Charge Transfer Driven Bond Weakening Underlies the Inverse Boron Content-Ideal Strength Relationship in Rhenium Borides.

Rhenium borides have emerged as promising candidates for superhard materials, yet the fundamental mechanisms governing their mechanical properties with varying boron content remain poorly understood. Through first-principles calculations, we investigate the ideal strength and deformation mechanisms of ReB2, ReB3, and ReB4 under multiple loading conditions. The results demonstrate an unexpected inverse relationship between boron content and hardness in rhenium borides contradicting conventional materials design principles. The stress-strain analyses identify failure mechanisms characterized by the rupture of critical B-B bonds within the boron networks. The anomalous behavior originates from charge redistribution effects, where all compounds show charge transfer from Re to B, yet the electron accumulation efficiency at boron sites diminishes with higher boron content. This reduced charge transfer leads to progressive weakening of essential B-B bonds, as quantitatively demonstrated through Crystal Orbital Hamilton Population (COHP) and Bader charge analyses. We highlight that optimal charge transfer rather than simple boron content determines mechanical performance. These findings offer new guidelines for developing advanced ultrahard materials.