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.