When Octahedra Learn to Be Prisms: Unlocking a Record Deep‑UV NLO Crystal

Deep ultraviolet (DUV) lasers are indispensable for cutting‐edge science and technology, yet their practical realization is severely hampered by the performance trade‐offs and growth limitations of existing nonlinear optical (NLO) crystals, such as the toxic and strongly layered KBe 2 BO 3 F 2 (KBBF). Here, we challenge the conventional octahedral coordination preference and propose a paradigm for optimizing the arrangement of π‐conjugated [B 3 O 6 ] rings by steering cation‐centered polyhedra toward triangular prismatic geometry. Guided by this design, we synthesize Na 2 CsCa 2 (B 3 O 6 ) 2 F (NCCBF), a new DUV NLO crystal that integrates a large second harmonic generation response (5.5 × KH 2 PO 4 (KDP)), a short ultraviolet (UV) cut‐off edge (180 nm), a suitable birefringence (0.090 @ 532 nm), and low refractive index dispersion. Remarkably, centimeter‐sized single crystals are grown via the top‐seeded‐solution‐growth technique, and the material achieves the shortest phase‐matchable wavelength among all known [B 3 O 6 ]‐based crystals. This work not only provides a promising candidate for high‐power DUV laser output but also establishes a new structural design principle, promoting octahedral distortion toward trigonal prismatic geometry for unlocking the full potential of π‐conjugated borate NLO crystals.