Series of SHG Materials Based on Lanthanide Borate–Acetate
Mixed Anion Compounds
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Abstract
The
first examples of lanthanide borate–acetate mixed anion compounds,
namely, Ln<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub>[B<sub>5</sub>O<sub>9</sub>(OH)]·H<sub>2</sub>O (Ln = La <b>1</b>; Ce <b>2</b>; Pr <b>3</b>), were synthesized under hydrothermal
conditions. These compounds are isostructural and crystallize in polar
space group <i>Cc</i>. They display a unique three-dimensional
(3D) framework built by a 3D network of lanthanide borate further
decorated by acetate anions. The borate anion exhibits a 2D layer
in the <i>ac</i> plane with large 9-member rings (MRs) which
are filled by lanthanide(III) ions into a {Ln[B<sub>5</sub>O<sub>9</sub>(OH)]}<sup>−</sup> 2D layer. Adjacent {Ln[B<sub>5</sub>O<sub>9</sub>(OH)]}<sup>−</sup> layers are bridged by remaining
lanthanide (III) ions to form a 3D network of lanthanide borate. It
is noteworthy that Ln<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub>[B<sub>5</sub>O<sub>9</sub>(OH)]·H<sub>2</sub>O (Ln =
La <b>1</b>; Ce <b>2</b>; Pr <b>3</b>) can be changed
into Ln<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub>[B<sub>5</sub>O<sub>9</sub>(OH)] (Ln = La <b>4</b>; Ce <b>5</b>; Pr <b>6</b>) under heating at 500 K. Compounds <b>1</b>–<b>4</b> display moderate SHG signals of about 2.0,
1.0, 1.4, and 2.5 times that of KH<sub>2</sub>PO<sub>4</sub>, respectively,
and they are phase matchable. Their SHG responses mainly arise from
the synergistic polarization effects of both asymmetric borate clusters
and π-conjugated CH<sub>3</sub>COO<sup>–</sup> anions
