Crystal structure, properties, and formation mechanism of hexagonal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>h</mml:mi> <mml:mi>P</mml:mi> <mml:mn>12</mml:mn> <mml:msub> <mml:mtext>-WN</mml:mtext> <mml:mi>x</mml:mi

Nitridation of electron-rich transition metals under pressure can form many binary nitrides with diverse compositions and crystal structures, especially for the W-N system with numerous newly discovered phases. However, because of their unique metastability, most reported W-N products involve complex atomic deficiencies, largely exacerbating the difficulties in structural resolution. Of particular interest is hexagonal $hP10\text{\ensuremath{-}}{\mathrm{W}}_{2}{\mathrm{N}}_{3}$, which has been predicted to be a soft van der Waals (vdW) material but experimentally has shown excellent mechanical properties, likely due to structural misassignment. Here we revisit the high-pressure synthesis and perform a comprehensive study to definitively solve the crystal structure and determine the key properties of this material. We find that it adopts $\mathit{hP}12$ symmetry rather than $\mathit{hP}10$, containing unusual W-W-W trimers. Intricate atomic deficiencies are revealed to be crucially important for stabilizing its crystal structure with an actual composition of $hP12\text{\ensuremath{-}}{\mathrm{W}}_{4.216}{\mathrm{N}}_{5.278}$. This material is intrinsically metallic and mechanically hard and brittle, opposite to that of a vdW material. It also exhibits superior thermal stability, making it highly suitable for many advanced applications.