Microscopic investigation of local structural and electronic properties of tungsten tetraboride: a superhard metallic material

Abstract

Tungsten borides, such as tungsten tetraboride ( WB4) exhibit a wide range of appealing physical properties, including superhardness, chemical inertness and electronic conductivity. Among the various tungsten borides, the most puzzling remains WB4, with its crystal structure to linger in question for over half a century ( Lech et al. in Proc Natl Acad Sci USA 112: 3223- 3228, 2015). In the present investigation, polycrystalline WB4 samples have been synthesized with two different methods and characterized at the atomic level by combining X- ray diffraction, scanning electron microscopy and nuclear magnetic resonance spectroscopy. The 11 B multiple quantum MAS experiment revealed a range of boron sites that were not resolved within the experiment. This result is in contrast to the 11 B MAS spectrum of WB2 with four resolved, discernible boron resonances. However, despite the structural complexity and boron- site variety in WB4, the detection of a single exponential of 11 B spin- lattice relaxation recovery suggested that all of the boron sites relaxed with a single time constant. The Knight shift ( K) was found to be independent of temperature while the T 1 1 was governed by the Korringa law with a Korringa product T1T = 350 sK across the entire temperature range ( 168- 437 K) of this study. The measured Korringa product was small, indicating substantial spin- lattice relaxation resulting from coupling with the conduction carriers. The abovementioned experimental results not only clearly rule out structures, such as the `` MoB4- type phase'' of WB4, with the resulting Fermi level in the pseudo- gap as has previously been predicted theoretically; but they also provide a comprehensible and valuable insight into the structural and electronic properties of WB4 at the atomic level.