Nonrandomly Distributed Tungsten Vacancies and Interstitial Boron Trimers in Tungsten Tetraboride

Tungsten tetraboride (WB₄) and its solid solutions represent one of the most promising candidates for superhard metals; however, the structural and bonding uncertainties regarding the fractionally occupied metal and boron sites have impeded an in-depth understanding of these compounds. Here, we examine the interstitial arrangements of boron atoms and polyhedral bonding in synthesized WB₄ using W L-edge X-ray absorption spectroscopy and X-ray photoemission spectroscopy. We identify a nonrandom distribution of W vacancies and B₃ trimers at the crystallographic W 2b site, instead of a full occupation. Furthermore, this peculiar structural arrangement is associated with two distinct sets of W and B binding states and a large value of density of states at the Fermi level (<i>E</i>_F), which suggests an inhomogeneous charge transfer at different crystallographic W and B sites with a preferred metallic bonding. Theoretical calculations elucidate that, while the B₃ trimers help form a three-dimensional covalent bonding network, the W vacancies are crucial to optimize the <i>E</i>_F location and thus enhance the bonding strength. Our findings provide key insights into the hardening mechanism in WB₄, which has broad implications for the rational design and synthesis of this class of materials.