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Mass spectrometric study of the vaporization behaviour of a-Na2NpO4: Thermodynamic investigation of the enthalpy of formation

The work herein presents the first high temperature Knudsen effusion mass spectrometric measurements carried out on the sodium neptunate α-Na2NpO4. The material's vaporization behaviour was investigated under vacuum conditions, in a tungsten metal cell up to 2700 K, and in an alumina cell up to 1900 K. Gaseous and condensed phases are in equilibrium under Knudsen conditions. The equilibrium decomposition reaction of α-Na2NpO4 to neptunium oxide, sodium oxide, and oxygen was brought to light in the temperature range 1030 to 1206 K. The enthalpy of formation of α-Na2NpO4 at 298.15 K was furthermore estimated at (-1761.0 ± 7.7) kJ.mol-1 using a second law analysis on the interval 1030 to 1115 K, in good agreement with the value reported by Goudiakas et al. (Goudiakas et al. 1991) (-1763.9 ± 7) kJ.mol-1, who used solution calorimetry. On the interval 1115 to 1206 K, where the vaporization of sodium oxide formed by decomposition of α-Na2NpO4 is considered, the analysis yielded (-1761.9 ± 20.4) kJ.mol-1 and (-1758.8 ± 22.3) kJ.mol-1 taking into account two different sodium oxide sublimation mechanisms sugggested by Hildenbrand and Lau (Hildenbrand and Lau 1993) and Steinberg and Schofield (Steinberg and Schofield 1991) respectively. Finally, the detection of the masses 253 and 292 in the temperature range 1350 to 1620 K suggests a probable sublimation of the high temperature tetragonal phase of the Na2NpO4 compound next to the decomposition. The method developed in the present paper holds promises for the thermodynamic investigation of other sodium neptunates and plutonates phases.