Possible coexistence of superconductivity and superhardness in BeB6- Dr. Huiyang Gou
NOVEMBER 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.
硼及其化合物的研究一直存在很多难题，主要是因为很难在实验上合成出高质量的硼化物晶体，这就使得其晶体结构无法确定，以及电子结构等一系列的物理性能无法准确理解。BeB6就是硼化物中典型的二元化合物，其首次被发现于1961年。至今对其精确的化学配比，晶体，电子结构等物理性能还未确定。北京高压科学研究中心的缑慧阳研究员一直致力于硼化物的研究。缑慧阳及其合作者利用理论计算的方法预测了BeB6常压，及其高压下的晶体结构等物理性能。他们惊奇的发现BeB6的常压相，命名为a- BeB6，具有类似于gama-B的硬度，46 GPa的维氏硬度。“此超高硬度是由其三维硼共价键决定的”，慧阳解释道。而且此硬度与其高压超导性能共存。其高压相，b- BeB6，也显示出很高的硬度维氏硬度(36 GPa）及共存的超导性。“这一意外的发现很不寻常，在类似的硼化物中只能实现其中的一种性能，比如MgB2只具有超导性而立方氮化硼只表现出超高硬度”，慧阳说到。希望我们的理论预言会对今后的硼化物的实验合成提供指导。