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Results for “solubility determination”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

High-throughput solubility determination for data-driven materials design and discovery in redox flow battery research

Solubility is crucial for redox flow batteries because it affects their energy density. A data-driven approach based on artificial intelligence/machine learning models can accelerate the development of highly soluble redox-active materials, but the lack of relevant, large-quantity data makes accurate solubility prediction difficult. To overcome this deficiency, we developed a high-throughput experimentation process that combines a robotically controlled platform with high-throughput methodology to collect large-scale and high-quality solubility data. We demonstrate the potential utility and applicability of this high-throughput process by measuring the aqueous and non-aqueous solubilities of redox-active materials and studying the effect of additives on their solubilities for both aqueous and non-aqueous redox flow battery applications. A redox flow battery based on our optimized negative electrolyte formulation and a ferrocyanide-positive electrolyte offers highly stable performance over 18 days (>100 cycles) with consistent capacity and a 24% boost in energy density.

25 ENERGY STORAGE↗
DOE OSTI2020

Nitroxide Radical Polymer–Solvent Interactions and Solubility Parameter Determination

Redox-active polymers such as macromolecular nitroxide radicals have been studied as electrode materials for organic batteries and electronics. Polymer–solvent interactions are essential to processing and device performance, but macromolecular nitroxide radical–solvent interactions are currently not well described. In this work, the Hildebrand and Hansen solubility parameters of poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), oxidized PTMA (PTMA + ), and PTMA’s precursor (PTMPM) are determined using both experimental and group contribution methods for the first time. This work indicates that the hydrogen-bonding Hansen solubility parameter (δ h ) and the solubility sphere radii (R) provide the best prediction of the macromolecular radical’s interaction with solvents. From these solubility parameters, the group contribution values for the nitroxide and oxoammonium cation were then calculated. It is shown that this information allows for the prediction of polymer–solvent interactions for other nitroxide-based macromolecular radicals. Lastly, the discovered solubility parameters are used to predict PTMA electrode formulations for batteries that outperform controls.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Determining the Solubility Behavior of Kogarkoite in Simulated Nuclear Waste

Kogarkoite (Na 3 FSO 4 ) is a sparingly soluble fluoride–sulfate double salt that has been identified in high level nuclear waste sludge at the Hanford Site and, more recently, in sludge batch compilation samples at the Savannah River Site (SRS). Due to its complex dissolution behavior, which exhibits an inverse dependence on sodium ion activity, the presence of this mineral poses significant challenges to waste retrieval and processing. Incomplete dissolution during sludge washing can lead to the retention of fluoride and sulfate in the high-level waste feed, potentially causing the formation of corrosive, immiscible molten salt layers, known as "glass gall,” in vitrification melters. Current efforts to optimize flowsheet parameters and wash-water volumes are hindered by the absence of a commercially available, certified reference material, which prevents the accurate calibration of analytical methods and the verification of dissolution kinetics. To address this critical gap, this research focuses on the laboratory synthesis of pure Kogarkoite to serve as a standard for comprehensive solubility and washing performance testing. A coupled synthesis and simulant campaign was executed using an evaporative crystallization protocol designed to replicate the dynamic concentration effects observed in tank farm operations. Thirteen simulant matrices were prepared by dissolving systematically varied ratios of sodium fluoride (NaF) and sodium sulfate (Na 2 SO 4 ) in deionized water under three distinct caustic regimes: 0.0 g (control), 4.0 g (~1 M), and 12.0 g (~3 M) sodium hydroxide (NaOH). While thermodynamic equilibrium models suggest that high-caustic environments should favor the stability of the double salt7, results from this evaporative study at 25 0 C revealed a distinct kinetic divergence. Simulants with high hydroxide loading predominantly yielded large, blocky crystals of sodium sulfate decahydrate (Na 2 SO 4 .10H 2 O). Successful synthesis of pure Kogarkoite was achieved exclusively in specific NaOH-free compositional windows, where the precipitate manifested as fine, opaque granular aggregates. Ion chromatography (IC) analysis confirmed phase purity through the simultaneous stoichiometric depletion of both fluoride and sulfate from the supernatant. This successful synthesis establishes a reproducible route to generate bulk Kogarkoite, enabling the subsequent phase of quantitative dissolution testing using inhibited water to optimize sludge-batch assembly.

