We present a study of the cohesive energy (${\mathit{E}}_{\mathrm{coh}}$) and the enthalpy of formation (${\mathrm{\Delta }}^0$H) 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 (${\mathit{E}}_{\mathrm{coh}}^{\mathrm{th}}$) and thermodynamic (${\mathit{E}}_{\mathrm{coh}}^{\mathit{e}}$) cohesive energies. The difference δ(${\mathit{E}}_{\mathrm{coh}}$)=${\mathit{E}}_{\mathrm{coh}}^{\mathrm{th}}$-${\mathit{E}}_{\mathrm{coh}}^{\mathit{e}}$ 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(${\mathit{E}}_{\mathrm{coh}}$) that is remarkably constant over a large part of the 3d- and 4d-transition-metal series. δ(${\mathit{E}}_{\mathrm{coh}}$) 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${\mathit{E}}_{\mathrm{coh}}$=${\mathit{E}}_{\mathrm{coh}}$(MC)-${\mathit{E}}_{\mathrm{coh}}$(MN), i.e., the difference between the cohesive energy of a carbide and a nitride of the same transition metal. Theoretical and thermodynamic Δ${\mathit{E}}_{\mathrm{coh}}$ 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 ${\mathit{E}}_{\mathrm{coh}}$ and ${\mathrm{\Delta }}^0$H 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.

• Received 4 March 1993

DOI:https://doi.org/10.1103/PhysRevB.48.11673