Ca₂B₅O₉Cl and Sr₂B₅O₉Cl: Nonlinear Optical Crystals with Deep-Ultraviolet Transparency Windows

M₂B₅O₉X is a prominent family with excellent nonlinear optical (NLO) responses, just as the Pb₂B₅O₉I crystal with a large second harmonic generation (SHG) of 13.5 times that of KH₂PO₄. However, most of these compounds are limited to ultraviolet and visible regions because of their long absorption edge (small band gap). Here, we report two members of this family, which change the situation. Using a high-temperature solution method, we obtain Ca₂B₅O₉Cl and Sr₂B₅O₉Cl crystals, which exhibit a deep-ultraviolet (DUV) absorption edge of 170 nm (band gap ≈ 7.29 eV). It is an important breakthrough in the DUV transparency of the M₂B₅O₉X family. Furthermore, Ca₂B₅O₉Cl crystals display a phase-matching SHG response under a 1064 nm laser, which is further confirmed by the balance between the suitable birefringence and the small dispersion of refractive indexes in the wavelength range of 1064-532 nm. Therefore, they are promising DUV transparency windows and NLO candidates.