Nonrandomly Distributed Tungsten Vacancies and Interstitial Boron Trimers in Tungsten Tetraboride

Abstract

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 (E 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 E 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.

Understanding the Hardness of Doped WB4.2

“…19,20 The presence of B 3 trimers is crucial in the formation of the interlayer 3D-covalent boron−boron bonding network (so-called "hourglass" structures, Figure 1B), which has been hypothesized to be responsible for the exceptional mechanical properties of the material, particularly preventing the slip along the most "slippery" slip system. 21,22 However, the disorder embedded in the system presents a considerable challenge to pinpoint the bonding effects behind material hardness, especially in the case of doped WB 4.2 . Little is known about the preferred locations of various transition metal dopants in the tungsten tetraboride lattice, as well as their influence on the key hypothesized hardening element in this latticethe hourglass structure.…”

“…WB 4.2 possesses a unique disordered crystal structure (Figure A). It consists of alternating layers of hexagonal boron sheets (B hex ) and W atoms, with some of W substituted by B 3 clusters (2 clusters per 3 unit cells). , The presence of B 3 trimers is crucial in the formation of the interlayer 3D-covalent boron–boron bonding network (so-called “hourglass” structures, Figure B), which has been hypothesized to be responsible for the exceptional mechanical properties of the material, particularly preventing the slip along the most “slippery” slip system. , However, the disorder embedded in the system presents a considerable challenge to pinpoint the bonding effects behind material hardness, especially in the case of doped WB 4.2 . Little is known about the preferred locations of various transition metal dopants in the tungsten tetraboride lattice, as well as their influence on the key hypothesized hardening element in this latticethe hourglass structure.…”

Synthesis, crystal structure, and properties of stoichiometric hard tungsten tetraboride, WB4

“…This fact motivates investigations of other potentially hard tungsten compounds, primarily, borides. [29][30][31][32][33][34][35][36][37] Among them, a phase referred to as "WB 4 ", [38][39][40][41][42][43][44][45][46][47][48][49] and its several derivatives (e.g., W 1Àx B 3 , WB 4Àx , WB 4+x , WB 4.2 , and WB 5Àx ) [50][51][52][53][54][55][56] were identied as highly promising candidates, whose mechanical characteristics (e.g. theoretical and experimental values of H V vary from 30 to 45 GPa (ref.…”

“…This fact motivates investigations of other potentially hard tungsten compounds, primarily, borides. 29–37 Among them, a phase referred to as “WB 4 ”, 38–49 and its several derivatives ( e.g. , W 1− x B 3 , WB 4− x , WB 4+ x , WB 4.2 , and WB 5− x ) 50–56 were identified as highly promising candidates, whose mechanical characteristics ( e.g.…”

Understanding Hardness of Doped WB4.2

“…1B), which has been hypothesized to be responsible for the exceptional mechanical properties of the material, and particularly preventing the slip along the most "slippery" slip system. 21,22 However, the disorder embedded in the system presents a considerable challenge to pinpoint the bonding effects behind materials hardness, especially in the case of doped…”