LAUSR

M2B5O9X is a prominent family with excellent nonlinear optical responses, just as Pb2B5O9I crystal with a large second harmonic generation of 13.5 times that of KH2PO4. However, most of these compounds are limited to ultraviolet and visible regions due to their long absorption edge (small band gap). Here, we report two members of this family, which change the situation. By a high temperature solution method, we obtain Ca2B5O9Cl and Sr2B5O9Cl crystals, which exhibit a deep-ultraviolet absorption edge of 170 nm (band gap 7.29 eV). It is an important breakthrough in the deep-ultraviolet transparency of M2B5O9X family. Furthermore, Ca2B5O9Cl crystals display a phase matching second harmonic generation 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 deep-ultraviolet transparency windows and nonlinear optical candidates.