolation method using the WinPlotr tool. The pseudo-Voigt function was employed to fit the background and diffraction peaks, respectively.³¹ The microstructural investigation was conducted through a SEM equipped with a Schottky thermal field emitter using an FEI Versa 3D Dual Beam system. Secondary electron (SE) imaging was recorded using an Everhart–Thornley Detector (ETD) while energy-dispersive spectroscopy (EDS) was performed to probe the micro-chemistry of the synthesized powder. EDS imaging was recorded using a silicon drift detector (Oxford instruments, UK). The as-synthesized MHP powders were dispersed onto a carbon tape and coated with a thin conductive layer of gold and platinum (a few nanometers thick) to prevent charging during SEM characterization. The thermal stability of the prepared MHP was evaluated by TGA. The measurements were performed using a DSC/TGA apparatus (SDT650, TA Instruments, DE, USA) with horizontal dual beams. Approximately 20–30 mg of sample powders were loaded in a standard alumina crucible (90 µL) and heated to 1000 °C with a heating rate of 10 °C min⁻¹. The measurement was performed in an argon environment maintained by a steady flow of 50 mL min⁻¹.

2.3. Consolidation of Na₂SnF₆

The as-synthesized MHP powders through OS and OS-routes at RT were consolidated into monolithic cylindrical pellets using spark plasma sintering (SPS) (Dr Sinter SPS 211-LX, Fuji, Saitama, Japan) in an argon environment. A low temperature consolidation process was optimized using DMSO as an additive to enhance diffusion and particle bonding during the SPS process at 200 °C. The powders were loaded into a graphite die set and sandwiched between two graphite punches. To prevent powder–die interaction and to facilitate easy removal from the die set, the powders were covered with graphite paper in a cup-like structure. This innovative cold sintering process (CSP) was conducted at 200 °C for 15 min under an applied pressure of 60 MPa, with heating and cooling rates of 25 °C min⁻¹.

3. Results and discussion

3.1. Structural analysis and phase behavior

Phase confirmation of the synthesized white precipitates in various reactions was carried out using XRD. Fig. 1 shows the representative diffraction patterns of the as-synthesized Na₂SnF₆. In both cases, either the OS route (Fig. 1(a)) or OS-ligand route (Fig. 1(c)), the diffraction patterns show that the primary phase corresponds to a tetragonal P4₂/mnm structure, matching the PDF reference pattern (01-080-1155) for Na₂SnF₆.³² Interestingly, some reactions at higher temperatures using micron-sized NaF show that the secondary phase belongs to a hexagonal symmetry of the P321 space group, referred to the PDF reference pattern (00-047-1082) for the secondary phase of Na₂SnF₆. This β-phase of Na₂SnF₆ was also reported by Grannec et al., who observed that it forms under rapid thermal quenching and is a high-temperature phase that remains metastable at room temperature, eventually transforming the P4₂/mnm structure.³³ In contrast, the reaction kinetics in the case of NaF with coarser powders (120–150 µm) was slower, although the reaction was carried out using only one-fifth of the amount of salts in the same volume of methanol (200 mL).

Fig. 1 XRD patterns of Na₂SnF₆ synthesized through (a) OS and (c) OS-ligand routes under various reaction conditions at different temperatures (RT, 35 °C and 60 °C) with different particle sizes (coarse, fine and nano-sized NaF). Enlarged patterns in (b) OS and (d) OS-ligand routes show no impurity of NaF and NaCl.

Therefore, in the OS-route, the β-phase of Na₂SnF₆ was observed only for the reaction in which NaF with 5 µm immobilized at 60 °C. In the OS-ligand route, the reaction kinetics was sufficiently fast to form the metastable phase for the reactions using micron-sized NaF at 35 and 60 °C, and nano-sized NaF at 60 °C, consistent with the ligand-assisted nucleation with fast reaction kinetics at higher temperature as compared with room temperature. However, nano-sized NaF (100–200 nm) was used in a smaller volume of methanol without a ligand in the OS route at 60 °C, and a longer duration was required for the reaction, resulting in Na₂SnF₆ crystallized in the tetragonal structure despite higher temperature and finer particle size. This indicates that the formation of the metastable β-phase is strongly influenced by reaction time in addition to governing solely by reaction kinetics alone. It must be noted that the diffraction patterns in Fig. 1(b) and (d) clearly show no impurity peaks of unreacted NaF and NaCl present in synthesized powders.

