【摘要】 红外非线性光学（IR NLO）晶体作为实现激光波长调谐的关键材料，在光通信、大气监测与遥感等领域发挥着至关重要的作用。然而，商业化红外非线性光学晶体材料，如AgGaS₂、ZnGeP₂等，普遍存在激光损伤阈值偏低或双光子吸收显著等问题，这在一定程度上限制了它们的进一步应用。基于此，本团队提出“孤对电子功能化超四面体”的设计策略，并采用高温固相法成功合成了新型红外非线性光学化合物LiPb₃GeS₄Br₃。该晶体属于六方晶系，空间群为P6₃，具有非中心对称结构。光学性能测试显示：相对于AgGaS₂（AGS）,LiPb₃GeS₄Br₃展现出更宽的光学带隙（2.66 eV vs. 2.58 eV）、更大的透光范围（0.43～21.8μm vs. 0.5～13.5μm）；在2100 nm激光激发下，其倍频强度达到AGS的1.2倍，激光损伤阈值为AGS的3.3倍。第一性原理计算进一步证实，其宏观光学性能主要源于孤对电子功能化的杂化超四面体[GePb₃S₈]与[LiBr₆]八面体的协同作用。本研究为设计高性能红外非线性光学晶体提供了新思路。更多还原

【Abstract】 Objective Infrared nonlinear optical（IR NLO） crystals are indispensable for laser frequency conversion in fields such as optical communications, atmospheric monitoring, and remote sensing. However, the practical application of commercial IR NLO materials, including AgGaS₂（AGS） and ZnGeP₂, is severely constrained by inherent shortcomings such as low laser-induced damage thresholds（LIDT） and pronounced two-photon absorption. To overcome these limitations and advance the development of high-performance IR NLO candidates, we introduce an efficient “lone-pair electron functionalized supertetrahedron” design strategy. Guided by this approach, an IR NLO compound, LiPb₃ GeS₄ Br₃, has been successfully synthesized via a high-temperature solid-state method. This compound crystallizes in the non-centrosymmetric hexagonal space group P6₃. Optical performance evaluations demonstrate that LiPb₃ GeS₄ Br₃ exhibits comprehensively superior properties relative to the benchmark material AGS. Notably, LiPb₃ GeS₄ Br₃ possesses a wider bandgap（2.66 eV vs. 2.58 eV）, an extended transmission window（0.43-21.8 μm vs. 0.5-13.5 μm）, a phasematchable second harmonic generation（SHG） response at 2100 nm with an intensity 1.2 times that of AGS, and a significantly enhanced LIDT, measured to be 3.3 times higher than that of AGS. First-principles calculations attribute these outstanding macroscopic optical properties to the synergistic effects between the lone-pair electron functionalized hybrid supertetrahedron [GePb₃S₈] and the [LiBr₆] polyhedron. This work not only presents a high-performance IR NLO material but also validates a general and effective design paradigm for the development of new excellent NLO crystals.Methods LiPb₃ GeS₄ Br₃ was synthesized via a high-temperature solid-state reaction. Stoichiometric mixtures of LiBr（99%）, PbBr₂（99.9%）, GeS₂（99%）, and PbS（99.9%） were weighed in a 1∶1∶1∶2 molar ratio in an argon-filled glovebox, mixed thoroughly, and sealed in an evacuated silica tube. The tube was heated at 375 ℃ for 20 h and then cooled to room temperature to obtain polycrystalline powder. For single-crystal growth, the reactant ratio was adjusted to 3∶3∶1∶2, sealed under vacuum, heated to 400 ℃ for 24 h, slowly cooled at 1 ℃/h to 325 ℃, and finally furnace-cooled to the yield yellow single crystal Li_0.8Pb_3.1GeS₄ Br₃. Optical spectra were recorded using UV-Vis-NIR diffuse reflectance（200-2000 nm） and FT-IR(400-4000 cm^-1) spectroscopy. Thermal analysis was carried out under N₂ from room temperature to 800 ℃ at a heating rate of 10 ℃/min. The second harmonic generation（SHG） response was evaluated via the Kurtz-Perry method using a 2100 nm Nd∶YAG laser with AGS as a reference. The laser-induced damage threshold（LIDT） was measured with a 1064 nm Q-switched Nd∶YAG laser. First-principles calculations based on density functional theory（DFT） were performed using VASP software packages with the pseudopotential method to investigate the electronic structure and linear/nonlinear optical properties of LiPb₃ GeS₄ Br₃.Results and Discussions Powder X-ray diffraction refinement analysis