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-K3B6O9F3, K1.2Rb1.8B6O9F3, K2.9Cs0.1B6O9F3, Rb2B6O9F(OH), KB3O4F2, and K5B11O18F2, were successfully synthesized in a closed
system. Among them, α-K3B6O9F3, K1.2Rb1.8B6O9F3, K2.9Cs0.1B6O9F3, and Rb2B6O9F(OH) are noncentrosymmetric and show evident
second-harmonic generation response (0.7–0.8 × KH2PO4). Specifically, Rb2B6O9F(OH) has a unique [B6O11F(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 KB3O4F2 has a large birefringence (0.096 at 546 nm) due to its rare
one-dimensional [B3O4F2]∞ chains, and K5B11O18F2 has a unique [B11O21F2] fundamental
building block that polymerizes to form the 14-membered ring channels.
These results facilitate the exploration of short-wavelength optical
materials in fluorooxoborates.