Direct experimental validation on the crystal structure, chemical bonding and magnetic properties of CrB2

The intrinsic brittleness, poor thermal shock and oxidation resistance of TMB₂ are the main obstacles that hinder their practical applications. It is well recognized that the macroscopic properties of ultra-high-temperature ceramics (UHTCs) are underpinned by their crystal structure, chemical bonding and microstructure features. Therefore, understanding microscopic information such as the electronic structure and chemical bonding of TMB₂ is essential not only for establishing the structure-property relationships but also for addressing the issue of how to tailor their properties from crystal structure and chemical bonding point of view, which is instructive to promote their practical applications in extreme aerospace environments. Although a number of theoretical works on the electronic structure of TMB₂ have been conducted using first-principles calculations based on density functional theory, direct observation of atom arrangement in TMB₂ was seldom conducted due to the limited resolution of transmission electron microscope. Recent advances in transmission electron microscopy have made it possible to directly observe the arrangement of atom columns in TMB₂ even for light elements such as B. Therefore, it is crucial to verify the crystal structure, chemical bonding and other structure information of TMB₂ through direct experiments.

Recently, a team of material scientists led by Yanchun Zhou from Zhengzhou University, China first reported direct experimental validation on the crystal structure, chemical bonding and magnetic properties of CrB₂, which is a member of TMB₂. This work provides a detailed analysis and demonstration of the structure and bonding of CrB₂ by combining theoretical calculations and experimental validations, which is important for understanding the structure-property relationship of CrB₂ and other TMB₂.

This work was available online on Journal of Advanced Ceramics on August 1, 2025

“In this work, a comprehensive study on crystal structure and chemical bonding properties of CrB₂ was conducted using a combinatorial of theoretical calculations and experimental validations. Theoretical calculations were mainly performed using first-principles calculations based on DFT, and the detailed electronic structure and chemical bonding of CrB₂ were systematically analyzed; whereas the experiments were mainly performed using spherical aberration-corrected transmission electron microscopy (AC-TEM) to observe the structure of CrB₂ at the atomic scale equipped with the electron energy loss spectroscopy (EELS), which will give direct evidence of the crystal structure and chemical bonding information. Finally, the hysteresis loop of CrB₂, which has not been reported to the best knowledge of the authors, was measured and its magnetic properties were analyzed in conjunction with theoretical calculations,” said Yanchun Zhou, professor at School of Materials Science and Engineering at Zhengzhou University (China), a senior expert whose research interests focus on the field of high-temperature ceramic materials.

“Theoretically, three types of chemical bonds, covalent, metallic and ionic, are predicted to exist in CrB₂. In detail, Cr is bonded to each other in (001) plane with metallic bonds, whereas B is bonded in (002) plane with a graphite-like six-membered ring due to sp² hybridization, which originates from the ionic bonding formed by the electron exchange between Cr and B, and forms covalent bonds in the (110) plane by hybridization with Cr 3d (t_2g and e_g),” said Yanchun Zhou.

“Experimentally, atomic-resolution HADDF and ABF images were obtained by aberration corrected transmission electron microscopy. The atomic-resolution HADDF image as well as the atomic-resolution EDS maps of CrB₂ were analyzed, and it is demonstrated that CrB₂ possesses an AlB₂-type structure in agreement with that determined from XRD (reciprocal lattice),” said Yanchun Zhou.

“The chemical bonding information of CrB₂ was obtained by the EELS accessory of STEM, and the peaks in the ELNES of B originate mainly from p_z and sp² hybridization. In addition, the ELNES as well as theoretically calculated chemical bonding information of CrB₂ is compared with that of MgB₂, which possesses fewer valence electrons. It is pointed out that the appearance of broader peaks in the ELNES of CrB₂ and other TMB₂ can be attributed to the covalent bonding between B and the transition metal, i.e., the resonance arising from the hybridization of B sp² and p_z with Cr 3d(t_2g) and 3d(e_g), which is absent in MgB₂ due to the lack of d orbitals.” said Yanchun Zhou.

“Theoretical calculations show that CrB₂ possesses a net magnetic moment, and the hysteresis loop of CrB₂ has been measured by VSM. The result shows that the molar magnetization of CrB₂ obtained from the hysteresis loop is in general agreed with that measured by Faraday's balance method in previous studies.” said Yanchun Zhou

Although the microstructure and chemical bond information of CrB₂ have been experimentally verified, research on tailoring the intrinsic brittleness and poor oxidation resistance of TMB₂ from a structural perspective still needs to be conducted. Additionally, the impact of CrB₂'s unique antiferromagnetic properties on its electromagnetic wave absorption performance, as well as its role as a component in the electromagnetic wave absorption performance of high-entropy transition metal diborides, still requires further investigation

Other contributors include Shuang Zhang, Huimin Xiang, Cheng Fang, Wei Xie from the School of Materials Science and Engineering at Zhengzhou University in Henan, China;

This work was supported by the National Natural Science Foundation of China (Nos. U23A20562 and 52302074). The authors would like to acknowledge Guogao Tang at Kaiple Company for TEM performance.

About Author

Yanchun Zhou holds a BSc in ceramics from Tsinghua University, and an M.S. in ceramics and Ph.D. in metals from Institute of Metal Research, Chinese Academy of Sciences. He was a visiting scientist at the Institute of Strength Physics and Materials, Russian Academy of Sciences, and a post doc at University of Missouri-Rolla in the 1990’s. He was Professor and Director of High-performance Ceramic Division, Shenyang National Laboratory for Materials Science before moving to Aerospace Research Institute of Materials and Processing Technology in 2010. He is now a professor at School of Materials Science and Engineering at Zhengzhou University in Henan, China

Zhou has discovered more than 20 new ternary carbides, nitrides and borides. His current interests and fields of research are designing, understanding the structural-property relations of damage tolerant ceramics for high and ultrahigh temperature applications. He has published more than 500 papers in peer-reviewed international journals with citations ca 29600 times with H-index of 91.

He has received numerous prizes and awards, and was elected Academician of the World Academy of Ceramics in 2009, Fellow of ACerS in 2010 and Academician of Asian-Pacific Academy of Material in 2013. He served as a member of the Advisory Committee of WAC (2010-2014), and a member of the Nominating Committee of WAC (2010-2014), Chairman of the International Committee of the ECD-ACerS, Chair of Ross Coffin Purdy Award Committee of ACerS (2015).  He also serves as editor-in-chief of Extreme Materials, editor-in chief of J Adv. Ceram., vice editor-in-Chief of JMST, editor of JACerS, and principal editor of JMR.

About Journal of Advanced Ceramics

Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen. JAC’s 2024 IF is 16.6, ranking in Top 1 (1/33, Q1) among all journals in “Materials Science, Ceramics” category, and its 2024 CiteScore is 25.9 (5/130) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508