KHg ₄ Ga ₅ S ₁₂ : A Diamond-like Tetrahedral Chalcogenide Exhibiting Giant Phase-Matching Second Harmonic Generation

Tetrahedra-based chalcogenides constitute the most abundant type for mid-infrared (IR) nonlinear optical (NLO) crystals. Meanwhile, different connection modes of tetrahedra will determine the mid-IR NLO properties, such as birefringence (Δ n ) and the second harmonic generation (SHG) response. In this research, we systematically investigate the structure–property modulated relationship between tetrahedra-based compounds and mid-IR NLO properties. Herein, three compounds, KHg 4 Ga 5 S 12, K 2 CdSi 4 S 10, and KCd 4 Ga 3 S 9, consisting of the tetrahedral units, have been successfully synthesized through a moderate-temperature solid-state reaction. Specifically, K 2 CdSi 4 S 10 and KCd 4 Ga 3 S 9 with the opposite polarity T2-supertetrahedra and random orientation helix tetrahedral chains exhibit a small Δ n and non-phase-matching (non-PM) SHG response. Notably, KHg 4 Ga 5 S 12 with the distorted diamond-like (DL) structure shows a large PM SHG response of 3.5 × AgGaS 2 (AGS), moderate Δ n of 0.042@2050 nm, and high laser-induced damage threshold (LIDT) of 4.1 × AGS. Therefore, the results indicate that KHg 4 Ga 5 S 12 has potential for application as a high-performance mid-IR NLO crystal. The consistent arrangement and distorted DL structure can be prioritized for the design of mid-IR NLO crystals.