LiPb ₃ GeS ₄ Cl ₃ : Quasi‐T ₂ ‐Supertetrahedral 3D Framework Enabling Superior Optical Performance

Abstract 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 (T 2 ). As a demonstration, we report a novel compound assembled by a quasi‐T 2 ‐supertetrahedron, LiPb 3 GeS 4 Cl 3 (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 T 2 ‐supertetrahedron [GePb(1) 3 S 8 ], while Pb(2) occupies an interstitial site with a labile coordination environment. As demonstrated by a series of samples Li x Pb 3.5‐ x /2 GeS 4 Cl 3 (L x PGSC, 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‐6 s 2 , S‐3 p , Cl‐3 p ) near the Fermi level. This activation significantly enhances the second‐order NLO susceptibility (LPGSC: d 33 = 36.01 vs. AGS: d 36 = 18.67 pm V −1 ) and polarity anisotropy (LPGSC: ∆n cal. = 0.09 vs. AGS: 0.03 at 546 nm).