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

CrB₂ crystallizes in an AlB₂-type crystal structure, and the chemical bonding in CrB₂ includes B sp²‒B sp² covalent bonds in the graphite-analogous six-numbered B ring, B p_z‒Cr 3d covalent‒ionic bonds, and Cr‒Cr metallic bonds from theoretical calculations. However, the crystal structure and chemical bonding properties have not been experimentally validated. To fill this research gap, herein, the crystal structure and chemical bonding of CrB₂ were evaluated for the first time via aberration-corrected transmission electron microscopy (AC-TEM) coupled with electron energy loss spectroscopy (EELS). Combined with first-principles calculations based on density functional theory (DFT), CrB₂ is confirmed to have an AlB₂-type structure, where Cr bonds to each other in the (001) plane via metallic bonding and where B bonds in the form of a graphite-like six-membered ring in the (002) plane through sp² hybridization, whereas Cr‒B ionic‒covalent bonding is formed in the (110) plane. A detailed analysis of the experimental and calculated results of the EELS of CrB₂ shows that the hybridization of Cr 3d and B has a significant effect on the EELS of transition metal borides (TMB₂). In addition, the hysteresis loop of CrB₂ was tested for the first time on the basis of theoretical calculations, and the molar susceptibility of CrB₂ was approximately 5.77×10⁻⁴ emu/mol. The present work is helpful for understanding the structure‒property relationships, which are essential for tailoring the properties from a crystal structure and chemical bonding point of view and promoting the practical application of TMB₂ in extreme aerospace environments.