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.