Sarker, Md Sharif [Florida International Univ. (FI↗
DOE OSTI2025

Composition and Plume Gas Interaction Control Iron Fractional Solubility More Than Particle Size in Volcanic Ash: Implications for Fertilization of the North Atlantic

Deposition of volcanic ash is thought to impact marine biogeochemical cycling by adding soluble iron (Fe) to the surface ocean. The magnitude of this input is a function of the amount of ash deposited, the total Fe content in the ash, and ash‐derived Fe's fractional solubility. However, the relative importance of chemical composition, acidic processing by the volcanic plume, and ash particle size in determining solubility is unclear. We paired an aerosol leach meant to provide an upper limit for fractional Fe solubility with chemical analyses of ash from the Cumbre Vieja (CV) and La Soufrière eruptions, which both impacted the North Atlantic in 2021. Fe in the ash samples is <6% soluble, but Fe fractional solubility in CV ash is approximately triple that of La Soufrière ash. Compared to La Soufrière, a larger proportion of the Fe in CV ash is in silicate rather than oxide minerals, which release more soluble Fe. Elevated levels of surficial fluorine (F) also suggest that CV ash was subjected to a more fluorine‐rich eruption plume and underwent more acidic processing. Particle size does not appear to be a primary control on Fe release. We estimate that the CV eruption had a much larger impact on dissolved Fe (DFe) concentration in the surface ocean than the La Soufrière eruption because of differences in soluble Fe content and particle deposition velocity. These differences may help explain why some eruptions elicit a biological response in the ocean while others do not.

Elliott, Hope E. [Univ. of Miami, FL (United State↗
DOE OSTI2025

Tailoring Electrolytes by Decoupling the Roles of Li + and Lithium Polysulfides in Li–S Batteries

Understanding the distinct roles of lithium ions (Li + ) and lithium polysulfide intermediates, Li 2 S x (LiPS) is critical for designing electrolytes that can extend the practical cycle life of lithium–sulfur (Li–S) batteries. Here, in this work, we decouple the solvation and solubility effects of Li + and LiPS and correlate them with electrochemical performance through a cosolvent strategy. Li + solubility and solvation primarily dictate the electrolyte’s ionic conductivity and the reversibility of lithium anode stripping/plating. In contrast, LiPS solvation governs the thermodynamics of sulfur (S 8 ), LiPS, and lithium sulfide (Li 2 S) interconversion, while the LiPS solubility determines their redox kinetics. By employing a fluorinated–glyme (F-glyme) cosolvent, specifically 1,2-bis­(2,2-difluoroethoxy)­ethane (F4DEE), that exhibits low LiPS solubility yet moderate Li + solvation, we designed an electrolyte that enhances lithium anode stability while maintaining sufficient sulfur cathode kinetics, thereby prolonging Li–S cell cycle life. This study provides mechanistic insights into the interplay between Li+ and LiPS in Li–S electrochemistry and offers design principles for next-generation electrolytes for Li–S batteries.

Yang, Jingtian [Argonne National Laboratory (ANL),↗
DOE OSTI2022

Data-Centric Development of Lignin Structure–Solubility Relationships in Deep Eutectic Solvents Using Molecular Simulations

Lignin is a natural source of aromatic chemicals with significant potential as an abundant, renewable feedstock for value-added products. Deep eutectic solvents (DES)–solvents composed of a hydrogen bond donor (HBD) and acceptor (HBA) in varying ratios–have emerged as a highly tunable class of solvents for lignin solubilization. However, the variety of possible DES compositions and limited molecular-scale understanding of lignin solubility makes solvent selection a challenge without laborious trial-and-error experimentation. To address these challenges, we use classical molecular dynamics (MD) simulations to study the interactions of lignin model compounds with various DES–water systems. Quantitative parameters (descriptors) were calculated by postprocessing the MD results and used to train a regression model that predicts experimentally determined solubilities of lignin model compounds. This approach revealed that the most important descriptors of solubility are the system temperature, solute hydrophilicity, and metrics quantifying hydrogen bonding. Maximizing the interactions between solute–HBD (hydrophobic group), water–HBD (hydrophilic group), and water–HBA molecules led to the highest model compound solubility. Our results support a hydrotropic mechanism in which extensive DES–water hydrogen bonding and favorable HBD interactions with the solute promote high solubility. We applied the regression model derived using model compounds to predict the solubility of representative lignin oligomers. The model predicted lignin oligomers’ solubilities in good agreement with experiments, indicating that the simulations of model compounds can be extended to predict the solubility of larger lignin compounds across a range of solvent compositions and temperatures. Furthermore, these findings provide new molecular-scale insight into lignin solubilization mechanisms and a new method for computationally screening potential solvent systems for lignin valorization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Experimental determination of the solubility constant of kurnakovite, MgB3O3(OH)5·5H2O

