Synthesis, Crystal Structure,
and Elastic Properties
of Novel Tungsten Nitrides
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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<sub>2</sub>N<sub>3</sub>. The final products are phase-pure and well-crystallized
in bulk forms. For hexagonal W<sub>2</sub>N<sub>3</sub>, hexagonal
WN, and cubic W<sub>3</sub>N<sub>4</sub>, 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