Highly efficient recovery of Nd into liquid metals of Bi and Sn was achieved in molten LiCl-KCl-NdCl₃ electrolyte at 773–973 K by leveraging the strong interactions of Nd with liquid metals. Based on the emf measurements of Nd-Sn and Nd-Bi alloys, the activity values of Nd were determined as low as 1.1‒5.8×10^–13 in both liquid metals at 973 K while the solubility of Nd was found to be 1.46 mol% in Sn and 5.65 mol% in Bi. Both liquid metals demonstrated high round-trip coulombic efficiencies (>99.3%) during deposition-removal cycles of 10‒50 mA cm^‒2 and high recovery capacity up to approximately 20 mol% Nd beyond the solubility limit. In addition, a high Nd recovery yield (84–90%) with respect to the applied charge was confirmed based on chemical analysis of electrolysis products in Bi after constant current electrolysis (–50 mA cm^–2) at 873‒973 K. Overpotentials during the Nd deposition process were attributed to charge-transfer and mass-transport resistances based on the current-potential curve and electrochemical impedance spectroscopy. The charge-transfer kinetics of Nd deposition into liquid metals was facile with high exchange current densities at ∼220 mA cm^‒2. The exceptionally high recovery efficiency for Nd in the molten chloride is thought to result from strong chemical interactions (i.e., low activity) of Nd in liquid metals that encourage one-step reduction, i.e., Nd³⁺ + 3e → Nd(in Bi or Sn) by effectively suppressing side reaction pathways from multivalent states (Nd²⁺ and Nd³⁺).

Using an electrochemical cell containing a two-phase Nd-Sn alloy (x_Nd = 0.10) in LiCl–KCl–NdCl₃ (1 mol%) as a reference electrode, coulometric titration emf measurements at various cathodic current densities (10–100 mA cm⁻²) were performed to evaluate the stability of the Nd-Sn electrode and corroborate emf values for Nd (in Bi) at T = 873 K. The potential difference between two identical Nd-Sn reference electrodes was measured before/after the electrochemical evaluation for 45 days, and the stability of the cell for the long-term operation was verified. The emf values of Nd (in liquid Bi) titrated at various current densities were consistent at each Nd concentration, demonstrating the reliability of the cell. 

Electromotive force measurements were used to examine pure rare-earth reference electrodes (Nd, Gd) in molten chloride salts (LiCl-KCl-RECl₃) over a temperature range of T = 400–800 °C to determine the effects of disproportionation reactions (i.e., Nd + NdCl₃ → NdCl₂) and active metal dissolution on their longevity and stability. Several methods for developing and calibrating a stable two-phase Gd-Bi electrode were investigated including the use of a pure Gd metal reference electrode continuously submerged in the electrolyte, a pure Gd metal reference electrode intermittently dipped into the electrolyte, and a transient technique that deposited pure Gd onto an inert W wire. Electromotive force measurements were also employed to examine the lifetime of the two-phase alloy as the reference electrode in comparison to a pure rare-earth metal as the reference electrode. Pure rare-earth metal did not exhibit stability for more than 15 h, whereas two-phase Gd-Bi electrodes were found to be stable for over 24 days at T = 500–700 °C.

Thermodynamic properties of the Nd-Bi system were investigated using a combination of experimental measurements, first-principles calculations based on density functional theory (DFT), data mining and machine learning (DM + ML) predictions, and calculation of phase diagrams (CALPHAD) modeling. The electromotive force (emf) of Nd-Bi alloys in molten LiCl-KCl-NdCl₃ at 773–973 K was measured via coulometric titration of Nd into Bi for the determination of thermochemical properties such as activity coefficients and solubilities of Nd in Bi. A new peritectic reaction of [liquid + NdBi₂ = Nd₃Bi₇] at 774 K was confirmed using differential scanning calorimetry, structural (X-ray diffraction), and microstructural (scanning electron microscopy) analyses. The unknown crystal structure of NdBi₂ was suggested to be a mixture of the anti-La₂Sb configuration and the La₂Te-type configuration based on ML predictions for over 26,000 data-mined AB₂-type configurations together with DFT-based verifications. Using the newly acquired experimental data and DFT-based calculations, the thermodynamic description of the Nd-Bi system was remodeled, and a more complete Nd-Bi phase diagram was calculated, including the Nd₃Bi₇ compound, invariant transition reactions, and liquidus temperatures.

