Thermodynamic properties of Barium-Bismuth alloys determined by emf measurements

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.