Thermodynamics for Actinide Monocarbides and Mononitrides

Thermodynamics of actinide monocarbides and mononitrides is studied from first-principles theory that includes relativistic electronic structure and anharmonic lattice vibrations. Density-functional theory (DFT) is expanded, when necessary, 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. For some systems, formation enthalpy, specific heat, and Gibbs energy calculated from the anharmonic model are compared to a calculation of phase diagram (CALPHAD) assessments. Overall, the theory reproduces CALPHAD results rather well, but the comparison is hampered by the sub-stoichiometric nature of the real systems.<br/>Acknowledgments: We thank B. Sadigh, A. Landa for helpful discussions. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.