Rb5Ba2(B10O17)2（BO2）:Theformationofunusualfunctional[BO2]-inborateswithdeep-ultraviolettransmissionwindow<iclass="icon-zqcb"></i>

The design and synthesis of new deep-ultraviolet （DUV） optical materials are important but challenging. It is well known that the polarizability anisotropy of linear [BO2]-is larger than those of [BO3]3-and [BO4]5-, which is the better choice for exploring excellent optical materials. Here, we search for such crystals in borate systems based on the unusual fundamental structural unit[BO2]-. By combining the linear [BO2]-with the traditional triangular [BO3]3-and tetrahedral [BO4]5-units, a new mixed metal borate, Rb5Ba2(B10O17)2（BO2） was obtained via high-temperature solution method. It exhibits excellent optical properties including a DUV transmission window （188 nm） and suitable birefringence （0.06）. It also has good thermal stability and its crystal plates can be prepared easily. Furthermore, the strategies of designing borates with [BO2]-units have been proposed from the transformation of crystal framework and the preparation method.