High-pressure synthesis, mechanical properties, and magnetic behavior of chromium diboride

Despite persistent synthetic challenges in realizing borophene, transition metal diborides, characterized by honeycomb borophene substructure and weak interlayer coupling, are attracting growing interest as potentially viable alternatives. Herein, we synthesized a bulk CrB2 sample with a honeycomb boron substructure using high pressure and high temperature method. CrB2 yields an asymptotic hardness of 12.3 GPa and a fracture toughness of 3.62 MPa·m1/2. First-principles calculations, corroborated by x-ray photoelectron spectroscopy, reveal that the high hardness stems from strong covalent bonding and a rigid boron network, while the enhanced fracture toughness arises from mechanical anisotropy and the metallic character within the layers. Chromium atoms stabilize the honeycomb structure through electron transfer that fills the empty π-bands, a process predominantly mediated by strongly directional Cr-3d a1g orbitals. Furthermore, CrB2 displays typical antiferromagnetic behavior with a Néel temperature of 96 K. The presence of defect structures in the sample influences its magnetic behavior, leading to a transition from ferromagnetic to antiferromagnetic, and then to paramagnetic behavior with increasing temperature. This work provides a valuable platform for understanding the mechanisms by which metals stabilize honeycomb boron substructures, paving the way for future borophene materials synthesis.