An ideal infrared (IR) nonlinear optical (NLO) crystal must have the advantages of a wide transmittance range, impressive laser-induced damage threshold (LIDT), sufficient birefringence index, bulk single-crystal form, and physicochemical stability.

However, there is often a trade-off between a strong NLO coefficient and a wide band gap toward high LIDT, making it challenging to achieve both properties in a single material. 

Recently, a research group led by Prof. GUO Guocong from the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, reported two novel non-centrosymmetric chalcogenides: M[M₄Cl][Ga₁₁S₂₀] (M = A/Ba, A = K, Rb) as excellent nonlinear optical crystals. 

The study was published in Small on Sept. 11.

M[M₄Cl][Ga₁₁S₂₀] (M = A/Ba, A = K, Rb) represent the first examples achieved by a cationic substitution strategy, resulting in salt-inclusion chalcogenides with diamond-like anionic frameworks. 

The researchers used to consider typical diamond-like chalcogenides as promising candidates for IR NLO materials; however, they often exhibit limited LIDTs due to their narrow band gaps. 

In this study, the researchers employed an unconventional cationic substitution strategy, [[SZn₄]S₁₂ + [S₄Zn₁₃]S₂₄ + 11ZnS₄ => MS₁₂+ [M₄Cl]S₂₄ + 11GaS₄], to create two novel salt-inclusion sulfides, M[M₄Cl][Ga₁₁S₂₀] (M = A/Ba, A = K, Rb). As anticipated, the introduction of mixed cations in the GaS₄ anionic frameworks resulted in wide band gaps (3.04 and 3.01 eV) and improved high LIDTs (9.4 and 10.3 × AgGaS₂@1.06 μm). 

Furthermore, the researchers found that the ordered arrangement of tetrahedral GaS₄ units favored strong second-harmonic generation intensities (0.84 and 0.78 × AgGaS₂@2.9 μm). 

This study is an example of employing a cationic substitution strategy based on diamond-like structures to create high-performance NLO materials.