Borate composed of π- and/or non-π-conjugated boron oxyanions offers wide tunability of its optical and electrical properties. Modifying the properties of borates begins with the initial design of their anionic framework and cationic component. However, these component-level designs are only part of the story, as the way these anions and cations are arranged in the lattice determines the key properties of borates, with this heavily influenced by microelectronic arrangement in the lattice. For short-wave ultraviolet and deep ultraviolet borates, the main optical-active component is their anionic frameworks, wherein the π-conjugated [BO₃] units with large hyperpolarizability, polarizability anisotropy, and HOMO–LUMO gap stimulated by the delocalized π electrons on all adjacent arranged p orbitals are the star primitives to design optical crystals.

Structurally, the most common structure type for borates is the 0D anionic framework, wherein those with π-conjugated B–O clusters are the research hotspots for optical applications, although varied types of them show significantly different structural characteristics, especially when considering the arrangement between and among these 0D clusters. Like, there is no distinct rule in the interarrangement (arrangement between two clusters) distribution, and the maximum dihedral angle between two π-conjugated [BO₃] units can change from 0 to 90°, suggesting that the [BO₃] unit is relatively flexible and the regulation of [BO₃]’ arrangement is controllable. This is completely different in 0D borates exclusively composed of [B₃O₆] units (abbreviated this type of borates as 0D e-[B₃O₆] borates). Statistics show that in all the available 0D e-[B₃O₆] borates, more than 90% have the minimum dihedral angle of zero, wherein the [B₃O₆] units are strictly parallelly arranged. The remaining are in near-plane arrangement with a maximum dihedral angle less than 13°. This means that in 0D e-[B₃O₆] borates, the structural features for both inside and between the [B₃O₆] clusters are fixed into planar or near planar configuration, and this is not a bad thing for pursuing high birefringence, since such an arrangement can reduce large optical anisotropy. However for nonlinear optical (NLO) crystals, the ideal standard for the birefringence is “moderate” to avoid nonphase matching (too small) behavior and walk-off effects (too large). With this, our preliminary review-type paper raises an interesting question: why are all of the [B₃O₆] units in the borates with only such isolated units aligned in a coplanar arrangement? Since that moment, we have focused on how to regulate the arrangement of [B₃O₆] units to achieve moderate birefringence in borate-based NLO crystals.

Based on this, a research group led by Prof. Shilie Pan at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science, break the inherent interarrangement of [B₃O₆] clusters. In this work, two new alkaline earth metal borate halides Ca₂B₃O₆X (X = Cl and Br) were reported to have a nonparallel arrangement of [B₃O₆] clusters with the highest dihedral angle of about ～76° among 0D e-[B₃O₆] borates. It is confirmed that the confinement effects of deformed ¹[ClCa₂] chains violate the natural interarrangement of [B₃O₆] clusters by compressing the existence space of [B₃O₆] primitives. The Ca₂B₃O₆X series’ higher dihedral angles between [B₃O₆] clusters set the birefringence to the optimal level of “moderate,” resulting in a short walk-off angle. A model is established to intuitively study the relationship between the arrangement of [B₃O₆] and the interaction of groups in the system, and the free energy reflected by the orbital hybridization between the s orbital of the Ca cations and 2p orbital of terminal O atoms on [B₃O₆] is confirmed to determine the size of a dihedral angle. At the same time, the Ca₂B₃O₆X series exhibits large SHG responses of 1.5–2 × KDP. This research offers a practical method for rearranging [B₃O₆] clusters in order to overcome their natural interarrangement for nonlinear optics.

The paper was published on J. Am. Chem. Soc. with the title of “Breaking the Inherent Interarrangement of [B₃O₆] Clusters for Nonlinear Optics with Orbital Hybridization Enhancement”. This work was financially supported by National Key R&D Program of China, Young Elite Scientist Sponsorship Program by CAST, National Natural Science Foundation of China, and the West Light Foundation of CAS.

Author List: Haotian Qiu, Fuming Li, Zhi Li, Zhihua Yang, Shilie Pan*, and Miriding Mutailipu*

Figure: Interaction model between cations and the [B₃O₆] units.

Author List: Haotian Qiu, Fuming Li, Zhi Li, Zhihua Yang, Shilie Pan*, and Miriding Mutailipu*

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