From AgGaS2 to AgHgPS4: vacancy defects and highly distorted HgS4 tetrahedra double-induced remarkable second-harmonic generation response

Metal chalcogenides perform well as mid-infrared nonlinear optical (NLO) materials, especially those with diamond-like (DL) structural features. Starting from the classical AgGaS2 (AGS), we propose a design strategy to enhance the second-harmonic generation (SHG) response by introducing vacancy defects and highly distorted NLO-active units. Hence, a new quaternary chalcogenide AgHgPS4 with a defect DL structure, crystallizing in the non-centrosymmetric monoclinic space group Pn, is successfully synthesized by the substitution of GaS4 units with HgS4 and PS4 units. Detailed experimental results verify that AgHgPS4 exhibits a much stronger SHG response (5.09 × AGS, 2.63 eV) without sacrificing the optical band gap compared with AGS. Moreover, the cation vacancy defects and highly distorted HgS4 units are identified as the crucial driving forces of the large NLO response by partial charge density distributions and dipole moment calculations.