Chemical Cosubstitution-Oriented Design of Rare-Earth Borates as Potential Ultraviolet Nonlinear Optical Materials

A chemical cosubstitution strategy was implemented to design potential ultraviolet (UV) and deep-UV nonlinear optical (NLO) materials. Taking the classic β-BaB₂O₄ as a maternal structure, by simultaneously replacing the Ba²⁺ and [B₃O₆]³⁻ units with monovalant (K⁺), divalent (alkaline earth metal), trivalent (rare-earth metal, Bi³⁺) ions, and the [B₅O₁₀]⁵⁻ clusters through two different practical routes, 12 new mixed-metal noncentrosymmetric borates K₇MIIRE₂(B₅O₁₀)₃ (MII = Ca, Sr, Ba, K/RE0.5; RE = Y, Lu, Gd) as well as K₇MIIBi₂(B₅O₁₀)₃ (MII = Pb, Sr) were successfully designed and synthesized as high-quality single crystals. The selected K₇CaY₂(B₅O₁₀)₃, K₇SrY₂(B₅O₁₀)₃, and K₇BaY₂(B₅O₁₀)₃ compounds were subjected to experimental and theoretical characterizations. They all exhibit suitable second-harmonic generation (SHG) responses, as large as that of commercial KH₂PO₄ (KDP), and also exhibit short UV cutoff edges. These results confirm the feasibility of this chemical cosubstitution strategy to design NLO materials and that the three selected crystals may have potential application as UV NLO materials.