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

Abstract

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.