Breaking the Inherent Interarrangement of [B3O6] Clusters for Nonlinear Optics with Orbital Hybridization Enhancement

The [B₃O₆] group as a prime functional unit provides borates with intrinsic properties that are modified by coordination to cations. Inherent [B₃O₆] cluster structures in borates exclusively made of them have a near-plane configuration, with more than 90% of them having a maximum dihedral angle of zero and the remaining ones being less than 13°. Although such an inherent configuration can produce considerable birefringence for good phase-matching ability, this is not conducive to obtaining high conversion efficiency and beam quality due to the walk-off effects in the nonlinear optical process. In this article, two new borate halides Ca₂B₃O₆X (X = Cl and Br) were reported, in which the confinement effects of distorted halogen-centered secondary building blocks compress the existence space of [B₃O₆] primitives, resulting in the nonparallel arrangement between [B₃O₆] clusters in this series. Both compounds show large second harmonic generation effects, and more importantly, the broken inherent interarrangement of [B₃O₆] clusters makes them a moderate birefringence and small walk-off angle. Their moderate birefringence is due to the large angular alignment between [B₃O₆] clusters, resulting from the orbital hybridization between the Ca s and the O p orbitals of the terminal O atoms on [B₃O₆] clusters. Our model supports this viewpoint and offers guidelines for rearranging [B₃O₆] clusters' arrangements in borates.