Unravelling the structure and strength of the highest boride of tungsten WB4.2

Abstract

Tungsten tetraboride is outstanding among transition-metal light-element compounds for its easy synthesis and superior mechanical properties. Its crystal structure, however, has eluded scientists for over half a century, impeding fundamental understanding and rational property optimization. Recent x-ray and neutron diffraction studies suggest rare boron trimers occupying vacant metal sites in the highest boride of tungsten ${\mathrm{WB}}_{4.2}$, but a viable crystal structure and key mechanical properties remain unresolved. Here we identify a ${\mathrm{WB}}_{4.2}$ phase in orthorhombic symmetry with a very large 104-atom unit cell using a tailored search algorithm treating boron trimer as a coherent unit. First-principles studies establish phase stability and unveil the mechanism for strength enhancement by newly identified bonding features. These findings solve a challenging crystal structure and elucidate its benchmark mechanical behaviors. This work offers powerful insights for exploring novel structures and properties of materials containing intricate multiatomic constituent structural units.