The diffraction patterns of synthesized MHPs were further analyzed through Rietveld refinement. Fig. 2 presents the refined structures of Na₂SnF₆ synthesized at 35 °C through the OS route and OS-ligand route. The diffraction peaks of Na₂SnF₆ synthesized through the OS route were fitted based on tetragonal symmetry of the trirutile type P4₂/mnm 136 space group with Z = 2. The cations occupy octahedral sites, and Na⁺ is coordinated with six F⁻ atoms to form NaF₆ octahedra and Sn⁴⁺ is bonded to six F⁻ atoms to form SnF₆ octahedra as shown in Fig. 2(c). Fig. 2(b) presents the refined structures of Na₂SnF₆ synthesized through the OS-ligand route with the introduction of both α and β-phases. The β-phase can be described by trigonal symmetry of the Na₂SiF₆-type (P321 space group) with Z = 3 in space group no. 150. In this hexagonally crystallized Na₂SnF₆ structure, two crystallographically independent Na⁺ ions are coordinated with six F⁻ ions to form distorted NaF₆ octahedra. The structure also contains two Sn⁴⁺ sites, each coordinated by six F⁻ ions to form two distinct [SnF₆]²⁻ octahedra.³⁴ Table 1 shows the refined structural parameters of synthesized Na₂SnF₆. The lattice parameters in all reactions were found to be smaller in the OS route than in the OS-ligand route. This can be attributed to the fact that, in the OS-ligand route, faster and more homogeneous nucleation produces crystals with fewer vacancies, resulting in slightly larger lattice parameters. Qiao et al. also reported that the unit cell of synthesized LiFePO₄ powders was larger when prepared using a surfactant-assisted chemical route compared to samples synthesized without a surfactant.³⁵ The XRD results clearly show that the reactions enriched with SnCl₄·5H₂O in both OS and OS-ligand routes successfully converted NaF into Na₂SnF₆. The different reaction conditions indicate that rapid precipitation or high supersaturation, driven by elevated reaction temperatures, particle size and ligand-assisted synthesis, promotes the crystallization of the metastable phase of Na₂SnF₆.

Fig. 2 Refined XRD patterns of Na₂SnF₆ synthesized through (a) OS route and (b) OS-ligand route at 35 °C and (c) crystal structure of Na₂SnF₆ crystallized with tetragonal symmetry of trirutile type P4₂/mnm.

Table 1 The refined structural parameters of synthesized Na₂SnF₆

MHP (Na2SnF6)	OS route	OS-ligand route
NaF_120 µm (60 °C)	a = b = 5.057 Å & c = 10.112 Å [100% tetragonal]	a = b = 5.071 Å & c = 10.148 Å [100% tetragonal]
NaF_5 µm (60 °C)	a = b = 5.067 Å, & c = 10.099 Å [61% tetragonal]	a = b = 5.073 Å, c = 10.103 Å [29% tetragonal]
	a = b = 9.412 Å & c = 5.181 Å [39% hexagonal]	a = 9.423 Å, c = 5.191 Å [71% hexagonal]
NaF_100–200 nm (60 °C)	a = b = 5.063 Å, & c = 10.114 Å [100% tetragonal]	a = b = 5.073 Å, c = 10.110 Å [73.7% tetragonal]
		a = b = 9.415 A, c = 5.309 A [26.3% hexagonal]
NaF_5 µm (35 °C)	a = b = 5.064 Å, & c = 10.114 Å [100% tetragonal]	a = b = 5.073 Å, c = 10.113 Å [51% tetragonal]
		a = 9.435 Å, c = 5.185 Å [49% hexagonal]
NaF_5 µm (RT)	a = b = 5.066 Å, & c = 10.102 Å [100% tetragonal]	a = b = 5.069 Å, & c = 10.104 Å [100% tetragonal]

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3.2. Microstructural and microchemical analysis

SEM was performed to examine the microstructure and microchemistry of the as-synthesized Na₂SnF₆. Fig. 3(a₁–e₁) and (a₂–e₂) show the microstructure of synthesized Na₂SnF₆ via the OS route and OS-ligand route, respectively. The powders appear to have good crystallinity, which exhibit agglomerated grain clusters ranging from hundreds of nanometers to a few micrometers in various reactions. Generally, grain clusters obtained from the OS-route are larger than that obtained from the OS-ligand synthesis route.

