Introduction of the Cl(-) anion in the borate systems generates a new perovskite-like phase, K(3)B(6)O(10)Cl, which exhibits a large second harmonic response, about four times that of KH(2)PO(4) (KDP), and is transparent from the deep UV (180 nm) to middle-IR region. K(3)B(6)O(10)Cl crystallizes in the noncentrosymmetric and rhombohedral space group R3m. The structure consists of the A-site hexaborate [B(6)O(10)] groups and the BX(3) Cl-centered octahedral [ClK(6)] groups linked together through vertices to form the perovskite framework represented by ABX(3).

Exploration on the compounds in the complex alkali metal borate system had resulted in the discovery of a class of deep-ultraviolet second-order nonlinear optical (NLO) materials LinMn−1B2n−1O4n−2 (M = Cs/Rb, n = 3, 4, 6).

The experimental and theoretical analysis of linear and nonlinear optical properties of K 3 B 6 O 10 Br (KBOB), with a moderate birefringence that is suitable for UV coherent light generation and optical parametric oscillators, is presented in detail. The second-order nonlinear optical coefficients were measured by the Maker fringe method and the refractive indices dispersion curves were deduced by the minimum deviation technique at 16 different monochromatic sources from UV to NIR, and then the type I and type II phasematching curves of second, third, and fourth harmonic generation (SHG, THG, and FHG) were calculated. Moreover, the correlations of crystallographic and crystallophysical axes were determined. On the basis of the density functional theory (DFT), the first-principles calculations have been employed successfully to study the structural and electronic properties of KBOB. In addition, to gain further insight into the structure−property relationship, the SHG density method was adopted to analyze the origin of the nonlinear optical response of KBOB.

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A series of K 3−x Na x B 6 O 10 Br (0.5 ≤ x ≤ 1.2) polycrystalline samples have been synthesized by the standard solid-state reaction method. Four stoichiometric crystals K 3−x Na x B 6 O 10 Br (x = 0.13, 0.67, 1.30, 2.20) have been successfully grown from the high-temperature solution, and the crystal structures were determined by single-crystal X-ray diffraction. Interestingly, the Na + concentration plays a profound role to influence the crystal structure. Up to about 23% (x = 0.7) K + ions can be substituted by Na + ions with the same noncentrosymmetric (NCS) crystal structure of K 3 B 6 O 10 Br (space group R3m) being retained, while a higher Na concentration would lead to it crystallizing in the centrosymmetric (CS) space group Pnma. Meanwhile, the second-harmonic generation (SHG) response of K 3−x Na x B 6 O 10 Br (x ≤ 0.7) is about 2.8 times that of KDP, while the SHG response decreases sharply when x > 0.7 (the SHG response is zero at x = 1.3). After careful structural analysis, we believe that the different Br-M (M = K/Na, K, or Na) lattices, which are influenced by the coordination environments of the cations, are responsible for the structural changes from NCS to CS.

Single crystals of Li 3 Cs 2 B 5 O 10 have been synthesized and its structure determined by single-crystal X-ray diffraction. Li 3 Cs 2 B 5 O 10 crystallizes in the non-centrosymmetric space group C222 1 (No. 20). The structure consists of isolated B 5 O 10 groups that are bridged by LiO n (n ¼ 4, 5) and CsO n (n ¼ 8, 10) polyhedra through their vertex oxygen atoms to form an infinite three-dimensional structure. Li 3 Cs 2 B 5 O 10 melts congruently and exhibits a short-wavelength absorption onset at 175 nm. As-prepared Li 3 Cs 2 B 5 O 10 is both non-centrosymmetric and phase matchable as supported by second harmonic generation measurements.

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