High-Pressure Synthesis of Ultra-Incompressible, Hard and Superconducting Tungsten Nitrides

Transition metal nitrides, particularly those of 5d metals, are known for their outstanding properties, often relevant for industrial applications. Among these metal elements, tungsten is especially attractive given its low cost. In this high-pressure investigation of the W–N system, two novel ultra-incompressible tungsten nitride superconductors, namely W₂N₃ and W₃N₅, are successfully synthesized at 35 and 56 GPa, respectively, through a direct reaction between N₂ and W in laser-heated diamond anvil cells. Their crystal structure is determined using synchrotron single-crystal X-ray diffraction. While the W₂N₃ solid's sole constituting nitrogen species are N^3- units, W₃N₅ features both discrete N^3- as well as N₂^4- pernitride anions. The bulk modulus of W₂N₃ and W₃N₅ is experimentally determined to be 380(3) and 406(7) GPa, and their ultra-incompressible behavior is rationalized by their constituting WN₇ polyhedra and their linkages. Importantly, both W₂N₃ and W₃N₅ are recoverable to ambient conditions and stable in air. Density functional theory calculations reveal W₂N₃ and W₃N₅ to have a Vickers hardness of 30 and 34 GPa, and superconducting transition temperatures at ambient pressure (50 GPa) of 11.6 K (9.8 K) and 9.4 K (7.2 K), respectively. Additionally, transport measurements performed at 50 GPa on W₂N₃ corroborate with the calculations.