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.