Beryllium-Free Nonlinear-Optical Crystals A₃Ba₃Li₂Ga₄B₆O₂₀F (A = K and Rb): Members of the Sr₂Be₂(BO₃)₂O Family with a Strong Covalent Connection between the _∞²[Li₂Ga₄B₆O₂₀F]^9– Double Layers

Two gallium-based borates, K₃Ba₃Li₂Ga₄B₆O₂₀F (I) and Rb₃Ba₃Li₂Ga₄B₆O₂₀F (II), have been successfully grown in the M₂O (M = K, Rb)–LiF–B₂O₃ flux. They can be recognized as the element cosubstitutions of Sr for K/Rb and Ba and of Be for Li and Ga from Sr₂Be₂(BO₃)₂O (SBBO), which uses poisonous beryllium oxide during the synthesis and crystal growth process and also exhibits the problem of structural instability. The isostructural borates crystallize into the noncentrosymmetric space group P6̅2c, with a = 8.6398(12) Å and c = 16.827(3) Å for I and a = 8.7214(12) Å and c = 17.180(3) Å for II. In the structures, the basic anionic units are the BO₃ triangles and GaO₄ and LiO₃F tetrahydra. These anionic units bond together through O atoms, forming the infinitely extended _∞²[LiGa₂B₃O₁₂]^8– single layer at the ab plane. The adjacent layers are further coupled to the _∞²[Li₂Ga₄B₆O₂₀F]^9– double layers by means of bridged O and F atoms. Then the adjacent double layers are strongly joined together via O atoms of the GaO₄ tetrahedra to form a three-dimensional skeleton, with K/Rb and Ba atoms occupying the network for charge balance. I and II have considerable second-harmonic-generation responses of about 0.7 and 0.5 as large as that of KH₂PO₄, respectively. In addition, the first-principles calculations were conducted to confirm that they address the structural instability issues in SBBO.