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

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 ¹¹B multiple quantum MAS experiment revealed a range of boron sites that were not resolved within the experiment. This result is in contrast to the ¹¹B 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 ¹¹B 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 T1-1 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. [ABSTRACT FROM AUTHOR]