Expanding the chemistry of borates with functional [BO2]\u2212 anions

Chunmei Huang,Kent J Griffith,Fangfang Zhang,Miriding Mutailipu,Kenneth R Poeppelmeier,Shilie Pan,Cong Hu,Zhihua Yang,John M Griffin

doi:10.1038/s41467-021-22835-4

Abstract

More than 3900 crystalline 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 motifs. Herein, a mixed metal K5Ba2(B10O17)2(BO2) with linear BO2 structural units was obtained, pushing the boundaries of structural diversity and providing a direct strategy toward the maximum thresholds of birefringence for optical materials design. 11B solid-state nuclear magnetic resonance (NMR) is a ubiquitous tool in the study of glasses and optical materials; here, density functional theory-based NMR crystallography guided the direct characterization of BO2 structural units. The full anisotropic shift and quadrupolar tensors of linear BO2 were extracted from K5Ba2(B10O17)2(BO2) containing BO2, BO3, and BO4 and serve as guides to the identification of this powerful moiety in future and, potentially, previously-characterized borate minerals, ceramics, and glasses.

Highlights

Results and discussionThe O atoms in isolated [BO2]− units are non-bridging atoms
More than 3900 crystalline borates, including borate minerals and synthetic inorganic borates, in addition to a wealth of industrially-important boron-containing glasses, have been discovered and characterized
The maximum thresholds of bandgap, birefringence, and nonlinear optical coefficient of A-borates comprising BO3 and BO4 units, are ~150 nm, ~0.07@1064 nm, and ~0.64 pm/V, respectively, assuming all the basic units are in optimally aligned configurations[18,27]

Summary

Results and discussion

The O atoms in isolated [BO2]− units are non-bridging atoms This low-frequency signal is better resolved at a high magnetic field (Supplementary Fig. 2a) and is resolved into three distinct sites (in 2:1:1 ratio) by multiple-quantum MAS (MQMAS) The overall MAS lineshapes could almost be reproduced by performing numerical simulations of the shift and quadrupolar parameters of the four BO4 and six BO3 sites directly from DFT calculations of the K5Ba2(B10O17)2(BO2) XRD structure model. After calculating the overall lineshape from BO3 and BO4 contributions, a small feature resembling a powder ‘horn’ line-shape was unaccounted for, which is most distinct at –12 ppm in the low-field MAS data where it does not overlap with other boron sites. Allowing the lineshape to be refined against the experimental data yielded an isotropic shift of 13.5(3) ppm, a reduced CSA (Haeberlen convention, see “Methods”) of –123(3) ppm, a quadrupolar coupling constant of 3.31(5) MHz, asymmetries near zero, and coincident shielding and quadrupolar tensors aligned along O–B–O

B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1

G Z RX G

Methods