In this work, we present a software package in Python for high-throughput first-principles calculations of thermodynamic properties at finite temperatures, which we refer to as DFTTK (Density Functional Theory ToolKit). DFTTK is based on the atomate package and integrates our experiences in the last decades on the development of theoretical methods and computational softwares. It includes task submissions on all major operating systems and task executions on high-performance computing environments. The distribution of the DFTTK package comes with examples of calculations of phonon density of states, heat capacity, entropy, enthalpy, and free energy under the quasi-harmonic phonon scheme for the stoichiometric phases of Al, Ni, Al₃Ni, AlNi, AlNi₃, Al₃Ni₄, and Al₃Ni₅, and the fcc solution phases treated using the special quasirandom structures at the compositions of Al₃Ni, AlNi, and AlNi₃.

Thermodynamic properties of binary Gd-Bi alloys (mole fraction, x_Gd = 0.02–0.35) were determined using electromotive force (emf) measurements in molten LiCl-KCl-GdCl₃ electrolyte in complement with structural, microstructural, and thermal characterization. X-ray diffractometry (XRD) and scanning electron microscopy (SEM) identified the phase constituents of Gd-Bi alloys (x_Gd = 0.02–0.35) as Bi matrix and GdBi compound. According to differential scanning calorimetry (DSC) measurements up to 1250 K, a eutectic transition [L = Bi + GdBi] was observed at 543 K but the reported peritectic transition [L + GdBi = GdBi₂] was not detected, suggesting the absence of the GdBi₂ compound. Based on structural, microstructural, and thermal characterization of Gd-Bi alloys (x_Gd = 0.02–0.35), the GdBi₂ compound was not detected, and its thermodynamic stability is questionable. The emf of Gd-Bi alloy (x_Gd = 0.16) at 700–1048 K relative to Gd(s) was measured by electrodepositing pure Gd metal at 25 K increments. The emf values of Gd-Bi alloys were determined via coulometric titration of Gd into liquid Bi using a two-phase [L + GdBi] alloy as a reference electrode at 773–973 K. The solubility of Gd in liquid Bi was also estimated to be 0.50 mol% (773 K), 0.99 mol% (873 K), and 2.21 mol% (973 K) and the excess partial molar Gibbs energy of liquid Gd-Bi alloys was as low as 65 kJ mol^–1, indicating strong chemical interactions between Gd and Bi.

Closing the nuclear fuel cycle requires recycling used nuclear fuel. Additional waste is generated during recycling due to fission products accumulating in processing salts (LiCl–KCl). Reducing the waste generated during recycling entails recovering alkaline-earth fission products (Ba²⁺/Sr²⁺) from molten chlorides with a minimal loss of bulk electrolyte constituents (Li⁺/K⁺). Electrochemical codeposition of Ba²⁺/Li⁺ and Sr²⁺/Li⁺ into liquid metal (Bi, Sb, Sn, and Pb) and alloy (Bi–Sb) electrodes was investigated in LiCl–KCl–(BaCl₂, SrCl₂) electrolytes at 500 and 650 °C. For the pure Bi (500 °C) and Sb (650 °C) electrodes, the greatest percentage of charge was used to deposit Ba and Sr. Effective recovery of Ba/Sr by liquid Bi and Sb electrodes is supported via experimentally determined activity values of Ba/Sr in Bi and Sb. Alloying Sb with Bi increased Ba recovery but decreased Sr recovery, compared to the recovery using a liquid Bi electrode. The results suggest that alkaline-earth fission products can be recovered from molten chlorides using liquid metal electrodes via electrochemical separation, thereby providing a method to reduce the generation of nuclear waste from nuclear fuel recycling.

