Anisotropic mechanical response and aliovalent doping-induced strengthening in CaF2 single crystals: a combined experimental and first-principles study

CaF₂ single crystals, despite their excellent optical properties, are limited in extreme environments by intrinsic brittleness and low mechanical strength. In this study, we investigate the anisotropic mechanical behavior and strengthening mechanisms of pure and Y³⁺-doped CaF₂ crystals. Nanoindentation and uniaxial compression tests reveal significant mechanical anisotropy: the <100> orientation exhibits the highest Young's modulus, compressive strength, and Vickers hardness, followed by the <110> and <111> orientations. This trend contrasts sharply with conventional face-centered cubic metals and covalent semiconductors. Importantly, Y³⁺ doping further elevates these properties. Notably, doping with about 1 at% Y³⁺ increases the compressive strength along the <111> direction by 94%, from 101 MPa to 196 MPa. We attribute the mechanical anisotropy to variations in Coulomb interactions between like-charged ions during compression along different crystallographic directions. The strengthening effect of Y³⁺ doping is explained by lattice distortion and enhanced Y–F bond strength compared to native Ca–F bonds. These findings provide valuable insights for understanding and designing optical devices for extreme conditions.