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Electron-Hole Excitation Induced Softening in Boron Carbide-Based Superhard Materials.

Photomechanical effect in semiconductors refers to a phenomenon that plastic deformation is influenced by light-induced electron-hole (e-h) excitation. To date, increasing amounts of theoretical and experimental studies have been performed to illustrate the physical origin of this phenomenon. In contrast, there has been little discussion about this effect in superhard materials. Here, we adopted constrained density functional theory simulations to assess how e-h excitation influences two boron-based superhard materials: boron carbide (B4C) and boron subphosphide (B12P2). We found that the ideal shear strengths of both systems decrease under e-h excited states. Under e-h excitation, the redistribution of electrons and holes contributes to the decreased strength, weakening the bonds initially broken under the shear deformation. The simulation results provide a fundamental explanation for the softening effects of superhard materials under e-h excitation. This study also provides a basis to tune the mechanical properties of superhard materials via light irradiation.