ZrB12: a kind of "metal-electric diamond"

The exploration of hard and superhard materials has always been an important research direction in the field of condensed matter physics, and it also has great application prospects in practical industrial production. Conventional superhard materials such as diamond, cubic boron nitride, etc., are usually composed of light elements (BCNO) in the form of covalent bonds. This strong BCNO covalent bond has good directionality and is resistant to isotropic compression. It is also resistant to shear in different directions and thus exhibits extremely high mechanical strength. However, the defects of traditional superhard materials are also very prominent: diamonds are prone to graphitization, and the synthesis of cubic boron nitride requires unusually harsh temperature and pressure conditions. In addition, pure covalent bonding forms often lead to their electrical extinction or broadband semiconductors, while industrial applications require materials with super-hard mechanical properties and good electrical properties (such as superhard) under many conditions. Coating, wire cutting, press hammer, etc.). Therefore, the search for ultra-hard low-resistance and even metallic materials has become an important research hotspot in recent years.
Recently, the Institute of Physics of the Chinese Academy of Sciences/Beijing National Laboratory of Condensed Matter Physics (Financing) Extreme Condition Laboratory EX6 Group Yu Xiaohui Associate Researcher and Surface Physics Laboratory SF10 Group Li Hui Associate Researcher, EX5 Group Yan Changqing Researcher, Jilin University Professor Zhu Pinwen, Professor Zhang Ruifeng from Beijing University of Aeronautics and Astronautics has made important progress in exploring the “metal-electric diamond”-ZrB12. They first successfully prepared pure phase ZrB12 samples by means of high temperature and high pressure, and refined the crystal structure by the crystal diffraction spectrum. As shown in Figure 1: ZrB12 is mainly composed of B network, and the distance between BB is only 1.78, which corresponds to the extremely strong BB bond. At the same time, the crystal is a face-centered cubic structure with high symmetry and no obvious slip direction. As shown in Fig. 2, the Vickers hardness in all directions of the crystal is almost the same, and the mechanical properties have good isotropy. In the case of 50g loading, ZrB12 has a hardness value of up to 40 GPa, which is the standard for superhard materials. At 500g loading, the hardness is still as high as 27 GPa, which is 2.5 times that of the most widely used hard WC material. . Theoretical calculations show that, as shown in Figure 3, ZrB12 has an ideal mechanical strength of 34.5 GPa, which is close to the traditional superhard material B6O formed by pure covalent bonds, and has good isotropy, which is consistent with our experimental results. This highly symmetrical three-dimensional BB network is the intrinsic structural origin of ZrB12 that exhibits high mechanical properties. By studying the low-temperature electrical transport properties of ZrB12 single crystals, they found that ZrB12 has very excellent metallic properties. As shown in Fig. 4, at room temperature, the resistivity is only 18 μΩ·cm, which is almost equivalent to the metal Pt. As the temperature decreases, the resistivity change of ZrB12 also exhibits metallic behavior, and is around 5.5 K. Superconductivity appeared. In addition, the seeback coefficient of ZrB12 at room temperature is only 2.0 μV·K-1, which indicates that the material has good metality. According to the structural data obtained by crystal diffraction refinement, they found that the structural body of ZrB12 is formed by the BB three-dimensional cage network, and the transition metal Zr is at the center of the {B}28 cage, and the distance from the adjacent Zr is as long as 5.2. That means there will be no direct metal orbital overlap; and the BB network exhibits exceptionally good mechanical stability, indicating that the BB bond is a local covalent form. To understand the superior metallity beyond the superhard properties of ZrB12, the researchers took a first-principles calculation simulation. They found that the Zr atom provides a large amount of electrons to the B orbital when bonded to B, and averaging 2.6 electrons per Zr atom to the Zr-B hybrid orbit. As shown in Fig. 5, further analysis of the energy band structure of ZrB12 reveals that the hybrid orbit of Zr-B can be superimposed on the BB three-dimensional orbital, forming a d-π-d bridge structure and the entire crystal structure. The through-field conductive path is formed, so that the ZrB12 as a whole exhibits an exceptionally superior metallic property. It can be said that the BB three-dimensional network is not only an important support for the crystal structure of ZrB12, but also a bridge for rapid electron conduction. The relevant results were recently published in "Advanced Materials" "DOI: 10.1002/adma.201604003" [Advanced Materials (2016)] This research work was funded by the National Natural Science Foundation of China.

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