Designing Deep‐UV Birefringent Crystals by Cation Regulation

Two new alkaline earth metal fluorooxoborates, CaB 4 O 6 F 2 and SrB 4 O 6 F 2 , have been synthesized and characterized. The title compounds are isostructural and crystallize in centrosymmetric space group P (No. 2). Structurally, the fundamental building blocks, B 4 O 8 F 2 groups , are connected by B−O bonds to form [B 4 O 6 F 2 ] ∞ layers, which stack parallelly along the c axis. In addition, BaB 4 O 6 F 2 , a known fluorooxoborate with different crystal structure, was also synthesized and characterized for comparison. The cation size influence on the structure of this MB 4 O 6 F 2 (M=Ca, Sr, Ba) family is discussed in detail. The diffuse‐reflectance spectroscopy measurements indicate that the cutoff edges of title compounds are below 190 nm. Moreover, first‐principle calculation results demonstrate that MB 4 O 6 F 2 (M=Ca, Sr, Ba) have large birefringence of 0.091, 0.089, and 0.085 at 1064 nm. Both experimental and theoretical studies suggest that MB 4 O 6 F 2 (M=Ca, Sr, Ba) are promising birefringent materials for deep‐UV applications.