Direct experimental validation on the crystal structure, chemical bonding, and magnetic properties of CrB ₂

CrB₂ crystallizes in an AlB₂-type crystal structure and the chemical bonding in CrB₂ includes B sp²-B sp² covalent bond in the graphite-analogous six-numbered B ring, B p_z-Cr 3d covalent-ionic bond, and Cr-Cr metallic bond from theoretical calculations. However, the crystal structure and chemical bonding properties were not experimental validated. To fill this research gap, herein the crystal structure and chemical bonding of CrB₂ were approved for the first time using aberration corrected transmission electron microscopy coupled with electron energy loss spectroscopy (EELS) accessory. 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 (001) plane by metallic bonding and B is bonding in the form of a graphite-like six-membered ring in (002) plane through sp² hybridization, while Cr-B ionic-covalent bonding is formed in (110) plane. A detailed analysis of the experimental and calculated results of EELS of CrB₂ show that the hybridization between Cr 3d and B has a significant effect on EELS of transition metal borides (TMB₂). In addition, hysteresis loop of CrB₂ was tested for the first time based on the theoretical calculation and the molar susceptibility of CrB₂ was about 5.77×10^-4 emu/mol. The present work is helpful for deep understanding the structure-property relationships which is essential to tailor the properties from crystal structure and chemical bonding point of view and promoting the practical applications of TMB₂ in extreme aerospace environments.