Researchers of Research Center for Crystal Materials of Xinjiang Technical Institute of Physics and Chemistry, CAS, have been committed to design, preparation, growth and performance study of new type of DUV NLO crystals. In their previous work, the ‘fluorination strategy’ was proposed by substituting fluorine for oxygen atoms in borates to regulate the structure of NLO crystals, to achieve the balance of the three fundamental parameters of DUV NLO crystals (Angew. Chem. Int. Ed., 2017, 56, 3916–3919). Based on this strategy, a series of deep-UV NLO crystals in the ABF family were obtained (J. Am. Chem. Soc., 2017, 139, 10645–10648； Angew. Chem. Int. Ed., 2017, 56, 14119–14123；Angew. Chem. Int. Ed., 2018, 57, 2150–2154).

Recently, the ‘fluorination strategy’ was further extended to the borophosphate system leading to a new fluoroborophosphate (NH₄)₃B₁₁PO₁₉F₃ (ABPF). It exhibits a KBBF-like structure with a unique [B₅PO₁₀F]_∞ layer formed by four different functional units: [BO₃], [BO₄], [BO₃F] and [PO₄]. It has a large SHG response (1.2 × KDP) and a sufficient birefringence (0.088 at 1064 nm) that enables the shortest phase-matching wavelength to reach the DUV region. Meanwhile, the introduction of strong B-O-P covalent bonds decreases the layered growth habit. Owing to a beryllium-free, no-layered growth habit, and excellent optical properties, ABPF has a promising future as DUV NLO crystal. Moreover, the authors propose that the introduction of strong covalent bonds between layers can enhance the interlayer interaction force while simultaneously maintaining the large optical anisotropy of layered structures. More importantly, this work proves the advancement of the ‘fluorination strategy’ in the exploration of DUV NLO materials. These findings will facilitate the discovery of more DUV NLO materials with optimal and practical performance.

This work was published in National Science Review as a Research Article (Natl. Sci. Rev., 2022, 9, nwac110). This work was supported by the National Key R&D Program of China (2021YFA0717800), the National Natural Science Foundation of China (61875229, 61922084 and 61835014), the West Light Foundation of Chinese Academy of Sciences (2020-JCTD-002) and the Instrument Developing Project of Chinese Academy of Sciences (GJJSTD20200007).