Researchers Crack Superhard Material Puzzle From the 60s

Skoltech researchers, together with industrial colleagues and academic partners, have recently solved a puzzle about the crystal structure of a superhard tungsten boride that has extremely useful industrial applications.

The research, published in the journal Advanced Science, details how the researchers cracked the puzzle which was first proposed in the 1960s.

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Impressive mechanical properties

Tungsten borides have long captured the interest of scientists due to several impressive mechanical properties, including their hardness. One long-held conundrum has been the crystal structure of the highest W-B phases, the so-called WB₄, which varied immensely between experimental models and theoretical predictions.

"Experimentally, the crystal structure is determined by X-ray structure analysis. But the large difference in atomic scattering cross-sections (heavy tungsten compared to light boron) renders positions of boron atoms in transition metal borides hardly discernable by X-ray diffraction," Alexander Kvashnin, Skoltech senior research scientist and first author of the study, explained in a press release.

"This can be resolved by neutron diffraction, but any diffraction method can only give the average structure." he continued. "If the material is disordered, the complete knowledge of its crystal structure (including the local arrangement of the atoms) can be obtained only using a combination of experimental techniques (X-ray, neutron diffraction) and computational methods of materials science." 

Searching for superhard materials

In 2017, Andrei Osiptsov and Artem R. Oganov at Skoltech proposed searching for superhard materials to be used for producing composite cutters installed on bits, which are used for drilling applications.

After the idea was well-received, researchers led by Artem R. Oganov of Skoltech and MIPT went after the creation of WB₅, tungsten pentaboride, which they expected to be harder than the widely used tungsten carbide at the same time as having a comparable fracture toughness.

In the new paper, Oganov and his colleagues show that the long-debated WB₄ and the newly predicted WB₅, which the researchers successfully synthesized at the Vereshchagin Institute, are actually the same material.

"This puzzle is solved in full detail. We have a detailed microscopic description of this material and its structure, we know the range of chemical compositions it can adopt and its properties. Other exciting puzzles are waiting for theorists' attention," said Artem R. Oganov.