Synthesis, Crystal Structures, Mechanical Properties, and Formation Mechanisms of Cubic Tungsten Nitrides

Abstract

Cubic tungsten nitrides with high elastic stiffness are promising replacements for metal carbides used in tool applications to achieve enhanced working efficiency. However, due to the difficulties in preparing these nitrides at ambient pressure, their crystal structures and mechanical properties remain largely elusive, which have limited the functionality of these materials. Here, we report a comprehensive study of cubic tungsten nitrides synthesized by a high-pressure method, leading to definitive structural identifications of rocksalt cF8-WN and NbO-type cP6-WN involving atomic deficiencies. Combined with calculations, we find that the structural stabilities of both nitrides are closely related to the atomic deficiency that prevents the filling of unfavorable W: 5d-t2g bands. The disordered N vacancies are decisive for stabilizing cF8-WN, while the ordering of W and N vacancies occurs at 3 GPa and relatively low temperatures and leads to the formation of nearly stoichiometric cP6-WN, rather than previously misassigned cP7-W3N4. Both nitrides exhibit similar excellent mechanical and thermal properties, rivaling and even exceeding WC. Besides, their formation mechanisms are also explored to be associated with atomic vacancies, shedding light on the rational design of functional nitrides by defect chemistry.

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

“…On the contrary, the phonon curves of cP6-WN and cP6-MoN, as shown in Figure c,d, respectively, have no imaginary frequencies, indicating that both cP6 phases are dynamically stable. These results on the instability of both cF8 phases accord with previously theoretical calculations − but are incompatible with the experimental observations. − In contrast, the conclusions on the highly stable cP6 phases well accord with previously theoretical and experimental results. − …”

“…Interestingly, the experimentally synthesized cP6-WN and cP6-MoN were also observed to possess some additional disordered vacancies. 22,24 We clearly see from Figure 3c,d that these disordered deficiencies lead to a gradual increase in the formation energy of both cP6 phases. That is to say, the experimentally observed disordered vacancies should not be intrinsic in cP6-WN and cP6-MoN.…”

“…It is well known that a series of transition-metal mononitrides (TMNs) crystallize in the rocksalt structure (i.e., the cF8 structure type, space group Fm 3̅ m ). − Among them, several experiments have reported the synthesis of cF8-WN (Figure a) under high pressure and temperature. − Also, it possesses a high bulk modulus of 422.9 ± 6.7 GPa and a Vickers hardness of 29 GPa at an applied load of 0.49 N. However, first-principles calculations have indicated that stoichiometric cF8-WN is both mechanically and thermodynamically unstable due to the calculated negative elastic constant C 44 and positive formation energy. − In contrast, a candidate cubic WN phase (i.e., the cP6 structure type, space group Pm- 3 m ), which is isostructural with NbO (Figure b), has been predicted to be very stable. More recently, Zhou et al have first synthesized cP6-WN under high pressure and temperature and also definitively identified cF8-WN involving atomic defects; subsequent measurements have shown that both phases exhibit similar excellent mechanical properties that rival and even exceed that of WC. This most recent experiment, together with earlier theoretical and experimental efforts, strongly suggests that the atomic vacancies should play a decisive role in the structural stabilities and mechanical properties of WN with the cF8 and cP6 types.…”

“…candidate cubic WN phase (i.e., the cP6 structure type, space group Pm-3m), which is isostructural with NbO (Figure 1b), has been predicted to be very stable. More recently, Zhou et al 22 have first synthesized cP6-WN under high pressure and temperature and also definitively identified cF8-WN involving atomic defects; subsequent measurements have shown that both phases exhibit similar excellent mechanical properties that rival and even exceed that of WC. This most recent experiment, together with earlier theoretical and experimental efforts, strongly suggests that the atomic vacancies should play a decisive role in the structural stabilities and mechanical properties of WN with the cF8 and cP6 types.…”

Polar Nitride Perovskite LaWN3‐δ with Orthorhombic Structure

“…The thus‐evaluated valence W 2+ is much distinct from the nominal valence W 4.8+ , further signaling the strong W‐N covalency. Due to the surface oxidation effect, an additional W4f doublet is also observed and likely associated with a WO 2+ x compound, [ 44 ] which can profoundly be reduced if the sample surface is etched and cleaned by Ar + irradiation before experiments (Figure 2b and Figure S4j), Supporting Information. By contrast, the La3d line are nearly intact after etching [Figure S4(k)], implying that LaWN 3‐ δ has a similar valence state to that of the surface La‐O oxide; this is not unexpected because La is so electropositive for donating the same number of electrons during the formation of LaWN 3‐ δ or surface La–O oxide.…”