Cohesive, Vibrational, and Chemical Bonding Features of 3d- and 4d-Transition Metal MX (X = C, N) Compounds: A Systematic Approach

The paper presents an ab initio approach to the cohesive, vibrational, and phase stability properties of the MX (cF8) NaCl-type structure compounds formed by M = Sc to Ni of the 3d-transition-metal series and M = Y to Pd of the 4d-transition-metal series, with X = C and N. The cohesive energy (E_coh) and a characteristic energy [E(0)] related to the vibrational entropy show a well-defined pattern of covariation with the average number of electrons per atom of the compound (n_e). Various types of homologous behavior of compounds formed by 3d- and 4d-elements from the same group of the periodic table are detected. In particular, a striking covariation of the E_coh [and E(0)] values for MX compounds formed by the pairs of elements Sc/Y to Ni/Pd with the increase in n_e is established. These various systematic findings are discussed in terms of chemical bonding features of the compounds, which include the hybridization of the p-electron states of C (or N) with the d-states of the transition metals. Using systematic calculations of the crystal Hamiltonian populations, a comprehensive explanation is developed, which relies on the progressive occupation of bonding and antibonding electronic orbitals.