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 (EF), 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 EF 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.