Vacancy Hardening and Ordering in Rhenium Tungsten Carbides

Abstract

Vacancies generally reduce the number of chemical bonds and hence cause structural softening. It is puzzling, however, that substoichiometric Re0.5W0.5C0.4 with 60% carbon vacancies was identified as a superhard material. Here, we report the underlying mechanism responsible for such anomalous vacancy-induced hardening in the Re0.5W0.5C1–x system via first-principles calculations. The shear stiffness and hardness increase consistently with rising carbon vacancy concentration in Re0.5W0.5C1–x and reach the maximum at about x = 40%. Such an unexpected hardening phenomenon originates from a gradual relief of the shear-unstable dd bonding and unfavorable pd antibonding interactions owing to the formation of C-vacancies. We further predict that the simultaneous ordered vacancies of W and C atoms can produce a cubic crystalline Re2/3W1/3C that is isomorphic with the NbO type. The calculations on vibrational, mechanical, and electronic properties reveal that this phase has considerable structural stability and is a hard metallic material. This work not only elucidates the intriguing mechanism responsible for the vacancy hardening in this class of systems but also provides a new principle for the structural stability of other transition-metal compounds.

Understanding the Role of Atomic Vacancies in the Stability and Hardening of Cubic Tungsten and Molybdenum Nitrides

“…In contrast to usual close-packed structures, this ordered "hole structure" was originally documented experimentally in NbO. 39 Several carbides and nitrides (i.e., WN, MoN, ReC, and Re 2/3 W 1/3 C) 20,27,40,41 were recently predicted to adopt this NbO type. Interestingly, the experimentally synthesized cP6-WN and cP6-MoN were also observed to possess some additional disordered vacancies.…”