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)|CaF 2-BaF 2 |Ba(in Bi) or Ba-Bi(x Ba = 0.05)|CaF 2-BaF 2 |Ba(in Bi) electrochemical cells at 723-1073 K. At 773K, 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 Ba 5 Bi 3 , 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)|CaF 2-SrF 2 |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 ± 5mV 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 Sr 2 Bi 3 and Sr 4 Bi 3 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.

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