Ba₄B₁₄O₂₅: A Deep Ultraviolet Transparent Nonlinear Optical Crystal with Strong Second Harmonic Generation Response...

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, Ba₄B₁₄O₂₅, 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 [B₁₄O₂₅]^8- constructed by the fundamental building blocks [B₁₄O₃₃]^24-. The high-density π-conjugated [BO₃]^3- groups and the fully closed-loop B-O-B connections make Ba₄B₁₄O₂₅ 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.