Rb5Ba2(B10O17)2(BO2): The formation of unusual functional [BO2]− in borates with deep-ultraviolet transmission window

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.