Temperature dependence of phase stabilities of hexagonal hemicarbides from first principles

Abstract

Transition metal hemicarbides, specifically M₂C (where M = V, Nb, Ta, Mo and W), have received considerable attention in the fields of catalysis and metallurgy. However, the determination of the exact phase of each compound can still be challenging due to the close energetic proximity of its various polymorphs. This study uses first-principles calculations to carefully consider the subtle differences between different polymorphs, with temperature as a key factor. It is found that the energies of each polymorph of M₂C are close, but can be distinguished when temperature effects are considered. This means that temperature plays an important role in the polymorph transformations. In addition, these phases exhibit dynamic stability at both zero and finite temperatures, in part due to the particular ordering of C atom occupancy in the metal lattice interstices. The study can provide calculations of a range of properties that help to identify stable structures and deepen the understanding of these materials in terms of chemical bonding, structural changes, lattice thermal vibrations and molecular dynamics.