Strong SHG Responses in a Beryllium‐Free Deep‐UV‐Transparent Hydroxyborate via Covalent Bond Modification

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Deep‐ultraviolet (deep‐UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep‐UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe2BO3F2, has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium‐free deep‐UV NLO material NaSr3(OH)(B9O16)[B(OH)4] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture‐stable NSBOH exhibits strong second‐harmonic generation (SHG) at 1064 nm (3.3 × KH2PO4) and 532 nm (0.55 × β‐BaB2O4), both amongst the largest powder SHG responses for a deep‐UV borate, with good phase‐matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B9O19]∞ honeycomb‐like framework with no layering. The Sr2+ and Na+ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO‐active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep‐UV NLO applications.