In this study, I present experimental results on the equilibrium between boracite [Mg 3 B 7 O 13 C1(cr)] and kurnakovite [chemical formula, Mg 2 B 6 O 11 ·15H 2 O(cr); structural formula, MgB 3 O 3 (OH) 5 ·5H 2 O(cr)] at 22.5 ± 0.5 °C from a long-term experiment up to 1629 days, approaching equilibrium from the direction of supersaturation, Mg 3 B 7 O 13 C1(cr) + H + + 2B(OH)$^-_4$ + 18H 2 O(1) ⇌ 3MgB 3 O 3 (OH) 5 ·5H 2 O(cr) + C1 - . Based on solubility measurements, the 10-based logarithm of the equilibrium constant for the above reaction at 25 °C is determined to be 12.83 ± 0.08 (2σ). Based on the equilibrium constant for dissolution of boracite, Mg 3 B 7 O 13 C1(cr) + 15H 2 O(1) = 3Mg 2+ + 7B(OH)$^-_4$ + C1 - + 2H + at 25 °C measured previously (Xiong et al. 2018) and that for the reaction between boracite and kurnakovite determined here, the equilibrium constant for dissolution of kurnakovite, MgB 3 O 3 (OH) 5 ·5H 2 O(cr) = Mg 2+ + 3B(OH)$^-_4$ + H + + H 2 O(1) is derived as -14.11 ± 0.40 (2σ). Using the equilibrium constant for dissolution of kurnakovite obtained in this study and the experimental enthalpy of formation for kurnakovite from the literature, a set of thermodynamic properties for kurnakovite at 25 °C and 1 bar is recommended as follows: Δ H $^0_f$ = -4813.24 ± 4.92 kJ/mol, Δ G$^0_f$ = -4232.0 ± 2.3 kJ/mol, and S 0 = 414.3 ± 0.9 J/(mol·K). Among them, the Gibbs energy of formation is based on the equilibrium constant for kurnakovite determined in this study; the enthalpy of formation is from the literature (Li et al. 1997), and the standard entropy is calculated internally with the Gibbs-Helmholtz equation in this work. The thermodynamic properties of kurnakovite estimated using the group contribution method for borate minerals based on the sums of contributions from the cations, borate polyanions, and structural water to the thermodynamic properties from the literature (Li et al. 2000) are consistent, within their uncertainties, with the values listed above. Since kurnakovite usually forms in salt lakes rich in sulfate, studying the interactions of borate with sulfate is important to modeling kurnakovite in salt lakes. Finally, for this purpose, I have re-calibrated our previous model (Xiong et al. 2013) describing the interactions of borate with sulfate based on the new solubility data for borax in Na 2 SO 4 solutions presented here.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Metastable solubility and local structure of amorphous calcium carbonate (ACC)

The properties of amorphous calcium carbonate provide the foundation for understanding mineralization of calcified skeletons but inconsistencies between studies suggest the physical picture is incomplete. This study quantifies the metastable solubility and structure of ACC produced under a series of chemical conditions. Using complementary in situ methods that quantify ACC structure (in situ PDF analysis) in parallel with chemical composition (TGA, ICP-OES), we find two short-range structures are produced that also show distinct morphological differences. Conditions with high carbonate ion concentrations stabilize a Ca-rich ACC with short-range order that is independent of Mg content. In contrast, low carbonate solutions favor Mg-rich ACC with mixed Ca- and Mg-short range order. Measurements of solubility product determined from supersaturated and undersaturated conditions find the distinctive structures are related to solution carbonate activity while solubility is primarily determined by the magnesium content of the solid. Bulk solubility is a composite that concurs with established values. The findings demonstrate the type of ACC that forms is tuned by Mg2+ and CO3-2- concentration and suggest ACC structure, not bulk composition, is a reliable indicator of intermediate phase. This chemical framework also provides a basis for interpreting apparent differences between observations of ACC in synthetic and natural systems that reconciles previous structural studies.