Fig. 3 SEM images of synthesized Na₂SnF₆ in various reactions from the (a₁–e₁) OS-route and (a₂–e₂) OS-ligand route.

SEM analysis concludes that the relatively smaller agglomerated grain clusters of MHP result from the increased number of nucleation centers provided by the surfactant, which hinders grain agglomeration. This can be explained by Ostwald ripening in which the large crystals grow at the expense of smaller ones due to the difference in solubility.³⁶ Yang et al. also reported that the use of oleic acid decreased the grain size of LiMnO₄ and increased the homogeneity.³⁷ As a result, the crystals remained smaller compared to those synthesized via the OS route under identical reaction conditions. Since particle morphology is controlled by the nucleation rate, supersaturation and crystal growth kinetics, increasing the reaction rate or supersaturation at higher reaction temperature or lower amount of used methanol helps to produce spherical homogeneous nanoparticles.³⁸ Na₂SnF₆ synthesized at 60 °C for 30 min using 5 µm NaF (0.904 g) in 200 mL and 100 mL methanol via the OS and OS-ligand routes, respectively, exhibits a spherical morphology as shown in Fig. 3(c₁ and c₂). In the OS route, nearly 1 µm spherical particles are obtained, whereas the OS-ligand route yields ∼1 µm spherical clusters composed of 200–300 nm spherical primary grains. In contrast, Na₂SnF₆ synthesized under similar reaction conditions using 120 µm NaF (0.181 g) in 200 mL and 100 mL methanol (OS and OS-ligand routes, respectively) shows the formation of flower-like nanostructures consisting of 100–400 nm crystallites as shown in Fig. 3(a₁ and a₂). These observations clearly demonstrate the effect of solution supersaturation on the nucleation and growth behavior of Na₂SnF₆. A high degree of supersaturation promotes a rapid and homogeneous burst of nucleation, resulting in the formation of spherical particles. Similarly, Na₂SnF₆ synthesized at 35 °C for 30 min also exhibits nearly spherical particles. In contrast, synthesis performed at room temperature for 1–2 h leads to irregularly shaped, agglomerated particles with sizes in the range of 1–5 µm, as shown in Fig. 3(e₁ and e₂).

EDS mapping shown in Fig. 4 and 5 validates the micro-chemistry in MHPs synthesized by both synthesis routes under various reaction conditions. Table 2 presents the EDS-derived empirical formula. Fig. 4 (OS route) and Fig. 5 (OS-ligand route) show the uniform elemental distribution throughout the powder sample. Overall, Na₂SnF₆ synthesized via the OS route exhibits a higher concentration of Na vacancies compared to that prepared by the OS–ligand route. This observation is consistent with the XRD results and supports the smaller unit–cell parameters estimated for the MHP synthesized through the OS route. EDS analysis yielded an atomic ratio of Na : Sn : F close to the composition Na₂SnF₆ within error due to limitations of EDS for light elements.

Fig. 4 EDS elemental mapping of Na, Sn, and F in synthesized Na₂SnF₆ in the OS-route under various reaction conditions: (a) NaF_120 µm (60 °C), (b) NaF_5 µm (60 °C), (c) NaF_100–200 nm (60 °C), (d) NaF_5 µm (35 °C), and (e) NaF_5 µm (RT).

Fig. 5 EDS elemental mapping of Na, Sn, and F in synthesized Na₂SnF₆ in the OS-route under various reaction conditions: (a) NaF_120 µm (60 °C), (b) NaF_5 µm (60 °C), (c) NaF_100–200 nm (60 °C), (d) NaF_5 µm (35 °C), and (e) NaF_5 µm (RT).

Table 2 EDS-derived empirical formula for all-synthesized Na₂SnF₆

MHP (Na2SnF6)	OS route	OS-ligand route
NaF_120 µm (60 °C)	Na1.88SnF6.02	Na2.01SnF6
NaF_5 µm (60 °C)	Na1.98SnF6.02	Na2SnF6
NaF_100–200 nm (60 °C)	Na2SnF6	Na2.03SnF6.08
NaF_5 µm (35 °C)	Na1.92SnF6.06	Na2SnF6.02
NaF_5 µm (RT)	Na2SnF6	Na1.99SnF6

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3.3. Thermal stability of MHPs by thermogravimetric analysis

