Synthesis, Crystal Structure, and Elastic Properties of Novel Tungsten Nitrides

Abstract

Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W−N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W 2 N 3 . The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W 2 N 3 , hexagonal WN, and cubic W 3 N 4 , they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production.

Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4

“…Interestingly, the b-axis is the least compressible although it has the largest lattice parameter (5.481 Å), whereas the most compressible axis, the c axis, corresponds to the shortest lattice parameter (c = 2.866 Å), which are in agreement with the recent results of first-principles simulations [28]. This anomalous compressive behavior has only sparsely been reported [20], and is likely to be associated with the peculiar atomic arrangements in orthorhombic CrB 4 . Vickers hardness measurements were performed on the recovered bulk sample with mixed phases of CrB 2 and CrB 4 .…”

“…Since 2007, inspired by frontier research on ReB 2 by Chung et al [7], there has been a new surge of research interest in searching for ultra-incompressible and superhard transition-metal (TM) borides, nitrides, and carbides. These efforts have led to a series of new system including diborides (ReB 2 [8][9][10] and RuB 2 [10]), tetraborides (WB 4 [11,12], FeB 4 [13], MnB 4 [14,15], and CrB 4 [16][17][18]), nitrides (Re x N [19] and WN x [20]), and carbides (Re 2 C [21]). …”

Nitrogen-rich transition metal nitrides

“…Most recently, Wang et al combined synthesis and computation studies to suggest two new phases of W 2 N 3 and one of W 3 N 4 (Table 1) [134]. The three new compounds were all reported to be produced at 5 GPa, but using different temperatures.…”

“…The computed aggregate shear modulus of c-W 3 N 4 is 183 GPa, which is quite high among the nitrogen-rich metal nitrogen compounds. Hence, it may be expected that c-W 3 N 4 will exhibit a Vickers hardness greater than 40 GPa [134]. Kroll gave a first account of a phase diagram of Re-N compounds with high metal oxidation state [22].…”

Vacancy-induced mechanical stabilization of cubic tungsten nitride

“…Theoretical studies have predicted the thermodynamically most stable WN phase to be the hexagonal close packed NiAs structure, 28 the simple hexagonal WC phase 29 which is also experimentally observed at a pressure of 5 GPa, 27 or more recently the NbO phase. 20,30 These latter studies have concluded that all phases except the NbO phase is thermodynamically unstable against phase separation into metallic W and molecular nitrogen.…”