(NH 4 ) 3 B 11 PO 19 F 3 : a deep-UV nonlinear optical crystal with unique [B 5 PO 10 F] ∞ layers.

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals that can extend the output range of coherent light below 200 nm are pivotal materials for solid-state lasers. To date, KBe 2 BO 3 F 2 (KBBF) is the only usable crystal that can generate DUV coherent light by direct second harmonic generation (SHG), but the layered growth habit and toxic ingredients limit its application. Herein, we report a new fluoroborophosphate, (NH 4 ) 3 B 11 PO 19 F 3 (ABPF), containing four different functional units: [BO 3 ], [BO 4 ], [BO 3 F] and [PO 4 ]. ABPF exhibits a KBBF-like structure while eliminating the limitations of KBBF crystal. The unique [B 5 PO 10 F] ∞ layers enhance ABPF's performance; for example, it has a large SHG response (1.2 × KDP) and a sufficient birefringence (0.088 at 1064 nm) that enables the shortest phase-matching wavelength to reach the DUV region. Meanwhile, the introduction of strong B-O-P covalent bonds decreases the layered growth habit. These findings will enrich the structural chemistry of fluoroborophosphate and contribute to the discovery of more excellent DUV NLO crystals.