Fig. 6(a) and (b) present the thermogravimetric analysis (TGA) results illustrating the thermal stability of the synthesized MHPs. Upon heating up to 400 °C, Na₂SnF₆ synthesized via the OS route at 60 °C using NaF with grain sizes of 120 µm, 5 µm and 100–200 nm exhibits weight losses of approximately 2.0%, 2.1%, and 2.0%, respectively. In addition, samples synthesized at 35 °C and room temperature (RT) using 5 µm NaF show weight losses of 2.2% and 2.25%, respectively. These minor weight losses are attributed to the removal of adsorbed moisture and volatile impurities. Similarly, in the OS-ligand route, Na₂SnF₆ synthesized at 60 °C using NaF with grain sizes of 120 µm, 5 µm and 100–200 nm undergoes weight losses of approximately 2.0%, 2.4%, and 2.6%, respectively; whereas samples synthesized at 35 °C and RT using 5 µm NaF exhibit weight losses of 1.4% and 2.5%, respectively. Overall, the MHPs synthesized via the OS-ligand route show a comparable thermal stability trend to those prepared by the OS route. A distinct weight-loss event was observed in the range of 340–360 °C, which is attributed to the decomposition and removal of residual oleic acid. The TGA results confirm that the synthesized Na₂SnF₆ exhibits excellent thermal stability up to 400 °C, irrespective of reaction temperature, NaF grain size, or the presence of surfactants.

Fig. 6 Thermal stability as evaluated by TGA testing (up to 600 °C with a ramping rate of 10 °C min⁻¹) for the synthesized Na₂SnF₆ samples under different reaction conditions via the (a) OS-route and (b) OS-ligand route.

3.4. Impacts of synthesis conditions on the reaction yield and scalability

Fig. 7 illustrates the impacts of temperature, grain size, and surfactant addition on the efficiency of NaF immobilization into Na₂SnF₆. Using 0.181 g of coarse-grained NaF (120–150 µm), effective immobilization was achieved at 60 °C with a reaction yield of 85.5% in 200 mL of methanol. Upon the addition of a surfactant, the methanol volume was reduced to 100 mL, while the reaction yield increased to 93.5% under the same conditions. When the grain size of NaF was reduced to 5–8 µm, a large amount of salt (1.25 times that of the coarser-grained NaF, i.e., 0.225 g) was immobilized at RT using only 50 mL of methanol, with a high reaction yield of 96.8% albeit with an extended reaction time of 1 h.

Fig. 7 (a) A comparison among reaction yields with respect to studied variable parameters of the immobilization process, (b) the effect of a ligand on the reaction yield, (c) the effect of grain size on reaction yield, and (d) the effect of reaction temperature on reaction yield.

Scaling up the same micron-sized feedstock to 5× (0.904 g) resulted in successful immobilization into MHP with reduced reaction yields of 84.2% and 85.4% at 35 °C and 60 °C, respectively. Further reducing the salt grain size to the nanoscale (100–200 nm) improved the reaction yield to 87% for the 5× scale (0.904 g) using 150 mL of methanol. Notably, the addition of a surfactant enhanced the efficiency of this scaled-up process, increasing the reaction yield to 97.2% while reducing the methanol requirement to only 75 mL, as shown in Fig. 7(a). Similar trends were observed for the micron-sized NaF under ligand-assisted synthesis conditions, where reaction yields exceeding 90% were achieved upon scaling the salt quantity to 5×, 5×, and 6.25× at 35 °C, 60 °C, and room temperature, respectively. Collectively, these results demonstrate a cost-effective, solution-based strategy for the immobilization of salt waste and highlight its strong potential for scalable processing. This systematic study concludes that the presence of a surfactant not only enhances the solubility of NaF in methanol but also facilitates the nucleation of Na₂SnF₆ by lowering the energy barrier and providing additional nucleation sites. Therefore, the reaction yield was found to be increased in the OS-ligand route compared the OS route as shown in Fig. 7(b). As grain size directly affects the solubility of NaF in methanol, larger-grained salt requires approximately five times more methanol than micron-grained salt to dissolve and immobilize the same amount of NaF as the solid dissolution rate is proportional to the surface area available for dissolution. Therefore, grinding the salt particles into fine particles helps to minimize the consumption of methanol. Fig. 7(c) clearly shows that the decreasing salt grain size helped to scale up the immobilization process with increased reaction yield in both OS and OS-ligand routes. Elevated reaction temperatures slightly improve the solubility of NaF and increase kinetic energy, promoting faster formation and nucleation of Na₂SnF₆. This reflects in increasing reaction yield from 35 to 60 °C as shown in Fig. 7(d). However, the reaction can also proceed effectively at room temperature, with longer processing times compensating for the lower kinetics, making it suitable for industrial-scale applications.

