Thermochemical studies on the system Na–U–O

Measurement of sodium vapor pressures and oxygen potentials over the three-phase fields 〈NaUO3〉–〈Na2U2O7〉–〈Na2UO4〉, 〈Na2U2O7〉–〈Na6U7O24〉–〈NaUO3〉 and 〈Na6U7O24〉–〈NaUO3〉–UO2 are made in the present study. Vapor pressure of sodium was measured by using a transpiration technique and the oxygen potential was measured by using emf (electro motive force) measurements with oxide and fluoride solid electrolytes. From the vapor pressure measurements a mean value of −1496.7 kJ mol−1 was obtained for the Δ f H 〈 NaUO 3 〉 ° at 298 K. The Δ f G 〈 Na 6 U 7 O 24 〉 ° was determined by measuring the sodium potential and the oxygen potential over the phase field 〈Na2U2O7〉–〈NaUO3〉–〈Na6U7O24〉. The Δ f G 〈 Na 6 U 7 O 24 〉 ° in the temperature range 774–1474 K was determined for the first time.