Abstract
Two-dimensional systems have strengthened their position as a key materials for novel applications. Very recently, boron joined the distinguished group of elements confirmed to possess 2D allotropes, named borophenes. In this work, we explore the stability and hardness of the highest borides of tungsten, which are built of borophenes separated by metal atoms. We show that the WB3+x compounds have Vickers hardnesses approaching 40 GPa only for small values of x. The insertion of extra boron atoms is, in general, detrimental to the hardness of WB3 because it leads to the formation of quasi-planar boron sheets that are less tightly connected with the adjacent tungsten layers. Very high concentrations of boron (x ≈ 1), give rise to a soft (Vickers hardness of ~8 GPa) and unstable hP20-WB4 structure that can be considered to be built of quasi-planar boron α-sheets separated by graphitic tungsten layers. By contrast, we show that the formation of tungsten vacancies leads to structures, e.g. W0.75B3+x, with Vickers hardnesses that are not only similar in value to the experimentally reported load-independent hardnesses greater than 20 GPa, but are also less sensitive to variations in the boron content.
Highlights
The highest boride of tungsten–often referred to as tungsten tetraboride–is recently best explored for its potential applications as superhard material, made its first appearance in the literature in 1961, when Chretien and Helgorsky[1] did the first attempt to find its structure
The highest borides of tungsten are obtained starting from WB3 in the hP16-WB3 structure by adding additional boron atoms at the positions shown in red in Fig. 1c and/or by selective removal of W atoms
This structure is nothing more than a sequence of quasi-planar boron α-sheets arranged in such a way that the boron atoms that stick out of the graphitic frames face each other forming dimers
Summary
Two-dimensional systems have strengthened their position as a key materials for novel applications. Very high concentrations of boron (x ≈ 1), give rise to a soft (Vickers hardness of ~8 GPa) and unstable hP20-WB4 structure that can be considered to be built of quasi-planar boron α-sheets separated by graphitic tungsten layers. The mechanical properties of WB4 were first determined by Gu et al.[3] who reported Vickers hardness (HV) values of 46.2 and 31.8 GPa under applied loads of 0.49 and 4.90 N, respectively, measured by the microindentation technic. The common description of hP20-WB4 that can be found in the literature is that this structure consists of graphitic boron layers separated by graphitic layers of W atoms like in the hP16-WB3 structure but with additional B2 dimers located between boron sheets and aligned along the c-axis (see Fig. 1a) This description, very elegant, is completely decoupled from more recent investigations related to 2D boron crystals[11, 12]. Since in the experiment WB3 is contaminated with boron atoms and, to some extent, possesses tungsten vacancies[8, 9, 16], in this work a more precise notation is used when referring to the highest boride of tungsten, namely W1–yB3+x, to underline the existence of W vacancies and explore their influence on the stability and properties of WB3+x
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