Thermodynamic properties of Nd–Bi and Nd–Sn alloys were determined via electromotive force (emf) measurements at 725–1075 K. The emf measurements of an Nd–Bi alloy at mole fraction xNd = 0.20 were conducted using a solid CaF2–NdF3 electrolyte relative to pure Nd(s). The emf values from the CaF2–NdF3 electrolyte were verified in separate experiments in molten LiCl–KCl–NdCl3 where pure Nd(s) was electrodeposited. The Nd–Bi (xNd = 0.20) exhibited two-phase behavior with a peritectic reaction (L + NdBi = NdBi2) at 926 K from differential scanning calorimetry. The two-phase Nd–Bi (xNd = 0.20) was employed as a stable reference electrode in molten LiCl–KCl–NdCl3 for emf measurements of Nd–Bi (xNd = 0.15–0.40) and Nd–Sn (xNd = 0.10) alloys. The emf measurements of these alloys were reproducible during thermal cycles over 50 h and were used to calculate thermodynamic properties, including the partial molar Gibbs energy, entropy, and enthalpy.

Electrochemical reduction of Sr²⁺ and Ba²⁺ into liquid Bi was investigated in dilute concentrations of SrCl₂/BaCl₂ (0‒5 mol%) in LiCl-KCl electrolytes at 500 °C to ascertain the limit of liquid Bi electrodes for alkaline-earth recovery. Analysis of the electrodes after constant current electrolysis to the specific charge of 270 C g^–1 showed Sr²⁺ ions consuming 29% of charge at 5 mol% before dropping to 8‒10% of the total charge at 0.45‒0.72 mol% SrCl₂ and Ba²⁺ ions consuming 54% at 5 mol% BaCl₂ before decreasing to 22‒24% at 0.42‒0.89 mol% BaCl₂; substantial co-deposition of Li was observed in all chemistries, consuming up to 53% of charge. At ~0.1 mol% SrCl₂/BaCl₂, these alkaline-earth ions consumed 1% of the total charge indicating ~0.4 mol% to be the lower limit of alkaline-earth chloride concentration to achieve appreciable deposition of alkaline-earths. The overpotentials of liquid Bi at 5 mol% of SrCl₂/BaCl₂ were evaluated by electrochemical impedance spectroscopy. The co-deposition of Sr and Li exhibited the largest increase in charge transfer resistances implying sluggish charge transfer kinetics whereas the co-deposition of Ba and Li exhibited a large increase in mass transport resistances due to the slow diffusion of Ba²⁺ ions in the electrolyte.

The thermochemical properties of Sr-Sn alloys were determined by electromotive force (emf) measurements to evaluate liquid tin as an interacting electrode material for separating alkaline-earth elements from molten salt solutions. A Sr(s)|CaF2-SrF2|Sr(in Sn) cell was used to measure emf values for twelve Sr-Sn alloys at mole fractions xSr = 0.02‒0.43, allowing the determination of thermochemical properties such as activity and partial molar quantities of Gibbs energy, entropy, and enthalpy of Sr at 730‒1110 K. Activity values of Sr in liquid Sn were as low as 6.9×10‒12 at 800 K and xSr = 0.02 indicating highly non-ideal solution behavior between Sr and Sn. Phase transitions were also determined from the emf data and were validated via differential scanning calorimetry (DSC). Through the combination of emf measurements for thermochemical properties, X-ray diffraction (XRD) for phase constituents, and DSC measurements for phase transitions, this work established more complete thermodynamic properties of the Sr-Sn binary system.

Fictive associates are widely used to describe and model liquid phases with strong ordering trends. However, little evidence is known about the assumed associates in most cases. In the present work, an ab initio molecular dynamics (AIMD) study is employed to investigate the characters of the Ba-Bi liquid, in which associates have been assumed in existing thermodynamic modeling. It is found that in the Ba rich melt, the Bi atoms are almost completely surrounded by Ba atoms. The Bi-centered coordination polyhedrons are strongly associated to crystalline structures of Ba₅Bi₃ and Ba₄Bi₃ with a longer lifetime than other polyhedrons during the AIMD simulations. In addition, these Bi-centered polyhedrons in Ba rich melt connect with each other through vertex, edge, face, and/or bipyramid sharing to form medium range orders (MRO). In the Bi rich melt, the Ba-centered polyhedrons also form MROs, but they are both structurally and compositionally diverse with a shorter lifetime. These findings from AIMD study provide evidences that there exist a strongly ordering Ba₄Bi₃ associate and a weakly ordering BaBi₃ associate in the Ba-Bi liquid. The predicted enthalpy of mixing in the liquid agrees well with the results by the CALPHAD modeling in the literature.

