Engineering an Excellent β‐BaB2O4‐ Inspired UV Nonlinear Optical Material through Secondary Building Unit Substitution

The development of nonlinear optical (NLO) crystals with tailored properties has been a long‐standing pursuit, yet remains challenging to achieve. Herein, we report the successful synthesis of a novel high‐performance UV NLO material, Li3[Be3(OH)3(C3H2O4)3]·7H2O (LBOC), by substituting the [B3O6] functional building units (FBUs) in the classic β‐BaB2O4 (β‐BBO) with meticulously designed [Be3(OH)3(C3H2O4)3]3‐ secondary building units (SBUs). The [Be3(OH)3(C3H2O4)3]3‐ SBUs within LBOC preserve the ordered arrangement of the [B3O6] FBUs found in β‐BBO, enabling LBOC to inherit the strong second harmonic generation (SHG) response from β‐BBO. Additionally, the increased dimensionality of the [Be3(OH)3(C3H2O4)3]3‐ SBUs, compared to the planar [B3O6] FBUs, leads to a reduction in optical anisotropy. Consequently, LBOC demonstrates a moderate birefringence of 0.067@589.3 nm, lower than that of β‐BBO (0.118@589.3 nm). Notably, bulk crystals of LBOC can be easily grown using a straightforward solution evaporation method, making it a promising candidate for UV NLO applications. This strategic design of the SBU opens new avenues for the rational development of NLO crystals with tailored properties.