LiPb₃GeS₄Cl₃: Quasi-T₂-Supertetrahedral 3D Framework Enabling Superior Optical Performance

The anisotropic structure building unit involving diverse chemical bond (ABUCB) has been proven effective in exploring high-performance nonlinear optical (NLO) materials, through simultaneously enhancing second harmonic generation (SHG) and birefringence (∆n). Building on this concept, we propose a strategy that employs different types of primary tetrahedra to construct anisotropic supertetrahedron (T₂). As a demonstration, we report a novel compound assembled by a quasi-T₂-supertetrahedron, LiPb₃GeS₄Cl₃ (LPGSC), exhibiting the superior optical properties among known thiogermanate halides and supertetrahedron-chalcogenides. In the structure, there are two crystallographically independent Pb sites: Pb(1) forms the heterometallic T₂-supertetrahedron [GePb(1)₃S₈], while Pb(2) occupies an interstitial site with a labile coordination environment. As demonstrated by a series of samples Li_xPb_{3.5- x /2}GeS₄Cl₃ (L_xPGSC, x = 1‒0), the Pb(2) site can be substituted by lithium. Together with theoretical studies, we demonstrate that lithium incorporation effectively activates non-bonding states (Pb-6s², S-3p, Cl-3p) near the Fermi level. This activation significantly enhances the second-order NLO susceptibility (LPGSC: d₃₃ = 36.01 vs. AGS: d₃₆ = 18.67 pm V^-1) and polarity anisotropy (LPGSC: ∆n_cal. = 0.09 vs. AGS: 0.03 at 546 nm).