Decentering the Symmetry Docking B and F in the KBeBOF-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 KBeBOF (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 BO groups or docking B and F atoms in known β-BeBOF (BBF). The first BBF structure with a 622 space group is designed by strictly aligning the BO groups of even layers and odd layers in β-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 β-BBF structure with the 3̅ space group convert to a noncentrosymmetric BBF structure having the 3 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 KHPO (KDP; = 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.