Noncentrosymmetric versus Centrosymmetric: Influence of the Na<sup>+</sup> Substitution on Structural Transition and Second-Harmonic Generation Property

A series of K<sub>3–<i>x</i></sub>Na<sub><i>x</i></sub>­B<sub>6</sub>O<sub>10</sub>Br (0.5 ≤ <i>x</i> ≤ 1.2) polycrystalline samples have been synthesized by the standard solid-state reaction method. Four stoichiometric crystals K<sub>3–<i>x</i></sub>Na<sub><i>x</i></sub>­B<sub>6</sub>O<sub>10</sub>Br (<i>x</i> = 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<sup>+</sup> concentration plays a profound role to influence the crystal structure. Up to about 23% (<i>x</i> = 0.7) K<sup>+</sup> ions can be substituted by Na<sup>+</sup> ions with the same noncentrosymmetric (NCS) crystal structure of K<sub>3</sub>­B<sub>6</sub>O<sub>10</sub>Br (space group <i>R</i>3<i>m</i>) being retained, while a higher Na concentration would lead to it crystallizing in the centrosymmetric (CS) space group <i>Pnma</i>. Meanwhile, the second-harmonic generation (SHG) response of K<sub>3–<i>x</i></sub>Na<sub><i>x</i></sub>­B<sub>6</sub>O<sub>10</sub>Br (<i>x</i> ≤ 0.7) is about 2.8 times that of KDP, while the SHG response decreases sharply when <i>x</i> > 0.7 (the SHG response is zero at <i>x</i> = 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.