In recent years, nonlinear optical (NLO) crystals become academic and commercial interest attributable to their prominent applications in the laser science and optoelectronic devices. However, the basic requirement for NLO materials is that they must crystallize in the noncentrosymmetric (NCS) space group.   

Prof. PAN Shilie’s group at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences（XTIPC）, has been obtained some good NLO materials with wide transparence window and large second harmonic generation (SHG) response by introducing the halogen atoms into borates. Recently, the group successfully synthesized a new deep UV NLO crystal Cs₂B₄SiO₉, with the large SHG response and deep UV cutoff edge.     

Based on the previous research, Prof. PAN Shilie’s group introduced a new rigid unit, AlO₄ tetrahedron, into B–O framework to design a [AlB₄O₉]_∞ framework similar to the [SiB₄O₁₀]_∞framework, whic h is responsible for the large SHG response and short UV cutoff edge of Cs₂B₄SiO₉. Besides, to better understand the effect of the rigid units on the symmetry of the framework, centrosymmetric NaBa₄(B₅O₉)₂F₂Cl are also synthesized by using B atoms to substitute Al atoms.  Researchers explained the basic building units of NaBa₄(AlB₄O₉)₂Br₃ are the isolated AlO₄ tetrahedra and B₄O₉ groups, which are composed of two BO₃ triangles and two BO₄ tetrahedra by sharing O atoms. Each B₄O₉ group is linked to four different AlO₄ tetrahedra through its terminal O atoms, and likewise each AlO₄ tetrahedron shares its four vertices with four neighboring B₄O₉ groups to form 3D [AlB₄O₉] backbone. The Na, Ba, and Br atoms are filled in the cavities of [AlB₄O₉] backbone (Fig.1).    

Fig.1 The structure of NaBa₄(AlB₄O₉)₂Br₃