Cohesive properties and electronic structure of 5d-transition-metal carbides and nitrides in the NaCl structure

We present a study of the cohesive energy (Ecoh) and the enthalpy of formation (Δ⁰H) of the NaCl-structure carbides and nitrides of the 5d transition metals, using ab initio linear-muffin-tin-orbitals total-energy calculations and an extensive analysis of thermochemical and phase-diagram data. The same approach has previously been applied to 3d- and 4d-transition-metal carbides and nitrides [Phys. Rev. B 43, 14 400 (1991); 45, 11 557 (1992)]. The results from our total-energy calculations and analyses of thermodynamic information are used in a detailed comparsion of theoretical (Ecohᵗʰ) and thermodynamic (Ecohᵉ) cohesive energies. The difference {δ}(Ecoh)=Ecohᵗʰ-Ecohᵉ is positive for all compounds considered here and it decreases on going from the 3d- to the 5d-transition-metal series. The origin of errors in ab initio calculated atomic total energies is discussed. We show that by correcting atomic energies using spectroscopic data, we get a {δ}(Ecoh) that is remarkably constant over a large part of the 3d- and 4d-transition-metal series. {δ}(Ecoh) is less regular in the 5d series, which reflects errors introduced by treating f electrons as valence states in the beginning of this series.Further insight into the effect of the systematic errors is obtained by studying {Δ}Ecoh=Ecoh(MC)-Ecoh(MN), i.e., the difference between the cohesive energy of a carbide and a nitride of the same transition metal. Theoretical and thermodynamic {Δ}Ecoh show very similar behavior along all three transition-metal series. This allows for estimates of unknown cohesive energies and enthalpies of formation. Thus, we predict Ecoh and Δ⁰H for LaC, ReN, OsN, IrN, and PtN. Apart from the presentation of new information on the 5d-series compounds, the paper summarizes results from our previous works and comparisons between all three transition-metal series are made.