Nonlinear optical (NLO) crystals are essential materials for generation of coherent UV light in solid state lasers. Therefore, the design of new nonlinear optical (NLO) materials with some properties, namely, large NLO coefficient, wide transparency range, good chemical and mechanical stability has been a hot topic. A successful example is KBBF, which is the only material that can achieve coherent light below 200 nm by direct second harmonic generation (SHG). However, its strong layer habits and the high toxicity of the beryllium oxide powders required for synthesis limit its application.  

Recently, Professor PAN Shilie and his Ph.D. students from Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences (XTIPC), designed and synthesized a new KBBF-like borate, Cs₃Zn₆B₉O_21, by using Zn atoms to substitute Be atom of KBBF and connecting the adjacent [Zn₂BO₃O₂]_∞ layers by B₃O₆ groups. Interestingly, Cs₃Zn₆B₉O₂₁ can not only be grown without the processing limitations of KBBF (strong layer habits and the high toxicity of the beryllium oxide powders), but it also exhibits the largest SHG response in the KBBF family, about 3.3 times that of KDP.  

Fig. 1 The structure comparison between KBBF and Cs₃Zn₆B₉O₂₁. (Image by XTIPC)

Based on the calculation of the dipole moment and pseudosymmetry analysis as well as the first principles calculations, Professor PAN, et al have realized that the large SHG response of Cs₃Zn₆B₉O₂₁ mainly come from the cooperation effect of the coparallel BO₃ triangles and the distorted ZnO₄ tetrahedra, which indicates that using Zn atoms to substitute the Be atoms of beryllium borates is a feasible method to synthesize some interesting NLO crystals or to increase the SHG response of the beryllium borates. 

 Fig. 2 Atomic distortion patterns obtained from the symmetry-mode analysis. （Image by XTIPC）

The research results have been published as a communication in the J. Am. Chem. Soc., 2014, 136(4), pp 1264. And the related Chinese invention patent Zinc borate compound cesium and cesium zinc borate nonlinear optical crystal and preparation method and uses  has also been applied (201210293692). 

The work has been supported by 973 Program of China, the National Natural Science Foundation of China, Main Direction Program of Knowledge Innovation of CAS, and the High Technology Research & Development Program of Xinjiang Uygur Autonomous Region of China, etc.