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

A series of K_3–<i>x</i>Na_<i>x</i>­B₆O₁₀Br (0.5 ≤ <i>x</i> ≤ 1.2) polycrystalline samples have been synthesized by the standard solid-state reaction method. Four stoichiometric crystals K_3–<i>x</i>Na_<i>x</i>­B₆O₁₀Br (<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⁺ concentration plays a profound role to influence the crystal structure. Up to about 23% (<i>x</i> = 0.7) K⁺ ions can be substituted by Na⁺ ions with the same noncentrosymmetric (NCS) crystal structure of K₃­B₆O₁₀Br (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_3–<i>x</i>Na_<i>x</i>­B₆O₁₀Br (<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