Synthesis, Crystal Structure, Stability, and Mechanical Properties of the Hexagonal Tungsten Nitride WN1-δ

The search for new binary tungsten nitrides, particularly those with a hP2-WC or hP4-NiAs structure, holds great promise for the attainment of mechanically excellent materials. However, due to the difficulties in forming those nitrides at ambient pressure, preparation of such a hexagonal WN compound has been rarely succeeded by conventional methods, leading to many ambiguities in its crystal structure, stabilities, and mechanical properties that have severely limited its practical applications. Here, we report the successful synthesis of a high-quality hexagonal WN through a high-pressure reaction route. Its crystal structure is definitively resolved to adopt the hP2 symmetry, rather than hP4, which is isotypic with that of WC. Combined with the calculations, the nitrogen deficiency is revealed to be intrinsic for stabilizing its crystal structure, which rationalizes its actual composition of WN0.764 with a nearly optimal electron filling of valence bands, similar to that of WC. Besides, this nitride is metallic and exhibits superior mechanical properties and thermal stabilities, rivaling or even exceeding those of WC, making it practical useful for the hard tool applications with sufficiently improved working efficiencies to replace carbides.