Geometries, electronic properties and stability of molybdenum and tungsten nitrides low-index surfaces

Motivated by the vital role played by transition metal nitride (TMN) composites in various industrial applications, the current study reports electronic properties, thermodynamic stability phase diagram, and vacancy formation energies of the plausible surfaces of NiAs and WC-type structures of δ₃-MoN and δ-WN hexagonal phases, respectively. Low miller indices of various surface terminations of δ₃-MoN and δ-WN namely, (100), (110), (111), and (001) have been considered. Initial cleaving of δ₃-MoN bulk unit cell offers separate Mo and N terminations signified as δ₃-MoN (100): Mo, δ₃-MoN(100):N, δ₃-MoN(111):Mo, δ₃-MoN(111):Mo, and δ₃-MoN(001):Mo. However, the (110) plane reveals mix-truncated with both molybdenum and nitrogen atoms i.e. δ₃-MoN (110): MoN. Likewise, the δ-WN faces incur analogous surface terminations. Ab initio atomistic thermodynamic analyses predict that, N-terminated (111) and (100) slabs to be the most energetically favourable surface terminations amongst the explored surfaces of δ₃-MoN and δ-WN, respectively. Evidenced by plotted density of states (DOS), bulk and surfaces of δ₃-MoN and δ-WN display a metallic character. In terms of surface relaxation and reconstructions, most investigated surfaces experience mainly downward displacements of their topmost layers. Most notably, the relaxed Mo-termination in (111) and (100) surfaces of δ₃-MoN demonstrate significant reconstructions resulted in the first layer to be solely truncated with nitrogen atoms instead of molybdenum in the un-relaxed geometry. Nevertheless, no surface reconstruction has been noticed in most of considered δ-WN configurations. Calculated Bader’s electronic charges reveal charge transfer from Mo/W atoms to N atoms, largely retaining the ionic bond nature in their bulk phases. Finally, vacancy formation energy (VFE) calculations showed that introducing nitrogen vacancies through the surface is an endothermic process. Furthermore, the energy required to create a vacant cite in the inner layers differ than that needed in the outer layers. Nitrogen-terminated slabs hold the highest concentrations. Results from this study should be useful when studying the activation of doubly and triply bonded molecules such as N₂ at surface vacancies.