Designing Deep-UV Birefringent Crystals by Cation Regulation

Two new alkaline earth metal fluorooxoborates, CaB₄ O₆ F₂ and SrB₄ O₆ F₂ , have been synthesized and characterized. The title compounds are isostructural and crystallize in centrosymmetric space group P1‾ (No. 2). Structurally, the fundamental building blocks, B₄ O₈ F₂ groups_, are connected by B-O bonds to form [B₄ O₆ F₂ ]_∞ layers, which stack parallelly along the c axis. In addition, BaB₄ O₆ F₂ , a known fluorooxoborate with different crystal structure, was also synthesized and characterized for comparison. The cation size influence on the structure of this MB₄ O₆ F₂ (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 MB₄ O₆ F₂ (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 MB₄ O₆ F₂ (M=Ca, Sr, Ba) are promising birefringent materials for deep-UV applications.