Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

Thermodynamic assessment of the neptunium-oxygen system: Mass spectrometric studies and thermodynamic modelling

Smith, A. L., Colle, J.-Y., Benes, O., Konings, R. J. M., Sundman, B., Gueneau, C.
J. Chem. Thermodyn. 2016, 103, 257-275
ABSTRACT
Knudsen effusion mass spectrometry measurements on neptunium dioxide are reported in this work, which have allowed to improve the understanding of its vapourization behaviour and solved discrepancies noticed in the literature: the enthalpy of formation of NpO2(g) has been re-assessed and the composition of neptunia at congruent vapourization has been determined at 2262 K. In addition, a thermodynamic model for the neptunium-oxygen system has been developed using the CALPHAD method. The non stoichiometric NpO2 x phase is described herein using the compound energy formalism with ionic constituents oNp3p; Np4pP1oO2 ; VaP2, while the liquid phase is represented with the ionic twosublattice model oNp4pPPoO2 ; VaQ ;OPQ . The reliability and consistency of all optimized Gibbs energies have been verified by calculating the phase equilibria, thermodynamic data, oxygen chemical potential and equilibrium partial pressures. Finally, a number of ill-defined data in the Np O system have been identified after critical review of the literature and comparison with the present experimental results and CALPHAD model.
Compounds
# Formula Name
1 NpO2 neptunium(IV) oxide
Datasets
The table above is generated from the ThermoML associated json file (link above). POMD and RXND refer to PureOrMixture and Reaction Datasets. The compound numbers are included in properties, variables, and phases, if specificied; the numbers refer to the table of compounds on the left.
Type Compound-# Property Variable Constraint Phase Method #Points
  • POMD
  • 1
  • Mass density, kg/m3 ; Crystal
  • Temperature, K; Crystal
  • Pressure, kPa; Crystal
  • Crystal
  • X-ray diffraction
  • 1
  • POMD
  • 1
  • Vapor or sublimation pressure, kPa ; Crystal
  • Temperature, K; Crystal
  • Crystal
  • Gas
  • Calculated from knudsen effusion weight loss
  • 72
  • POMD
  • 1
  • Vapor or sublimation pressure, kPa ; Crystal
  • Temperature, K; Crystal
  • Crystal
  • Gas
  • Calculated from knudsen effusion weight loss
  • 44