Band structure and cohesive properties of 3d-transition-metal carbides and nitrides with the NaCl-type structure

We study the cohesive energy of NaCl-structure MX compounds with X=C and N and M=Sc, Ti, V, Cr, Mn, Fe, Co, and Ni, by combining ab initio total-energy calculations with an analysis of thermodynamic information. Using the linear muffin tin orbital method, we calculate the cohesive energy (E₂₎₇) of carbides and nitrides and establish trends as a function of the average number of valence electrons per atom in the compound (n₄). These results are compared with values derived by us from thermodynamic information. Since most of the compounds considered here are metastable, we apply interpolation and extrapolation procedures to get information on their E₂₎₇. This allows us to extend the comparison between theory and thermodynamic data to a wider range of n₄ values than covered in previous work. We find a remarkable agreement between theoretical and experimental trends. There is, however, a systematic difference between linear muffin tin orbital and thermodynamic values comparable to that observed in calculations for the pure 3d transition metals. The general variation of the cohesive energy is discussed and interpreted in the light of our band-structure results. A more detailed study is carried out for CrN and the NaCl-structure carbide and nitride of Mn.