Fluorooxoborate Promoting the Exploration of Short-Wavelength Linear and Nonlinear Optical Crystals with Expected Properties and Versatile Structures

Short-wavelength optical crystals are the key components of optical devices, but only a few crystals can be used in practical applications. In recent years, the emergence of fluorooxoborate has expanded the source of optical crystals. Herein, a series of new fluorooxoborates, α -K 3 B 6 O 9 F 3, K 1.2 Rb 1.8 B 6 O 9 F 3, K 2.9 Cs 0.1 B 6 O 9 F 3, Rb 2 B 6 O 9 F(OH), KB 3 O 4 F 2, and K 5 B 11 O 18 F 2, were successfully synthesized in a closed system. Among them, α -K 3 B 6 O 9 F 3, K 1.2 Rb 1.8 B 6 O 9 F 3, K 2.9 Cs 0.1 B 6 O 9 F 3, and Rb 2 B 6 O 9 F(OH) are noncentrosymmetric and show evident second-harmonic generation response (0.7–0.8 × KH 2 PO 4 ). Specifically, Rb 2 B 6 O 9 F(OH) has a unique [B 6 O 11 F(OH)] fundamental building block composed of four different anionic units, and its shortest phase-matching wavelength is 244 nm based on theoretical calculations. The centrosymmetric KB 3 O 4 F 2 has a large birefringence (0.096 at 546 nm) due to its rare one-dimensional [B 3 O 4 F 2 ] ∞ chains, and K 5 B 11 O 18 F 2 has a unique [B 11 O 21 F 2 ] fundamental building block that polymerizes to form the 14-membered ring channels. These results facilitate the exploration of short-wavelength optical materials in fluorooxoborates.