Noncentrosymmetric Rare-Earth Borate Fluoride La₂B₅O₉F₃: A New Ultraviolet Nonlinear Optical Crystal with Enhanced...

In crystal engineering, it is an effective and controllable approach to modify the electronic band structure and optimize crystal performances using rational chemical cosubstitution in a classic structure model. Herein, the noncentrosymmetric (NCS) rare-earth borate fluoride La₂B₅O₉F₃ was designed and synthesized successfully based on the extraordinarily stable M₂B₅O₉X (M = Ca, Sr, Ba, Sn, Pb, and Eu; X = Cl, Br, and I) template. Moreover, all 70 rare-earth borate halides were discussed, and the ratio of crystallization in NCS group is only 17.1%, much lower than 34.9% in all anhydrous borates. Benefiting from the substitution of [MOX] by [LaOF] polyhedra with improved hyperpolarizability and anisotropy of polarizability, compared with the M₂B₅O₉X family, La₂B₅O₉F₃ with optimized band structure exhibits the suitable SHG response (1.2 × KH₂PO₄ (KDP) @ 1064 nm), large band gap (6.58 eV), and moderate birefringence, which well achieves the optimal balance among the three critical parameters mentioned above for nonlinear optical (NLO) applications in the short-wavelength region. This work expands the research field of NLO materials to rare-earth borate fluorides and can lead to a better understanding of the role of rare-earth metal cations.