Nonlinear optical (NLO) crystals possess the significant frequency conversion capability. Noncentrosymmetry (NCS) is a prerequisite for second-order NLO materials. How to rationally design NCS structures is still a challenging task. 

In the study published in J. Am. Chem. Soc., a group led by Prof. GUO Guocong at Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences, reported two novel NCS salt-inclusion chalcogenides: [ABa₂Cl][Ga₄S₈] (A = Rb, Cs), which are the first examples achieved through polycation-substitution-induced centrosymmetry (CS)-to-NCS transformation and NLO-functional motif ordering. 

The researchers constructed the [Ga₄S₈]^4- layers in RbGaS₂ by apex-sharing T2-supertetrahedra Ga₄S₁₀. Although the [Ga₄S₈]^4- unit is NLO-active, RbGaS₂ cannot produce second harmonic generation (SHG) efficiency because the neighboring [Ga₄S₈]^4- layers in its structure stack in a back-to-back style, which results in a CS space group of C2/c and cancels out their hyperpolarizabilities. 

Therefore, the researchers succeeded in replacing the Rb⁺ in RbGaS₂ with acentric polycation [ClA₂Ba₃]⁷⁺ by using salt-inclusion synthesis method, affording two new NCS sulfides, [ABa₂Cl][Ga₄S₈] (A = Rb, Cs).  

They unveiled that the orderly arrangement of NLO-active T2-supertetrahedral Ga₄S₁₀ motifs resulting from the template effect of polycation [ClA₂Ba₃]⁷⁺ is responsible for the remarkable SHG intensities (10.4–15.3× KH₂PO₄ (KDP) at 1064 nm; 0.9–1.0×AgGaS₂ at 1910 nm).  

Those above-mentioned experimental results together with high laser-induced damage thresholds (11–12×AgGaS₂), wide transparent window (0.4-12.3 μm), and phase-matchable behavior, indicate that [ABa₂Cl][Ga₄S₈] (A = Rb, Cs) are promising wide-spectrum NLO materials used in both Vis and IR regions. 

This study provides an effective approach for designing new NLO materials.

Schematic illustration of reconstructing NLO-functional motif in a parallel fashion via polycation-substitution-induced CS-to-NCS transformation strategy (Image by Prof. GUO’s group)