Thermodynamic properties of Ca–Pb alloys are investigated by electromotive force (emf) measurements to determine equilibrium cell potentials and phase properties for their application in energy storage systems such as liquid metal batteries. Using the electrochemical cell Ca(s) | CaF₂(s) | Ca(in Pb) at 700–1060 K, cell emf is measured for thirteen Ca–Pb alloys at mole fractions, x_Ca = 0.06–0.80. At 873 K, the equilibrium potentials of liquid Ca–Pb alloys are 0.57–0.62 V versus Ca and the activity values are as low as a_Ca = 6.2 × 10⁻⁸ at x_Ca = 0.06. In addition, the emf values as a function of temperature provide partial molar quantities (entropy and enthalpy) as well as phase transitions which are corroborated by determining transition temperatures and phase constituents using differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD). This work establishes the fundamental data necessary for the design of Pb-containing liquid metal electrodes through the integration of electrochemical, thermal, and structural properties of Ca–Pb electrodes.

Thermodynamic properties of Ba–Sn alloys were determined for Ba mole fractions (x_Ba = 0.03–0.69) by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and electromotive force (emf) measurements. Six intermetallic compounds (BaSn₅, BaSn₃, Ba₃Sn₅, BaSn, Ba₅Sn₃, and Ba₂Sn) were identified by XRD and the phase transition temperatures were determined by DSC up to x_Ba = 0.50, which were used to delineate phase boundaries for constructing an experimentally-determined phase diagram. In addition, thermochemical solution properties were calculated by measuring emf values of Ba–Sn alloys using the following electrochemical cell: Ba(s) | CaF₂–BaF₂ | Ba(in Sn) over 713–1063 K, including activity and partial molar quantities of Gibbs energy, entropy, and enthalpy. The activity of Ba in Sn was as low as 1.5 × 10⁻¹¹ at x_Ba = 0.03 and 923 K, indicating highly non-ideal solution behavior with a large excess partial molar Gibbs energy (−164 kJ mol⁻¹). By integrating solution properties from emf measurements with the phase behavior by DSC and XRD, a reliable description of the Ba–Sn system was established.

The thermodynamic properties of Ba-Pb alloys (activities, phase transitions, and partial molar quantities of Gibbs energy, entropy and enthalpy) were determined based on electromotive force (emf) measurements for thirteen mole fractions (x_Ba = 0.05–0.76) from 700–1060 K using an electrochemical cell: Ba(s) | CaF₂-BaF₂ | Ba(in Pb). The activity of Ba in liquid Pb was found to be as low as 2.0 × 10^–10 at x_Ba = 0.05 and 973 K, reflecting a large excess partial molar Gibbs energy of –156 kJ mol^–1 due to strong chemical interactions between Ba and Pb. Emf measurements were most reliable in the presence of a liquid phase, but were less reliable for high-melting compositions due to the formation of metastable phases during thermal cycling, confirmed by thermal and structural analyses using differential scanning calorimetry and X-ray diffraction. The experimentally-determined activity values of Ba-Pb alloys provide a new, reliable data set for further refinement in the assessment of thermodynamic properties for the Ba-Pb system.

The thermodynamic properties of Sr-Sb alloys were determined by electromotive force (emf) measurements to understand the solution and phase behavior of Sr in Sb. A Sr(s) | CaF₂-SrF₂ | Sr(in Sb) electrochemical cell was used to measure emf values at 833–1113 K for Sr mole fractions over x_Sr = 0.03–0.84. The activity of Sr in liquid Sb was as low as 6.9 × 10⁻¹³ at 973 K and x_Sr = 0.03, implying strong chemical interactions. Six intermetallic compounds were confirmed by X-ray diffraction (XRD): SrSb₂, Sr₂Sb₃, Sr₁₁Sb₁₀, Sr₁₆Sb₁₁, Sr₅Sb₃, and Sr₂Sb. Phase transition temperatures were verified by differential scanning calorimetry (DSC) for x_Sr = 0.03–0.48. Integrating the thermodynamic properties of the Sr-Sb system from emf, XRD, and DSC measurements, three characteristic transition temperatures were newly defined and an experimentally-determined Sr-Sb phase diagram was constructed up to x_Sr = 0.50.

