Ln(HCOO) ₃ (Ln = Y, Gd, Ce, and La) With Triangular Configuration Exhibits Excellent UV Nonlinear Optical Performance

Abstract Identifying an ideal structure is essential for developing outstanding nonlinear optical (NLO) crystals, which typically involve the selection of microscopic functional building blocks (FBUs) and constructing excellent crystal structures. This often presents significant challenges. Herein, the long‐neglected π ‐conjugated [HCOO] ⁻ is selected as the UV NLO‐active FBU, and a series of isomorphic anhydrous rare earth formates, Ln(HCOO) ₃ (Ln = Y, Gd, Ce, and La) with the space group R 3m, are synthesized and characterized. This crystal series exhibits a desired delicate balance between strong second‐harmonic generation (SHG) effects (2.41–3.89 × KH ₂ PO ₄ ), large band gaps (4.20–5.45 eV), and suitable birefringence (0.064–0.081@1064 nm). Besides, such a unique structure exhibits a significant structure‐property correlation, revealing that the ionic radius of Ln ³⁺ is inversely related to the SHG effect, and a smaller ionic radius corresponds to stronger SHG effects.