AgGaSe₂‑Inspired Nonlinear Optical Materials: Tetrel Selenides of Alkali Metals and Mercury

High-performance infrared nonlinear optical (IR NLO) materials are crucial devices in tunable IR solid-state lasers, and the functional-group cosubstitution strategy was selected to design and explore outstanding IR NLO crystals. For that reason, taking the famous AgGaSe₂ as the template, five new mercury-based IR NLO selenides, Li₂HgMSe₄ and Na₂Hg₃M₂Se₈ (M = Si, Ge, Sn), were successfully designed and synthesized through concurrently replacing the cation (Ag⁺) and GaSe₄ unit with the alkali metal (Li⁺ or Na⁺) and anionic groups (HgSe₄ and MSe₄) to optimize crystal structures and performances. All of them exhibit extremely strong powder second-harmonic generation (SHG) responses (3.6–6.0 × commercial AgGaS₂) with the essential phase-matching behavior. Note that Li₂HgSnSe₄ exhibits the largest SHG response (6.0 × AgGaS₂) among the known Hg-based chalcogenides without disorder structures, and its millimeter-level single-crystals were successfully grown by the Bridgman method. Theoretical analysis further illustrates that the different arrangement modes of HgSe₄ units offer considerable but distinguishing SHG contributions, such as Li₂HgMSe₄ (53–55%) and Na₂Hg₃M₂Se₈ (19–23%). This research result highlights the practicability of the functional group cosubstitution-oriented design strategy and Hg-based selenides could be viewed as the optimal system for future exploration of large SHG crystals.