Solid-state ultraviolet (UV) lasers are widely used for their var-ious applications in commercial and scientific researches.The non-linear optical (NLO) crystals with high NLO coefficients and good quality are key factors for the harmonic generation of UV light sources [1-3].The fourth harmonic generation (FHG) using NLO crystals is an effective method to achieve a high-power UV laser at 266 nm [4,5].The borate crystals possessing wide transparency range,good chemical stability and high damage threshold,are promising candidates for the NLO applications [4-7].As a good UV material,several basic conditions should be satisfied:(1) a wide transparency window (absorption edge ＜ 300 nm);(2) a moderate birefringence (0,07-0.10);(3) a large second-harmonic generation(SHG) coefficient (＞1 × KH2PO4 (KDP)).Owing to the mutual restriction among the band gap,birefringence,and NLO effect,achieving a balance among these three parameters in the design of new materials is still a huge challenge.In the past decades,many borates with excellent properties have been discovered [8-11].Among them,KBe2BO3F2 (KBBF) is the only NLO crystal that can generate SHG wavelength as short as 161 nm.Furthermore,only two NLO crystals are currently commercially available for the FHG at 266 nm:β-BaB2O4 (BBO) and CsLiB6O10 (CLBO) [4,5].