Experimental and theoretical studies on the NLO properties of two quaternary non-centrosymmetric chalcogenides: BaAg2GeS4 and BaAg2SnS4

New middle and far-infrared (MFIR) nonlinear optical (NLO) chalcogenides have been receiving increasing attention for their great importance in military and civil fields. In addition, the current challenge in the efforts for identifying a promising MFIR NLO material lies in achieving simultaneously large second-harmonic generation (SHG) intensity and high laser-induced damage threshold (LIDT) in the same material. In this study, two quaternary non-centrosymmetric (NCS) sulfides, BaAg2GeS4 (1) and BaAg2SnS4 (2), were synthesized from a high-temperature solid-state reaction using BaCl2 flux in evacuated closed silica tubes. Although 1 and 2 show identical stoichiometry, they crystallize in different NCS space groups, tetragonal I $(4) over bar $ 2m (no. 121) and orthorhombic I222 (no. 23), respectively, based on the results of crystal structure solution. In their structures, highly distorted AgS4 tetrahedra interconnect together via corner-sharing to form two-dimensional (2D) layers, which are further bridged with isolated GeS4 or SnS4 units to produce a three-dimensional (3D) framework structure with Ba cations lying in the tunnels. Remarkably, they not only possess phase-matchable (PM) abilities but also exhibit a good balance between strong SHG responses (1.7x and 0.4x AgGaS2) and high LIDTs (3.2x and 1.5x AgGaS2). Moreover, theoretical calculations based on density functional theory (DFT) methods have aided the understanding of energy bands, electronic structures, and linear and NLO properties.