Refractive Index Modulates Second-Harmonic Responses in RE₈O(CO₃)₃(OH)₁₅X (RE = Y, Lu; X = Cl, Br): Rare-Earth Halide Carbonates as Ultraviolet Nonlinear Optical Materials

The first series of rare-earth chloride/bromine carbonate nonlinear optical (NLO) crystals, RE₈O­(CO₃)₃(OH)₁₅X (RE = Y, Lu; X = Cl, Br), have been synthesized with a facile hydrothermal method. All of the title compounds were isostructural with the noncentrosymmetric space group P6₃ (no. 173), and their structures can be described as the [REO₄(CO₃)₃] units connect the adjacent infinite [RE₇O₂₅]_∞ layers in the a–b plane to form an intricate three-dimensional framework, and the Br^– or Cl^– anions were located in the center of equilateral triangles formed by the RE(3)­O₇ polyhedra. The measured second-harmonic generation (SHG) effects of Y₈O­(CO₃)₃(OH)₁₅Cl, Lu₈O­(CO₃)₃(OH)₁₅Cl, Y₈O­(CO₃)₃(OH)₁₅Br, and Lu₈O­(CO₃)₃(OH)₁₅Br were 1.65, 2.22, 1.83, and 3.00 times that of KH₂PO₄, respectively. Our study revealed that the difference of SHG efficiency was mainly derived from the different refractive indices of crystals when the orientation of [CO₃]^2– groups that had a relationship with the SHG coefficient was almost identical. Furthermore, the birefringences of the four crystals were successively measured to be 0.045, 0.062, 0.073, and 0.088 (λ = 546.1 nm), which were very favorable for realizing phase-matching in the visible and UV region. In addition, with a wide transparent region from UV to near infrared, they can be excellent UV NLO materials.