Large second-harmonic generation (SHG) response and high laser-induced damage threshold (LIDT) are two crucial performances of outstanding nonlinear optical (NLO) crystal materials. Magnetic NLO materials, particularly those containing d-block metals, are an important subgroup of NLO materials and have large NLO efficiency due to d-involved NLO-active transitions.  

However, such type of compounds has been frequently neglected by researchers because d-block metals also have well-known d-d transitions that usually lead to strong optical absorptions and narrow band gap, which decrease LDT and seriously hinder their practical applications.  

In a study published in Materials Horizons, the research team led by Prof. GUO Guocong from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, achieved broad transparency and wide band gap in magnetic infrared (IR) NLO chalcogenide by suppressing d-d transitions. 

The researchers found that [K₃Cl][Mn₂Ga₆S₁₂] exhibits a rare nanoporous [MnGa₃S₆]^- framework with tunnels of inner diameter 9.0 Å, and the 1-D [K₃Cl]²⁺ chains constructed by face-sharing [K₆Cl]⁵⁺ octahedra was embedded in the tunnels.  

According to the Laporte selection rule, the d-d transitions of a magnetic ion are forbidden in a centrosymmetric (CS) local coordination. The magnetic ions Mn²⁺ (d⁵) in [K₃Cl][Mn₂Ga₆S₁₂] are octahedrally coordinated by six S^2- , leading to the forbidden of d-d transitions, alleviating the negative influence of d-d transition on transmission range and band gap. 

Although the second-order Jahn–Teller effect leads to the slight centrosymmetric breaking of the octahedral coordination of Mn²⁺ (d⁵), the d-d transitions of Mn²⁺ are effectively suppressed, achieving a broad transparency (0.39–25.0 mm) and the widest band gap (3.17 eV) among all magnetic IR NLO chalcogenides. 

In addition, the researchers found that [K₃Cl][Mn₂Ga₆S₁₂] exhibits a strong phase-matchable second-harmonic generation intensity of about 0.8 times that of AgGaS₂ at the incident laser radiation of 1910 nm. The LIDT estimated for the compound is 100.0 MW/cm² at 1064 nm, approximately 12.5 times that of AGS (8.0 MW/cm²).  

Theoretical calculation showed that the SHG contributions of Mn atoms (7.0%) are smaller than those of S (37.0%), Cl (16.9%), and Ga (36.7%) atoms but larger than that of K atoms (2.5%).  

Therefore, the SHG contributions of Mn atoms are maintained in [K₃Cl][Mn₂Ga₆S₁₂] with the condition that d-d transitions are greatly suppressed, which makes it fulfill the important criteria of a promising IR NLO candidate (NLO > 0.5×AgGaS₂, Eg > 3.0 eV).  

This study provides an avenue for designing magnetic NLO materials for practical applications.