Ba2ScBSi2O9: A Mixed-Coordination Borosilicate with a Low B/Si Ratio Exhibiting Enhanced Second Harmonic Generation Response

Rational chemical substitution is an effective way to regulate structure and enrich property. Herein, a new noncentrosymmetric borosilicate, Ba₂ScBSi₂O₉, was successfully synthesized by substituting CaO₆ units in Ba₂CaB₂Si₄O₁₄ with ScO₆ octahedra, with comparatively strong covalency. This substitution not only effectively prevents polymerization of the B-O groups, resulting in an intriguing structural transformation from tetrahedral-coordinated borosilicate of Ba₂CaB₂Si₄O₁₄ to mixed-coordinated borosilicate Ba₂ScBSi₂O₉, but also enhances its second harmonic generation response (2 × KDP), that is nearly four times higher than its parent structure while keeping a short ultraviolet (UV) cutoff edge (λ_cutoff < 190 nm). In addition, the polar space group of Pca2₁ for Ba₂ScBSi₂O₉ achieves its ferroelectric polarization reversal capability, which makes quasi-phase-matching technology possible to counteract the nonphase-matching caused by small birefringence of silicates. This work indicates the unique role of heterovalent substitution in regulating structure and performance, providing new insights for exploring borosilicate with versatile functionality.