In crystal engineering, modification of the crystal structure by cation substitution is one of the most important and efficient methods to obtain new structures with different properties. It is well known that cations with similar ionic radius or coordinate environment are easily to replace each other without changing the host crystal structures.  

In order to design and discover new nonlinear optical（NLO）crystals, approaches by cations substitution are often adopted. A group of researchers lead by professor PAN Shilie from Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences (XTIPC), designed and synthesized four stoichiometric crystals,  K_3-xNa_xB₆O₁₀ (x=0.13, 0.67, 1.30, 2.20), by introducing Na cations into K₃B₆O₁₀Br. Interestingly, they have the similar formula type of K_3-xNa_xB₆O₁₀Br, however, they crystallize in a different space group, R3m and Pnma for K_3-xNa_xB₆O₁₀Br (x=0.13, 0.67) and K_3-xNa_xB₆O₁₀Br (x=1.30, 2.20), respectively (Fig.1). The second harmonic generation (SHG) response of K_3-xNa_xB₆O₁₀Br (x=0.13, 0.67) is about 2.8 times of that of KDP (Fig.2). 

 

Fig.1 The 3D framework of K_3-xNa_xB₆O₁₀Br (x=0.13, 1.3) 

 

Fig.2 Phase-matching, i.e., particle size vs. SHG intensity, datas for K_2.87Na_0.13B₆O₁₀Br, K_2.33Na_0.67B₆O₁₀Br, K₃B₆O₁₀Br and KDP  (Image by XTIPC)

Detailed structural analyses suggest that the coordination environments of cations play critical roles to determine the symmetry of the Br-M  (M=K/Na, K, or Na) lattices, which influences the overall structure of the crystal. 

The research results have been published in the Cryst. Growth Des., 2014.This work is supported by the “National Natural Science Foundation of China”, the High-level Professional and Technical Personnel of Autonomous region, etc.