Mergelsberg, Sebastian T.↗
DOE OSTI2022

The solubility of titanite in silicate melt determined from growth and dissolution experiments

The solubility of titanite (CaTiSiO 5 ) in Si-rich melts was measured experimentally through growth experiments at 800–1000 °C, 0.5–1.0 GPa, log fO 2 ~ CCO–0.8, t = 72–168 h, and H 2 O = 0 to 4 wt.%, and in dissolution experiments at 925–1300 °C, 0.8 GPa, t = 18–118 h, and H 2 O = 1–10 wt.% in a piston-cylinder apparatus. Run product glasses in growth experiments were homogeneous, and iron loss suppressed ilmenite crystallization. Saturation concentrations in dissolution experiments were estimated by fitting measured diffusion profiles. Titanite solubility increases with increasing temperature and melt composition parameter \(M = {\text{ molar }}\left( {{\text{Na }} + {\text{ K }} + {\text{ 2Ca}}} \right)/\left( {{\text{Al }} \times {\text{ Si}}} \right)\) . Multiple linear regression of glass composition data from growth and dissolution experiments ( n = 29) plus 39 experiments from the LEPR database (Hirschmann et al. 2008 ) yielded the titanite solubility equation (adj. r 2 = 0.95): \(\left( {TiO_{2} } \right)^{melt} \left( {wt.\% } \right) = 0.978 \times M + 0.0048 \times T\left( K \right){-}5.90\) . This model correctly predicted undersaturation in 95% of 2344 experiments from the LEPR database that did not have titanite or rutile. Application to natural rocks yields saturation temperatures that are similar to independent temperature estimates. Finally, this equation should be useful for constraining the temperatures of titanite-saturated melts, for determining whether titanite saturation in magmatic source regions is likely, and for determining when titanite can crystallize and begin to exert an influence on melt geochemistry.

58 GEOSCIENCES↗
DOE OSTI2021

Vapor-Liquid Partitioning of Methylmercury Compounds: Fundamental Data to Support the Savannah River Site Liquid Waste System: Henry's Law, Solubility and Vapor Pressure Determination for Representative Methylmercury Compounds

The Savannah River Site (SRS) Liquid Waste System (LWS) contains approximately 66 tons of mercury within the liquids, salts, and sludges that are currently being processed into final wasteforms for disposal. Mercury concentrations within the system exceed those typically experienced in environmental or industrial systems; thus, management of mercury compounds continues to be a priority for SRS. In the LWS, waste is vitrified into a borosilicate glass wasteform that contains most of the radioactivity, while the lower activity solutions are dispositioned in a low-level grout wasteform, or “saltstone”. The alkaline, high ionic strength caustic wastes are pumped, evaporated, and otherwise managed throughout the LWS and Defense Waste Processing Facility (DWPF) as they are stored and prepared for conversion to the final wasteforms. Because of the complexity of this system, a key component of effective mercury management in the LWS requires analysis of mercury in various physical phases. The high concentration of mercury within the SRS LWS has the potential to generate vapor-phase contamination. Elemental mercury (Hg 0 ), dimethylmercury ((CH 3 ) 2 Hg), and methylmercury (CH 3 Hg + ) are among species known or suspected to contribute to the flux of mercury from liquid to vapor phase (Iverfeldt and Lindquist, 1982). Chemical speciation affects not only mercury behavior in LWS operations but may also affect the performance of mercury treatment and removal technologies in the LWS.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2026

Mobility of heavy rare earth elements in magmatic-hydrothermal systems: Experimental determination of DyPO 4 solubility in supercritical fluids