3.5. Consolidation and densification of Na₂SnF₆ into monolithic waste forms

It is essential to consolidate the MHP into the dense pellets as waste forms for the permanent immobilization of the waste elements. Conventional high-temperature sintering is unsuitable for this material due to the risk of MHP degradation or loss. A cost-effective sintering process (CSP) is required to fabricate monolithic, densified Na₂SnF₆ samples at temperatures significantly lower than the temperature leading to the loss of highly volatile waste elements. To overcome this limitation, a novel cold sintering process (CSP) was developed. Assisted by SPS, the CSP employed dimethyl sulfoxide (DMSO) as a transient solvent to partially dissolve the synthesized Na₂SnF₆ and promote densification at low temperature. Initial attempts using deionized water or methanol as transient solvents were unsuccessful, as Na₂SnF₆ could not be effectively densified. In contrast, the addition of a few drops of DMSO into the Na₂SnF₆-loaded graphite die enabled effective cold sintering, resulting in pellets with densities reaching ∼90% of the theoretical value.

The CSP was conducted at 200 °C for 15 min under an applied pressure of 60 MPa, with heating and cooling rates of 25 °C min⁻¹. The measured pellet densities were ∼89% and ∼91% of the theoretical density for Na₂SnF₆ synthesized at room temperature via the OS and OS-ligand routes, respectively. The enhanced densification is attributed to a dissolution–reprecipitation mechanism inherent to the cold sintering process, wherein partial dissolution of Na₂SnF₆ in DMSO facilitates elemental transport and atomic diffusion within the transient solvent phase. Consequently, the addition of a small amount of DMSO markedly improves pellet consolidation at temperatures as low as 200 °C, while preserving the structural integrity of the MHP phase. Fig. 8(a) presents the XRD patterns of the densified pellets, confirming the retention of the tetragonal Na₂SnF₆ phase. Fig. 8(b–e) show SEM images of the polished pellet surfaces prepared from Na₂SnF₆ synthesized at room temperature via the OS and OS-ligand routes, respectively, clearly illustrating particle bonding and neck formation achieved through the CSP. The homogeneous spatial distribution of Na, Sn, and F, as revealed by EDS elemental mapping (Fig. S2, SI), confirms that crystal chemistry remains chemically inert during the CSP.

Fig. 8 (a) XRD patterns of monolithic pellets of Na₂SnF₆ synthesized at RT through OS and OS-ligand routes. (b–e) The SEM images of monolithic pellets of Na₂SnF₆ synthesized at RT through OS and OS-ligand routes, respectively.

4. Conclusion

In conclusion, the present study reports a cost-effective, scalable, and low-temperature solution-based approach for the immobilization of fluoride salt waste into a MHP-structured compound, e.g., NaF as a representative system into Na₂SnF₆. The immobilization process addresses a critical need in nuclear waste management by enabling high fluoride loading (up to 57.4 wt%) with minimal byproduct formation. Systematic optimization of immobilization parameters including different reaction temperatures (60 °C, 35 °C, and RT), salt grain sizes (120 µm, 5 µm, and 100–200 nm), and the use of surfactants consistently yielded reaction efficiencies above 90%. Reducing grain size of salt and the addition of a surfactant significantly enhanced the reaction yield and reduced the methanol consumption. Structural and chemical characterization studies using XRD, SEM, EDS, and TGA confirmed the formation of thermally stable, homogeneously distributed Na₂SnF₆ with excellent crystallinity and elemental uniformity. Microstructural analysis of the synthesized waste forms revealed that the presence of a surfactant significantly enhanced the reaction yield by promoting additional nucleation sites for waste form precipitation and reducing grain agglomeration. An innovative cold sintering process integrated with SPS was developed, enabling the consolidation of synthesized Na₂SnF₆ at 200 °C while achieving 89–91% of the theoretical density and minimizing the loss of highly volatile waste elements, thereby demonstrating its practical viability as a waste form material.

Conflicts of interest

The authors declare that they have no known competing financial interests.

Data availability

All data supporting this article have been included in the manuscript. Additional data are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ta00002a.

Acknowledgements

This work was supported by a DOE ARPA-E program under the award number DE-AR0001615.

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