The electrochemical behavior of Te^2– ions was investigated in the LiCl–Li₂Te binary on glassy carbon electrodes at 650 °C as a means to understand the fundamental thermodynamic and mass transport properties of Te^2– ions. Cyclic voltammetry and constant-potential electrolysis confirmed an electrochemically reversible, two-electron soluble–insoluble reaction of Te^2–/Te(l). The formal potential for the Te^2–/Te(l) reaction was determined to be 1.744 V vs Li⁺/Li(l), and the diffusivity of Te^2– ions was about 0.44–1.25 × 10^–5 cm² s^–1. The low value for diffusivity relative to those of other cations in molten salts suggests the possibility of forming complex ions such as [Li₈Te]⁶⁺ due to strong chemical interactions with Li⁺ ions in the electrolyte. The anodic polarization of Te(l) indicated that the formation of TeCl₂(g) and the cathodic polarization of Te(l) involved the codeposition of Li metal into the Te(l). The results of this work provide essential knowledge in developing electrochemical processes for separation of tellurium as well as in mitigating the degradation reactions with structural materials.

The standard Gibbs free energy of formation (Delta G (o) (f, 298)) of bastnasite, (Ce, La, Nd, Pr)FCO3, was estimated using four different methods. The obtained average values were – 1589.5 kJ/mol for CeFCO3, – 1599.5 kJ/mol for LaFCO3, – 1589.1 kJ/mol for NdFCO3, and – 1595.4 kJ/mol for PrFCO3. The value for cerium bastnasite, CeFCO3, was used to construct a potential vs. pH (Eh-pH) diagram to investigate the stability relations of cerium bastnasite. The diagram shows that CeFCO3 is stable in neutral to basic conditions (pH similar to 6.5-11).

The thermodynamic properties and phase stability of the Ba-Bi system are investigated computationally and experimentally in the present work. The enthalpies of formation and the finite temperature thermodynamic properties of seven compounds are predicted by first-principles calculations based on density functional theory (DFT), indicating five compounds (BaBi₃, Ba₁₁Bi₁₀, Ba₄Bi₃, Ba₅Bi₃, and Ba₂Bi) to be stable. Phase relations at 773 K and 858 K with composition x_Ba = 0.90 are established by isothermal annealing and powder X-ray diffraction (XRD) to clarify the previously observed phase transition at 796 K. The extremely low chemical activity of Ba in liquid for a wide range of temperatures and compositions indicates very strong short-range ordering in the liquid phase which is modeled in the present work by introducing the Ba₄Bi₃ and BaBi₃ associates in the liquid phase. Both thermodynamic and phase equilibrium data are then used to evaluate the model parameters in Gibbs energy functions of the five stable compounds and three solution phases of liquid, bcc, and rhombohedral phases by the CALPHAD (CALculation of PHAse Diagram) technique. The present work shows that the Ba-Bi system consists of three eutectic reactions, two peritectic reactions, one peritectoid reaction, and two congruent reactions, as well as that the concentrations of associates are very high in the liquid phase with very low concentration of atomic Ba, which provides the fundamental understanding as to why Bi can be used to remove Ba ions from molten salt solutions.

The thermodynamic properties of Sr-Pb alloys were determined by electromotive force (emf) measurements. A Sr(s)|CaF₂-SrF₂|Sr(in Pb) electrochemical cell was used to measure emf values at 773–1073 K for Sr-Pb alloys at mole fractions x_Sr = 0.07–0.59. These emf measurements were used to determine thermodynamic properties of Sr-Pb alloys, including activity, partial molar entropy, and partial molar enthalpy. At 873 K, activity values of Sr in Pb were as low as a_Sr = 1.72 × 10^–9 at mole fraction x_Sr = 0.07, implying strong atomic interactions between Sr and Pb. Phase transition temperatures of Sr-Pb alloys, observed during emf measurements, were corroborated by thermal analysis (0.07 ≤ x_Sr ≤ 0.34) by differential scanning calorimetry (DSC), and the phase constituents of Sr-Pb alloys (0.07 ≤ x_Sr ≤ 0.75) were characterized using X-ray diffraction (XRD). Experimentally-determined thermodynamic properties were compared to the assessed thermodynamic properties of the Sr-Pb system, confirming the phase transition temperatures and highlighting discrepancies in solution properties (activity and excess Gibbs energy).

