COHESIVE PROPERTIES OF 4D-TRANSITION-METAL CARBIDES AND NITRIDES IN THE NACL-TYPE STRUCTURE

We present a study of the cohesive energies (E(coh)) of all 4d-transition-metal carbides and nitrides in the NaCl-type structure using ab initio linear-muffin-tin-orbitals type total-energy calculations and an extensive analysis of thermodynamic data. Many of the carbides considered here are metastable and their cohesive energy is not known from direct measurements. In these cases, we estimate E(coh) using thermodynamic model calculations and analyses of phase diagrams. This information allows us to perform a detailed comparison of theoretical (E(coh)th) and thermodynamic (E(coh)e) cohesive energy values. The difference E(coh)th - E(coh)e is positive for all carbides and nitrides, and we discuss the various sources of error in the theoretical approach. Trends in theoretical E(coh) values are remarkably improved when spectroscopic data are used to correct calculated atomic total energies. We also show that most of the discrepancy between theoretical and thermodynamic results cancels in the difference DELTA-E(coh) between a carbide MC and a nitride MN of the same transition-metal M, and excellent agreement with experiments is obtained for the systems where thermodynamic information is available, viz., for M = Zr, Nb, and Mo. This fact allows us to estimate the unknown cohesive energies and enthalpies of formation for YN, TcN and for the metastable NaCl structures of RuN, RhN, and PdN, from a combination of ab initio results and assessed thermodynamic information for the corresponding carbides. Our method of theory-based predictions of thermodynamic quantities is compared with the semiempirical method of Miedema. We also discuss the role of extrapolation methods related to ab initio results in the estimation of thermodynamic information that is used in the modeling of alloy phase diagrams.