From Ce(IO₃)₄to CeF₂(IO₃)₂: fluorinated homovalent substitution simultaneously enhances SHG response and bandgap for mid-infrared nonlinear optics

Efficient mid-infrared (mid-IR) nonlinear optical (NLO) crystals are highly desirable, but it remains extremely challenging to simultaneously achieve a strong second harmonic generation (SHG) response, a large optical bandgap, and wide optical transparency in a material. In this work, a fluorinated homovalent substitution strategy has been put forward and this strategy leads to a new cerium fluorinated iodate, CeF₂(IO₃)₂, which is designed based on a noncentrosymmetric cerium iodate, Ce(IO₃)₄. The introduction of F⁻ions enables CeF₂(IO₃)₂to achieve the simultaneous enhancement of an enlarged phase-matchable SHG response that is 8 times that of KH₂PO₄(KDP) and a bandgap of 2.90 eV compared to its parent Ce(IO₃)₄(0.9× KDP and 2.17 eV, respectively). CeF₂(IO₃)₂exhibits a high thermal stability (∼430 °C) and a wide transparent region (0.43-6.46 μm), suggesting that CeF₂(IO₃)₂is a promising mid-IR NLO material. First-principles simulations reveal that the difference in linear and nonlinear optical properties between Ce(IO₃)₄and CeF₂(IO₃)₂is mainly attributed to the divergent anisotropies of the cerium-centered polyhedra.