The identification of materials capable of catalyzing the oxygen evolution reaction (OER) in highly acidic electrolytes is a critical bottleneck in the development of many water-splitting technologies. Bulk-scale solid-state compounds can be readily produced using high-temperature reactions and therefore used to expand the scope of earth-abundant OER catalysts capable of operating under strongly acidic conditions. Here, we show that high temperature arc melting and powder metallurgy reactions can be used to synthesize electrodes consisting of intermetallic Ni₂Ta that can catalyze the OER in 0.50 M H₂SO₄. Arc melted Ni₂Ta electrodes evolve oxygen at a current density of 10 mA/cm²for >66 h with corrosion rates 2 orders of magnitude lower than that of pure Ni. The overpotential required for pellets of polycrystalline Ni₂Ta to produce a current density of 10 mA/cm² is 570 mV. This strategy can be generalized to include other first-row transition metals, including intermetallic Fe₂Ta and Co₂Ta systems.

Electrochemical behavior of Ni alloys (Ni, β-NiAl, β-NiAl/Cr) was investigated in LiCl-KCl-Na₂SO₄ electrolyte at 700 °C under three gaseous atmospheres (Ar, O₂, O₂-0.1%SO₂). In oxidizing atmospheres, Ni rapidly degraded due to instability of NiO, and alumina-rich scale on β-NiAl provided limited protection against hot corrosion (e.g., cracks in the scale under O₂-0.1%SO₂); however, the addition of both Al and Cr resulted in enhanced corrosion resistance by forming a mixed-oxide (Al₂O₃-Cr₂O₃) scale in oxidizing atmospheres. In hot corrosion processes of Ni alloys, the formation and stability of oxide scales in the molten salt were influenced by gaseous atmosphere and alloying elements.

Electrochemical deposition of Sr and Ba into liquid Bi metal was investigated in LiCl-KCl-SrCl₂-BaCl₂ electrolytes at 500 °C as a means to separate stable alkaline-earth ions from the molten salts (eutectic LiCl-KCl) utilized for recycling used nuclear fuel, by leveraging the strong chemical interactions between alkaline-earth metals and liquid Bi. The liquid Bi electrodes were subjected to cathodic discharge up to 270 C g^–1 at a constant current density of 50 mA cm⁻² in eutectic LiCl-KCl with the addition of 5 mol% total of SrCl₂ and/or BaCl₂. The use of Bi resulted in complex electrode reactions, leading to co-deposition of Sr (2.0–6.5 mol%), Ba (4.1–12.8 mol%), and Li (5.9–16.2 mol%), and coulombic efficiencies of 63–67% were achieved. The observed co-deposition was also supported via thermodynamic analyses of electrode potentials by incorporating the experimentally determined activity values of each alkali/alkaline-earth metal in Bi. The results of this work suggest that alkaline-earth fission products accumulated in molten salts (Sr²⁺ and Ba²⁺) can be recovered into liquid Bi by electrochemical separation, which could be employed as a critical step for recycling the process salt (LiCl-KCl) in order to minimize the generation of additional nuclear wastes.

The electrochemical reduction behavior of molten Na₂SO₄ was investigated at 700 °C in LiCl-KCl-Na₂SO₄ (5–15 mol%) electrolyte under two different gaseous atmospheres of inert argon and oxidizing O₂-0.1% SO₂. Sulfate ions (SO₄^2–) were directly reduced into sulfur (S) and sulfide (S^2–) ions at large negative overpotentials in both gaseous atmospheres. During the transition from inert to oxidizing atmosphere, open circuit potentials of a platinum electrode (vs. Ag/Ag⁺) were shifted in the positive direction by more than 0.60 V, implying the presence of stronger oxidants which can be coupled with the metal oxidation reactions during the hot corrosion processes.

