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.