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.