Lattice dynamics and thermal properties of thorium metal and thorium monocarbide

Abstract

Thorium is a chemical element that is beginning to attract attention because of its potential use as a nuclear fuel. It is not easy to carry out experiments because of its radioactive nature, and therefore theoretical works are highly appreciated. Thorium contains only a small number of the 5f states, and it is generally accepted that these states are itinerant, that they form a chemical bond, and that their nature does not need to be corrected with the Hubbard model. On the other hand, there is a well-known problem with the description of the 6p(1/2) states when the spin-orbit coupling is added as the perturbation to a scalar-relativistic calculation. Electronic, elastic, phonon, and thermodynamic properties are analyzed in terms of the importance of the spin-orbit coupling acting on the 6d and 5f states. The importance of the spin-orbit coupling acting on the semicore 6p states is discussed. The same properties are analyzed for thorium monocarbide, and a difference caused by adding a carbon atom into the structure is discussed. Detailed analysis of the thermal conductivity (both phonon and electronic contributions) is also included. We have given extra attention to the thermal conductivity of ThC to explain why the optical phonon modes account only for approximately 6% of the phonon thermal conductivity.