Abstract
Density-functional theory (DFT) is employed to investigate the thermodynamic and ground-state properties of bulk uranium tri-iodide, UI3. The theory is fully relativistic and electron correlations, beyond the DFT and generalized gradient approximation, are addressed with orbital polarization. The electronic structure indicates anti-ferromagnetism, in agreement with neutron diffraction, with band gaps and a non-metallic system. Furthermore, the formation energy, atomic volume, crystal structure, and heat capacity are calculated in reasonable agreement with experiments, whereas for the elastic constants experimental data are unavailable for comparison. The thermodynamical properties are modeled within a quasi-harmonic approximation and the heat capacity and Gibbs free energy as functions of temperature agree with available calculation of phase diagram (CALPHAD) thermodynamic assessment of the experimental data.
Highlights
Uranium tri-iodide, UI3, is a rare anti-ferromagnetic inorganic compound that forms in thePuBr3 -type, Cmcm, orthorhombic structure [1,2]
We are not focused on band gaps or spectroscopical properties in this report, but on high-temperature thermodynamical properties that can be derived from energetics that are less sensitive to unoccupied energy levels associated with band gaps
We have explored the rare uranium tri-iodide compound from first-principles theory and calculation of phase diagram (CALPHAD)
Summary
PuBr3 -type, Cmcm, orthorhombic structure [1,2]. This structure has the same space group number (#63). Aside from its crystal structure and magnetic configuration, UI3 ’s physical properties have not received a lot of attention compared to those of other uranium compounds. As with most uranium compounds, UI3 has substantial magnetic moments that have been reported in the range 1.97–3.65 μB on the uranium atoms [2,3]. These moments are reported [2] to order in a complex non-intuitive anti-ferromagnetic fashion. In addition to its structure and magnetic properties, the energy of formation (or formation enthalpy) has been measured (−98.8 [7] and −111.6 kcal/mol [8])
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