Abstract
Thermodynamics of plutonium monocarbide is studied from first-principles theory that includes relativistic electronic structure and anharmonic lattice vibrations. Density-functional theory (DFT) is expanded to include orbital-orbital coupling in addition to the relativistic spin-orbit interaction for the electronic structure and it is combined with anharmonic, temperature dependent, lattice dynamics derived from the self-consistent ab initio lattice dynamics (SCAILD) method. The obtained thermodynamics are compared to results from simpler quasi-harmonic theory and experimental data. Formation enthalpy, specific heat, and Gibbs energy calculated from the anharmonic model are validated by direct comparison with a calculation of phase diagram (CALPHAD) assessment of PuC and sub-stochiometric PuC0.896. Overall, the theory reproduces CALPHAD results and measured data for PuC rather well, but the comparison is hampered by the sub-stoichiometric nature of plutonium monocarbide. It was found that a bare theoretical approach that ignores spin-orbit and orbital-orbital coupling (orbital polarization) of the plutonium 5f electrons promotes too soft phonons and Gibbs energies that are incompatible with that of the CALPHAD assessment of the experimental data. The investigation of PuC suggests that the electronic structure is accurately described by plutonium 5f electrons as “band like” and delocalized, but correlate through spin polarization, orbital polarization, and spin-orbit coupling, in analogy to previous findings for plutonium metal.
Highlights
The physics of the actinides and their compounds are fascinating and somewhat controversial.The controversy is primarily focused on the nature of the actinide 5f electrons and their degree of correlation
Strong electron correlation manifests itself as localization of the 5f electrons on the actinide atom and for the actinide-oxide compounds this localization leads to band gaps in the electronic structure [1,2]
In the present report we focus on ground-state and high-temperature thermodynamical properties of plutonium monocarbide, PuC, from first-principles theory
Summary
The physics of the actinides and their compounds are fascinating and somewhat controversial. The controversy is primarily focused on the nature of the actinide 5f electrons and their degree of correlation. Strong electron correlation manifests itself as localization of the 5f electrons on the actinide atom and for the actinide-oxide compounds this localization leads to band gaps in the electronic structure [1,2]. For the first four, there is consensus that they are appropriately described by band-like 5f electrons [3,4]. In terms of theoretical approaches, weak or intermediate electron correlation implies that density-functional theory is a viable starting point and methods aimed for stronger electron correlations, assuming explicitly an intra-atomic Coulomb interaction with a Hubbard U parameter (density-functional theory (DFT) + U), is not necessary
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