Dysprosium (Dy) is part of the heavy rare earth elements (REE) and a critical component in the production of high-performance super magnets used in wind turbines, electric vehicles, and other green technologies. The HREE phosphate, xenotime, is commonly associated with hydrothermal systems, where the mobility, fractionation, and deposition of REE are controlled by the stability of aqueous complexes and solubility of REE minerals which depend on temperature, pressure, and fluid composition (salinity, pH, and ligand availability). Here, we conducted a series of synthetic DyPO 4 solubility experiments in NaCl-HCl-bearing aqueous solutions (pH of 2 at 25 °C) from 500 to 700 °C and 1.5 kbar using externally heated pressure vessels. DyPO 4 displays a strong prograde solubility with temperature across all studied fluid salinities (0–0.5 m NaCl). The DyPO₄ solubility isotherms at 500 °C, 600 °C, and 700 °C show a complex relationship with salinity where DyPO₄ solubility is highest (39.2–1552 ppm at 500–700 °C) in 0 m NaCl solutions, decreases strongly (21–331 ppm at 500–700 °C) in the 0.06 m NaCl solutions and remains relatively constant in the 0.25 m (42.8–248 ppm) and 0.5 m NaCl (40.2–124 ppm) solutions. Thermodynamic modeling suggests that pH increases as a function of temperature and salinity, and controls solubility due to the stability of chloride complexes (i.e., DyCl +2 and DyCl 2 + ) at 500–600 °C, transitioning to hydroxyl complexes (i.e., Dy(OH) 3 0 ) dominating at 700 °C and relatively alkaline pH. Comparison with NdPO 4 solubility data indicates that the light REE in monazite is significantly more soluble than the heavy REE in xenotime in saline fluids at high temperature (500–700 °C), implying efficient light/heavy REE fractionation in supercritical fluids. The lower solubility of DyPO 4 relative to NdPO 4 suggests that at high temperatures, acidic (pH 2–4), low-salinity (∼5 wt% NaCl) fluids will favor the precipitation of xenotime over monazite as it will cool from 700 °C to below 500 °C. These observations align with paragenetic mineral assemblages in NYF pegmatites (e.g., Baveno, Italy), where xenotime commonly immobilizes heavy REE during the high-temperature hydrothermal stages.

58 GEOSCIENCES↗
DOE OSTI2025

NdPO 4 solubility and aqueous Neodymium speciation in supercritical fluids: An experimental study at 500–700 °C and 1.7 kbar

A key aspect in the formation of rare earth elements (REE) deposits is the role of REE transport as aqueous REE complexes in supercritical hydrothermal solutions, where the nature of the aqueous complex is controlled by solution composition, temperature and pressure. Despite chloride being considered as one of the most abundant transporting ligands in magmatic-hydrothermal fluids, experimental investigations on the stability of aqueous REE chloride complexes are scarce above 300 °C. In this study, synthetic NdPO 4 crystals were reacted with non-saline and saline (0, 0.05 and 0.5 mNaCl), acidic (0.01 mHCl) aqueous solutions in a series of solubility experiments conducted at 500–700 °C and 1.7 kbar, where the solubilities were determined using a stable Nd isotope ( 145 Nd isotope spike) dilution technique. NdPO 4 solubility ranges between 28 ppm and 10,858 ppm, where solubility increases with both temperature and salinity. At 500 °C, log mNdPO 4 increases from –3.93 to –1.60 and there is a strong correlation between NdPO 4 solubility and NaCl concentrations (slope of 1.2 ± 0.3), indicating stabilization of the Nd chloride aqueous complexes with a stoichiometry corresponding to NdCl 2+ . At 600 °C, this correlation is weaker (slope of 0.4, log mNdPO 4 increases from –2.63 to –1.88) indicating the stabilization of both Nd chloride and hydroxyl species controlling solubility. At 700 °C, NdPO 4 solubility is largely independent of NaCl concentration indicating that solubility is controlled by Nd hydroxyl complexes, where stoichiometry suggests the neutral Nd(OH) 3 0 species is dominant. The solubility product (Ksp) of NdPO4 is derived from experimental data with the relation: log K sp = -41.81 – 0.057T – 20987/T, with T temperature in Kelvin. Comparison of the measured Nd phosphate solubility to thermodynamic predictions using the available Helgeson-Kirkham-Flowers equation of state parameters for aqueous Nd complexes indicate that predictions are up to three orders of magnitude lower compared to experimental observations. This discrepancy is most pronounced in saline solutions, suggesting that thermodynamic properties of the REE chloride species in supercritical fluids require revision. Numerical simulations of fluid-rock interaction between acidic, saline fluids and a Strange Lake felsic mineral assemblage demonstrates that NdPO 4 solubility predictions from models are four to six orders of magnitude lower than those calculated based on empirical fits from experiments, which suggests that acidic, saline fluids may play an important role in mobilizing large amounts of light REE from 450 to 700 °C.

