Borates are in the spotlight of research fields owing to their continuous extended structural diversity and diverse applications in the design of photonic and optoelectronic devices, nuclear waste separation and sequestration, commercial glasses, amorphous oxide catalysts, as well as Li-ion and Mg-ion rechargeable batteries. Consequently, considerable effort has been put toward the search for borates with distinctive atomic structures and properties. Therefore, more than 3900 borates, including borate minerals and synthetic inorganic borates, in addition to a wealth of industrially-important boron-containing glasses, have been discovered and characterized. Of these compounds, 99.9 % contain only the traditional triangular BO₃ and tetrahedral BO₄ units, which polymerize into superstructural units. There is an absence of studies of borate-based optical materials with BO₂ units. Structurally, the realization of borates with the BO₂ functionality will dramatically enrich the structural diversity of this technologically-important family of materials and offer a path toward improving the three critical optical parameters in borates. This basis stimulates the exploration of borates with BO₂ configurations and routes toward a more fundamental understanding of the origin of this unique building unit.

Recently, Prof. Shilie Pan (Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science) and Prof. Kenneth R. Poeppelmeier (Northwestern University) have successfully obtained a new borate with linear BO₂ structural units. K₅Ba₂(B₁₀O₁₇)₂(BO₂) and its BO₂ anions push the boundaries of structural diversity and provide a direct strategy toward the maximum thresholds of birefringence for optical materials design. The ¹¹B solid-state Nuclear Magnetic Resonance (NMR) is a ubiquitous tool; here, density functional theory-based NMR crystallography guided the characterization of BO₂ structural units in any material. The full anisotropic shift tensor and quadrupolar tensor of linear BO₂ were extracted from K₅Ba₂(B₁₀O₁₇)₂(BO₂) containing BO₂, BO₃, and BO₄ and serve as a guide to the identification of this powerful moiety in future and, potentially, previously-characterized borate minerals, ceramics, and glasses. The study was published in Nature Communications (Nature Communications, 2021, 12, 2597).

This work was financially supported by National Natural Science Foundation of China, National Key Research Project, and Xinjiang Key Research and Development Program.

https://www.nature.com/articles/s41467-021-22835-4