Fluorooxoborates: A precious treasure of deep-ultraviolet nonlinear optical materials

As the key components of all-solid-state deep-ultraviolet (DUV, λ ​< ​200 ​nm) laser sources, exploring DUV nonlinear optical (NLO) materials has been a research hotspot for a long time. However, as the only material that has been practically used to generate DUV coherent light through the direct second harmonic generation (SHG) process, KBe2BO3F2 (KBBF) still meets bottlenecks, namely, severe layered crystal growth tendency, for extensive application. In the search for the next generation of DUV NLO materials, fluorooxoborates are highly expected due to their excellent NLO performances, flexible structures, and rich structural chemistry. So far, a large number of materials with prominent properties have emerged in the fluorooxoborate system. By investigating the differences in structures and properties among fluorooxoborates and borates and borate fluorides, the excellent NLO performances of fluorooxoborates are attributed to functionalized B–O/F chromophores. In this review, by analyzing the published fluorooxoborates, the fluorooxoborate system is a precious treasure for exploring DUV NLO materials. The synthesis, structures, and properties of some representative fluorooxoborates in the research history are introduced and summarized. Finally, we put forward the opportunities and challenges of future research in fluorooxoborate DUV NLO crystals.