A Deep-Ultraviolet Nonlinear Optical Crystal: Strontium Beryllium Borate Fluoride with Planar Be(O/F)₃ Groups

A new strontium beryllium borate fluoride, Sr₃[(Be_<i>x</i>B_1–<i>x</i>)₃B₃O₁₀]­[Be­(O_1–<i>x</i>F_<i>x</i>)₃] <i>x</i> = 0.30 (SBBOF), designed to be used in the deep-UV nonlinear optical (NLO) application, was grown by the spontaneous crystallization of a molten flux of SrO–B₂O₃–LiF. It crystallizes in the space group <i>R3m</i> (No. 160) with the following unit cell dimensions: <i>a</i> = 10.3179(11) Å, <i>c</i> = 8.3958(13) Å, <i>V</i> = 774.1(2) ų, and <i>Z</i> = 3. SBBOF consists of [(Be_<i>x</i>B_1–<i>x</i>)₃B₃O₁₀] anionic groups and isolated [Be­(O_1–<i>x</i>F_<i>x</i>)₃] planar groups. Importantly, a new strategy to improve the birefringence was introduced by changing the local configuration of isolated structural units from trigonal pyramids to planar triangles. UV–vis diffuse reflectance spectroscopy indicates that the short-wavelength absorption edge of SBBOF is below 200 nm. The band structure and refractive index were calculated. Second harmonic generation (SHG) was measured using the Kurtz and Perry technique, which showed that SBBOF is a phase-matchable material in both visible and UV regions, and its measured SHG coefficient is 2.2 times as large as that of <i>d</i>₃₆ (KDP) at 1064 nm