Ba 4 B 14 O 25 : A Deep Ultraviolet Transparent Nonlinear Optical Crystal with Strong Second Harmonic Generation Response Achieved by a Boron-Rich Closed-Loop Strategy.

Discovering new deep ultraviolet (DUV) nonlinear optical (NLO) materials is the current research hotspot. However, how to perfectly integrate several stringent performances into a crystal is a great challenge because of the natural incompatibility among them, particularly wide band gap and large NLO coefficient. To tackle the challenge, a boron-rich closed-loop strategy is supposed, based on which a new barium borate, Ba 4 B 14 O 25 , is designed and synthesized successfully via the high-temperature solid-state melting method. It features a highly polymeric 3D geometry with the closed-loop anionic framework [B 14 O 25 ] 8- constructed by the fundamental building blocks [B 14 O 33 ] 24- . The high-density π-conjugated [BO 3 ] 3- groups and the fully closed-loop B-O-B connections make Ba 4 B 14 O 25 possess excellent NLO properties, including short UV cutoff edge (<200 nm), large second harmonic generation response (3.0 × KDP) and phase-matching capability, being a promising DUV-transparent NLO candidate material. The work provides a creative design strategy for the exploration of DUV NLO crystals.