Engineering an Excellent <i>β</i> ‐BaB ₂ O ₄ ‐Inspired UV Nonlinear Optical Material Through Secondary Building Unit Substitution

Abstract 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, Li 3 [Be 3 (OH) 3 (C 3 H 2 O 4 ) 3 ]·7H 2 O (LBOC), by substituting the [B 3 O 6 ] functional building units (FBUs) in the classic β ‐BaB 2 O 4 ( β ‐BBO) with meticulously designed [Be 3 (OH) 3 (C 3 H 2 O 4 ) 3 ] 3− secondary building units (SBUs). The [Be 3 (OH) 3 (C 3 H 2 O 4 ) 3 ] 3− SBUs within LBOC preserve the ordered arrangement of the [B 3 O 6 ] 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 [Be 3 (OH) 3 (C 3 H 2 O 4 ) 3 ] 3− SBUs, compared to the planar [B 3 O 6 ] FBUs, leads to a reduction in optical anisotropy. Consequently, LBOC demonstrates a moderate birefringence of [email protected] nm, lower than that of β ‐BBO ([email protected] 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.