Skoltech researchers, together with colleagues from academic institutions and the industry, have solved the mystery of the 1960s about the crystal structure of potentially superhard tungsten boride, which can be extremely useful in a wide variety of applications, including drilling technologies. An article about the scientific work supported by the Gazprom Neft Scientific and Technical Center has been published in the journal Advanced Science. 

Tungsten borides first attracted the attention of scientists in the middle of the 20th century due to their hardness and other interesting mechanical properties. One of the long—standing mysteries associated with these compounds is the crystal structure of the so-called compound “WB4”, a higher tungsten boride, which differs greatly in experimental models and theoretical predictions of different groups.

"Experimentally, the crystal structure is determined by X-ray diffraction analysis. But the large difference in atomic effective scattering cross-sections (heavy tungsten compared to light boron) makes the positions of boron atoms in transition metal borides barely distinguishable for such an analysis. This problem can be solved by neutron diffraction, and this has been done recently, but any diffraction method can only give an average structure. If the material is disordered, a complete understanding of its crystal structure, including the local arrangement of atoms, can be achieved only through a combination of experimental and computational methods," said Alexander Kvashnin, senior researcher at Skoltech and the first author of the article.

In 2017, Skoltech employees Andrey Osiptsov and Artyom Oganov proposed the idea of searching for new superhard materials for composite chisel cutters used for drilling oil and gas wells. Gazprom Neft's Scientific and Technical Center liked this idea - that's how its cooperation with Skoltech began. The group led by Oganov predicted the existence of WB5 tungsten pentaboride, which surpassed the widely used tungsten carbide in hardness and was comparable to it in crack resistance. The compound was eventually successfully synthesized at the Vereshchagin Institute of High Pressure Physics.

In a new study, Oganov and his colleagues show that the mysterious “WB4” and the new tungsten pentaboride WB5 are actually the same material.

"We studied the tungsten-boron system to predict the existence of a stable structure of higher tungsten borides, as we knew about this long-standing mystery. The prediction of the WB5 structure came as a surprise to us, especially because of its amazing properties like Vickers hardness and crack resistance, as well as stability at very high temperatures. We decided that this material should be used in industry, and our colleagues from the Vereshchagin Institute synthesized it. The diffraction pattern corresponded very well to theoretical predictions with the exception of a few weak peaks, which were in theory but not in experiment. Our predicted WB5 has an ideal monocrystalline structure, but, as we were able to show, in experiments we obtained a very disordered WB5-x very close to it," Kvashnin explained.

Scientists synthesized a new material, studied its properties and discovered an unexpected connection between two compounds: the crystal structure of this higher tungsten boride is similar to the structure of WB5 with some disorder and nonstoichiometry (the latter means that the proportions of chemical elements in its composition cannot be represented by small integers). Therefore, the new connection was designated not as WB4, but as WB5−x. Its crystal structure was predicted using the USPEX evolutionary algorithm developed by Oganov and his students, and developed using a microscopic lattice model.

Since WB5-x is fairly easy to synthesize, its excellent mechanical properties and stability at high temperatures make it a promising alternative to tungsten carbide composites, which have been most often used in many technologies over the past 90 years.

"The mystery of WB4 has been solved completely: we have a detailed description of this material and its structure, we know the entire range of chemical compositions that it can have, and its properties. The theorists have other interesting puzzles ahead," Artyom Oganov concluded. 

Specialists from the A.M. Prokhorov Institute of General Physics of the Russian Academy of Sciences and the L. F. Vereshchagin Institute of High Pressure Physics of the Russian Academy of Sciences also participated in the study.