Sublimation studies of NpO2F2

Using Knudsen effusion mass spectrometry, we have identified the following reaction as the sublimation decomposition mechanism of NpO2F2(s): 2NpO2F2(s)=NpO2(s)+O2(g)+NpF4(g). From second-law analysis of the measured pressures of NpF4(g) over the temperature range 820–985 K the derived enthalpy change at 298 K is 556.8±12.2 kJ/mol and the entropy change is 342.4 ±13.8 J/K mol. From these values and enthalpies of formation and entropies reported in the literature for the other species we calculate the enthalpy of formation of NpO2F2(s) to be −1608±10 kJ/mol and the entropy to be 147±9 J/K mol.