Phase stability and electronic structures of stoichiometric tantalum mononitrides

Using the density functional methods, we study electronic and structural properties of stoichiometric phases of tantalum mononitride (TaN). The ε phase is found to be most stable, closely followed by the θ phase. Interestingly, we find that the cubic δ phase of the stoichiometric TaN undergoes a spontaneous distortion into a tetragonal structure. This is attributed to a Jahn–Teller type distortion that lifts degeneracy at the Fermi level. The electronic structures indicate that the largest carrier density is expected for δ-TaN while θ-TaN is close to a quasi-one-dimensional conductor.