A novel electrochemical cell based on a CaF₂ solid-state electrolyte has been developed to measure the electromotive force (emf) of binary alkaline earth-liquid metal alloys as functions of both composition and temperature in order to acquire thermodynamic data. The cell consists of a chemically stable solid-state CaF₂-AF₂ electrolyte (where A is the alkaline-earth element such as Ca, Sr, or Ba), with binary A-B alloy (where B is the liquid metal such as Bi or Sb) working electrodes, and a pure A metal reference electrode. Emf data are collected over a temperature range of 723 K to 1,123 K in 25 K increments for multiple alloy compositions per experiment and the results are analyzed to yield activity values, phase transition temperatures, and partial molar entropies/enthalpies for each composition.

The thermodynamic properties of Ba-Sb alloys, including the activity, partial molar entropy, and partial molar enthalpy, were determined using the electromotive force (emf) technique for thirteen compositions spanning the composition range of x_Ba = 0.03–0.77. Emf measurements were performed at ambient pressure under an inert argon atmosphere using a Ba-Bi(x_Ba = 0.05)|CaF₂-BaF₂|Ba(in Sb) electrochemical cell at 723–1073 K. At 923 K, activity values of Ba in Sb were as low as 2.0 × 10⁻¹⁵ at mole fraction x_Ba = 0.05 and approached unity for mole fractions x_Ba ≥ 0.77. Phase characterization using X-ray diffraction (XRD) was performed on compositions of x_Ba = 0.03–0.77 and thermal characterization was performed on compositions up to x_Ba = 0.40 using differential scanning calorimetry (DSC). Integrating the results from emf, XRD, and DSC measurements, an experimentally determined Ba-Sb phase diagram was constructed.

Thermodynamic properties of Ba-Bi alloys, including the activity, partial molar entropy and enthalpy, were determined using the electromotive force (emf) technique for fourteen compositions, x_Ba = 0.05–0.80. Emf measurements were performed at ambient pressure using a Ba(s)|CaF₂-BaF₂|Ba(in Bi) or Ba-Bi(x_Ba = 0.05)|CaF₂-BaF₂|Ba(in Bi) electrochemical cells at 723–1073 K. At 773 K, activity values of Ba were as low as 6.6 × 10⁻¹⁶ at mole fraction x_Ba = 0.05 and approached unity for mole fractions x_Ba ≥ 0.80. Stable emf values were observed at mole fractions x_Ba = 0.05–0.25, exhibiting less than a 5 mV difference between the heating and cooling cycles. Mole fractions x_Ba ≥ 0.30 exhibited increased hysteresis or had an unexpected emf profile due to the formation of metastable phases such as Bi and Ba₅Bi₃, confirmed by X-ray diffraction. The Ba-Bi alloys were further characterized using differential scanning calorimetry over the entire composition range. Based on these data, a revised Ba-Bi binary phase diagram is proposed.

The thermodynamic properties of Sr-Bi alloys were determined by electromotive force (emf) measurements to evaluate the viability of liquid bismuth metal as a medium for separating alkali/alkaline-earth fission products from molten salt electrolyte. A Sr(s)|CaF₂-SrF₂|Sr(in Bi) cell was used to measure emf values at 748–1023 K for thirteen Sr-Bi alloys at mole fractions 0.05 ≤ x_Sr ≤ 0.75. Activity values of strontium in bismuth were determined at 788 K, 888 K, and 988 K as well as the partial molar entropy and enthalpy at each composition. Reproducible emf values within ± 5 mV were obtained up to x_Sr = 0.35 during cooling-heating cycle. At higher mole fractions (x_Sr ≥ 0.40), the emf values exhibited increased hysteresis during the thermal cycles due to the strong tendency of the alloys to form meta-stable phases. The non-equilibrium phase behavior of Sr-Bi alloys was verified by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and microstructural analyses. Compared to the existing equilibrium phase diagram, two additional phases of Sr₂Bi₃ and Sr₄Bi₃ were identified and discussed. Liquid-state solubility of Sr was 15–40 mol% at 788–988 K and the activity values were as low as 10⁻¹³ at 788 K, implying strong chemical interactions between Sr and Bi.