Effect of Rigid Units on the Symmetry of the Framework:
Design and Synthesis of Centrosymmetric NaBa<sub>4</sub>(B<sub>5</sub>O<sub>9</sub>)<sub>2</sub>F<sub>2</sub>Cl and Noncentrosymmetric
NaBa<sub>4</sub>(AlB<sub>4</sub>O<sub>9</sub>)<sub>2</sub>Br<sub>3</sub>
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Abstract
Two similarly stoichiometric borate
halides, NaBa<sub>4</sub>(AlB<sub>4</sub>O<sub>9</sub>)<sub>2</sub>Br<sub>3</sub> and NaBa<sub>4</sub>(B<sub>5</sub>O<sub>9</sub>)<sub>2</sub>F<sub>2</sub>Cl, have been successfully designed
and synthesized, and their structures were determined by single-crystal
X-ray diffraction. Their crystal structures feature the [AlB<sub>4</sub>O<sub>9</sub>]<sub>∞</sub> and [B<sub>5</sub>O<sub>9</sub>]<sub>∞</sub> networks, respectively. NaBa<sub>4</sub>(AlB<sub>4</sub>O<sub>9</sub>)<sub>2</sub>Br<sub>3</sub> is noncentrosymmetric
and crystallizes in polar space group <i>P</i>4<sub>2</sub><i>nm</i>, while NaBa<sub>4</sub>(B<sub>5</sub>O<sub>9</sub>)<sub>2</sub>F<sub>2</sub>Cl is centrosymmetric and crystallizes
in monoclinic space group <i>P</i>2<sub>1</sub>/<i>n</i>. Powder second-harmonic generation (SHG) measurements
reveal that NaBa<sub>4</sub>(AlB<sub>4</sub>O<sub>9</sub>)<sub>2</sub>Br<sub>3</sub> has an optical nonlinearity comparable to that of
KH<sub>2</sub>PO<sub>4</sub> (KDP) and is type I phase-matchable.
In addition, infrared and UV–Vis–NIR diffuse reflectance
spectroscopy, as well as electronic band structure calculations, were
performed on the reported materials
