Possible coexistence of superconductivity and superhardness in BeB6- Dr. Huiyang Gou
Beijing Nov. 21, 2016 — New work co-led by HPSTAR scientist, Dr. Huiyang Gou, theoretically predicts the structural and physical properties of beryllium hexaboride (BeB6) at ambient and high pressures. The ambient phase of BeB6 shows a Vicker's hardness comparable to that of γ-B/cBN and coexists with superconductivity, which is unusual and exciting. This study provides new insights into the bonding mechanism for design and synthesis of novel functional materials.
The element boron—carbon’s neighbor in the periodic table and a nonmetallic, hard material—possesses structural complexity, electron deficiency, unusual binding situations, and a rich variety of compounds. Higher hardness, possible superconductivity, and better thermoelectricity make boron-rich compounds attractive for materials research and industrial applications.
“Many of the fundamental questions regarding boron and boron-rich compounds are still mysteries after decades”, said Huiyang. “The greatest challenges to boron chemistry are the synthesis of pure products, concise crystal-structure determination, and the related understanding of their electronic structures”.
In the latest issue of the Journal of Physical Chemistry Letters (DOI: 10.1021/acs.jpclett.6b02444), Huiyang and his coworkers used a theoretical global structural search to predict the structural and physical properties of a typical boron binary compound, BeB6, which was discovered in 1961.
“The structure of borides determines the physical properties of the boron-boron and metal-boron bonds, which boride chemical stability depends on”, Huiyang added. Using the particle swarm optimization CAPLYSO method for structural predictions combined with density functional calculations, they probed the stable phases of BeB6 up to 400 GPa. They found that the ambient phase, α-BeB6, was the hardest among the three BeB6 phases and comparable to g-B/cBN(cubic boron nitride). This implies that α-BeB6 may be a new, low-density superhard material. Interestingly, another high-pressure phase of b-BeB6 showed a relatively higher Tc of 24 K.
“The prominent, strong and uniformly distributed 3D covalent network in a-BeB6 explains its superhard nature”, Huiyang explained. “This coexistence is unusual. Superconductivity is a coherent phenomenon that deals with the collective motion of delocalized electrons, which is intrinsically opposite to the strong covalent conditions required for superhardness. Over the years, boride-based materials have been shown to have either one property or another, for example superconductivity in MgB2 or superhardness in cBN. We show that these two properties can co-exist in the same material, beryllium hexaboride (BeB6). This is achieved by the unique ‘electron deficient’ environment in BeB6 that allows metallicity as well as a strongly covalent bonded lattice (where B uses sp3 orbitals for bonding)”.
Caption: Crystal structure of α-BeB6 and b-BeB6 with coherent superhigh/high hardness and superconductivity.