American Chemical Society
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Noncentrosymmetric versus Centrosymmetric: Influence of the Na+ Substitution on Structural Transition and Second-Harmonic Generation Property

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posted on 2014-04-02, 00:00 authored by Shujuan Han, Ying Wang, Shilie Pan, Xiaoyu Dong, Hongping Wu, Jian Han, Yun Yang, Hongwei Yu, Chunyan Bai
A series of K3–xNax­B6O10Br (0.5 ≤ x ≤ 1.2) polycrystalline samples have been synthesized by the standard solid-state reaction method. Four stoichiometric crystals K3–xNax­B6O10Br (x = 0.13, 0.67, 1.30, 2.20) have been successfully grown from the high-temperature solution, and the crystal structures were determined by single-crystal X-ray diffraction. Interestingly, the Na+ concentration plays a profound role to influence the crystal structure. Up to about 23% (x = 0.7) K+ ions can be substituted by Na+ ions with the same noncentrosymmetric (NCS) crystal structure of K3­B6O10Br (space group R3m) being retained, while a higher Na concentration would lead to it crystallizing in the centrosymmetric (CS) space group Pnma. Meanwhile, the second-harmonic generation (SHG) response of K3–xNax­B6O10Br (x ≤ 0.7) is about 2.8 times that of KDP, while the SHG response decreases sharply when x > 0.7 (the SHG response is zero at x = 1.3). After careful structural analysis, we believe that the different Br-M (M = K/Na, K, or Na) lattices, which are influenced by the coordination environments of the cations, are responsible for the structural changes from NCS to CS.