Decentering the Symmetry via Docking B and F in the KBe2BO3F2-Family Structure.

Deep-ultraviolet (DUV; <200 nm) nonlinear optical crystals arouse a great research interest in modern optical material science as they are key parts of solid-state lasers. Since KBe2BO3F2 (KBBF) still is the only crystal which can generate DUV coherent light practically, exploring other available structures in the DUV region by revising KBBF-family structure templates is necessary. In this paper, two KBBF-family structures are designed by adjusting the directions of BO3 groups or docking B and F atoms in known ß-Be2BO3F (BBF). The first BBF structure with a P6322 space group is designed by strictly aligning the BO3 groups of even layers and odd layers in ß-BBF. It is very interesting that docking B and F atoms forms novel B-F bonds, which destroy the inversion symmetry of the structure, making the centrosymmetric ß-BBF structure with the R3̅c space group convert to a noncentrosymmetric BBF structure having the R3c space group. This noncentrosymmetric BBF structure exhibits a DUV cutoff edge shorter than 150 nm and a second harmonic generation effect comparable to that of KH2PO4 (KDP; d36 = 0.39 pm/V), indicating its feasibility as a frequency-doubling crystal in the DUV region. Our conversion design provides an effective way for uncovering noncentrosymmetric structures in KBBF-family structures.