reveals that LiPb₃ GeS₄ Br₃ crystallizes in the noncentrosymmetric hexagonal space group P6₃, featuring a three-dimensional [ framework interconnected with onedimensional [ LiBr₃]^2-_∞ chains along the c-axis. The framework is constructed by hybrid supertetrahedron [GePb₃S₈] units exhibiting C₃ symmetry, connected via corner-sharing along the c-axis and edge-sharing within the ab-plane. Theoretical calculations demonstrate that the [GePb₃S₈] unit exhibits remarkable enhancement in polarizability anisotropy and second-order hyperpolarizability, measuring 6.5 and 2.9 times higher than the reference [HgGa₃S₁₀] unit, respectively. This significant improvement stems from the incorporation of Pb²⁺ with stereochemically active lone-pair electrons, which alters the tetrahedral connectivity from cornersharing in T_d-symmetric [HgGa₃S₁₀] to edge-sharing in C₃-symmetric [GePb₃S₈], effectively reducing structural symmetry and enhancing optical anisotropy. Comprehensive characterization confirms exceptional material properties. Rietveld refinement of powder XRD patterns validates phase purity with good agreement factors(R_wp=5.02%, R_p=3.34%). EDS confirmes the elements present, and the spectrum shows a wide transmission range of 0.43-21.8 μm. With a bandgap of 2.66 eV, it demonstrates phase-matchable second harmonic generation at 2100 nm with intensity 1.2 times that of AGS. These findings collectively validate the “lone-pair electron functionalized supertetrahedron” strategy as a highly effective approach for developing high-performance infrared nonlinear optical crystals, achieving simultaneous enhancement of multiple optical properties through rational structural design. First-principles calculations on LiPb₃ GeS₄ Br₃ reveal that its optical properties are primarily governed by the synergistic interplay between the hybrid supertetrahedron [GePb₃S₈] and the [LiBr₆] structural units. The calculations further identify a localized asymmetric electronic environment, generated by the combined effect of non-bonding orbitals and the stereochemically active lone-pair electron on Pb²⁺. This specific electronic structure is shown to contribute to the enhancement of the material’s second-harmonic generation response.Conclusions This study successfully demonstrates the “lone-pair electron functionalized supertetrahedron” strategy by synthesizing LiPb₃ GeS₄ Br₃ through a high-temperature solid-state reaction. The crystal adopts a non-centrosymmetric structure in the P6₃ space group, constructed by hybrid [GePb₃S₈] supertetrahedron and [LiBr₆] octahedra forming a three-dimensional network. Theoretical calculations confirm that the optical properties originate predominantly from the [GePb₃S₈] unit with synergistic contributions from the [LiBr₆] polyhedra. The wave function distribution at CBM and VBM, and ELF verify the presence of non-bonding S 3p/Br 4p orbitals and a localized asymmetric electronic environment induced by the stereochemically active lone-pair electron on Pb²⁺, which collectively enhance the second-harmonic generation response. The compound exhibits outstanding comprehensive optical properties, including a wide bandgap（2.66 eV）, moderate birefringence（0.04@546 nm）, phase-matchable SHG output at 2100 nm（1.2 times that of AGS）, a broad transmission range（0.43-21.8 μm）, and a significantly improved laser damage threshold（3.3 times that of AGS）. This work validates the effectiveness of the “lone-pair electron functionalized supertetrahedron” approach and provides valuable guidance for designing high-performance infrared nonlinear optical materials.更多还原