Na 4 [B 6 O 9 (OH) 3 ](H 2 O) Cl: A Deep-Ultraviolet Transparent Nonlinear Optical Borate Chloride with {[B 6 O 9 (OH) 3 ] 3- } ∞ Chains.

The development of nonlinear-optical (NLO) crystals with short ultraviolet cutoff edges is significant and challenging. Here, a new sodium borate chloride, Na 4 [B 6 O 9 (OH) 3 ](H 2 O) Cl, was successfully obtained by the mild hydrothermal method, which crystallizes in a polar space group Pca 2 1 . The structure of the compound is characterized by {[B 6 O 9 (OH) 3 ] 3- } ∞ chains. Measurements of optical properties indicate the compound exhibits a deep-ultraviolet (DUV) cutoff edge (≤200 nm) and moderate second harmonic generation response (0.4 × KH 2 PO 4 ). It presents the first DUV hydrous sodium borate chloride NLO crystal and the first sodium borate chloride possessing a one-dimensional B-O anion framework. Probing into the connection of structure and optical properties has been performed based on theoretical calculations. These results are instructive for designing and obtaining new DUV NLO materials.