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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.
Qiu et al. (Tue,) studied this question.