Theory of phase stabilities and bonding mechanisms in stoichiometric and substoichiometric molybdenum carbide

First principles, total energy methods have been applied to predict the relative stabilities of the four experimentally verified MoC phases: the cubic delta(NaCl) phase and the three hexagonal gamma(WC), eta and gamma'(TiAs) phases. The effect of vacancies on the relative stability of these four phases was investigated using a model structure with ordered vacancies within the carbon sublattice. For stoichiometric MoC, the gamma phase was found to be the most stable followed by gamma', delta, and eta, but for substoichiometric MoC0.75, the order of relative stability was changed and the substoichiometric delta phase was found to have the lowest energy followed by gamma' and gamma. A study of the electronic structure revealed vacancy induced peaks in the density of state and the electron density attached to these peaks was analyzed and found to emanate from unscreened Mo-Mo bonds through the carbon vacancy site. Finally, the oxygen stabilization of the gamma' MoC phase was studied. (C) 1999 American Institute of Physics. [S0021- 8979(99)02519-0].