Series of SHG Materials Based on Lanthanide Borate–Acetate Mixed Anion Compounds

The
first examples of lanthanide borate–acetate mixed anion compounds,
namely, Ln2(CH3CO2)2[B5O9(OH)]·H2O (Ln = La 1; Ce 2; Pr 3), were synthesized under hydrothermal
conditions. These compounds are isostructural and crystallize in polar
space group Cc. 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 ac plane with large 9-member rings (MRs) which
are filled by lanthanide­(III) ions into a {Ln­[B5O9(OH)]}− 2D layer. Adjacent {Ln­[B5O9(OH)]}− layers are bridged by remaining
lanthanide (III) ions to form a 3D network of lanthanide borate. It
is noteworthy that Ln2(CH3CO2)2[B5O9(OH)]·H2O (Ln =
La 1; Ce 2; Pr 3) can be changed
into Ln2(CH3CO2)2[B5O9(OH)] (Ln = La 4; Ce 5; Pr 6) under heating at 500 K. Compounds 1–4 display moderate SHG signals of about 2.0,
1.0, 1.4, and 2.5 times that of KH2PO4, respectively,
and they are phase matchable. Their SHG responses mainly arise from
the synergistic polarization effects of both asymmetric borate clusters
and π-conjugated CH3COO– anions.