58 GEOSCIENCES↗
DOE OSTI2022

The S solubility of Cr and Al containing simulated low-activity waste glass

Here, the presence of sulfur-containing molten salt phases during vitrification is a particular problem which can be detrimental to the melter. Borosilicate compositions were designed to explore both the effects of a fixed amount of Cr, as Cr 2 O 3 or Na 2 CrO 4 , and the impact of Al 2 O 3 , on sulfur solubility as determined by progressively adding elemental S. The NABS series (with Al 2 O 3 ) has a higher SO 3 solubility of ~3.5 mol% while the NBS series (without Al 2 O 3 ) has a SO 3 solubility of ~2 mol%. Crystalline Cr 2 O 3 formed in the NABS series upon the addition of S but is not present in the NBS series. At the highest S addition, a separate salt phase always forms, and was identified as Na 2 (S,Cr)O 4 via electron dispersive spectroscopy, ultraviolet-visible spectroscopy, x-ray diffraction, and thermal analysis. Magic angle spinning nuclear magnetic resonance identified no change in the B coordination in either series as S is added, suggesting that the Na is scavenged from other locations, such as the Si sites, to charge compensate sulfate, whether in the glass or in the salt. The Cr precursor identity did not affect the observed overall behavior regarding sulfur, but did impact Cr redox as identified with Raman spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Data-driven organic solubility prediction at the limit of aleatoric uncertainty

Abstract Small molecule solubility is a critically important property which affects the efficiency, environmental impact, and phase behavior of synthetic processes. Experimental determination of solubility is a time- and resource-intensive process and existing methods for in silico estimation of solubility are limited by their generality, speed, and accuracy. This work presents two models derived from the FASTPROP and CHEMPROP architectures and trained on BigSolDB which are capable of predicting solubility at arbitrary temperatures for a wide range of small molecules in organic solvent. Both extrapolate to unseen solutes 2–3 times more accurately than the current state-of-the-art model and we demonstrate that they are approaching the aleatoric limit (0.5–1$$\log S$$ log S ) of available test data, suggesting that further improvements in prediction accuracy require more accurate datasets. The FASTPROP-derived model (called FASTSOLV) and the CHEMPROP-based model are open source, freely accessible via a Python package and web interface, highly reproducible, and up to 2 orders of magnitude faster than current alternatives.

Science & Technology - Other Topics↗
DOE OSTI2024

High Sodium Solubility in Magnesiowüstite in Iron‐Rich Deep Lower Mantle

Abstract (Mg,Fe)O ferropericlase‐magnesiowüstite has been proposed to host the majority of Earth's sodium, but the mechanism and capacity for incorporating the alkali cation remain unclear. In this work, experiments in the laser‐heated diamond anvil cell and first‐principles calculations determine the solubility of sodium and favorability of sodium incorporation in iron‐rich magnesiowüstite relative to (Mg,Fe)SiO 3 bridgmanite. Reaction of Mg/(Mg + Fe) (Mg#) 55 and 28 olivine with NaCl at 33–128 GPa and 1600–3000 K produces iron‐rich magnesiowüstite containing several percent sodium, while iron‐rich bridgmanite contains little to no detectable sodium. In sodium‐saturated magnesiowüstite, sodium number [Na/(Na + Mg + Fe)] is 2–5 atomic percent at pressures below 60 GPa and drastically increases to 10–20 atomic percent at deep lower mantle pressures. For these two compositions, there is no significant dependence of the results on Mg#. Our calculations not only show consistent results with experiments but further indicate that such an increase in solubility and partitioning of Na into magnesiowüstite is driven by the spin transition in iron. These results provide fundamental constraints on the crystal chemistry of sodium at lower‐mantle conditions. If the sodium capacity of (Mg,Fe)O is not strongly dependent on Mg#, (Mg,Fe)O in the lower mantle may have the capacity to store the entire sodium budget of the Earth.

Geochemistry & Geophysics↗