SHG Materials SnGa4Q7 (Q = S, Se) Appearing with Large Conversion Efficiencies, High Damage Thresholds, and Wide Transparencies in the Mid-Infrared Region

Two new ternary noncentrosymmetric (NCS) isostructural compounds, SnGa4S7 (1) and SnGa4Se7 (2), have been synthesized by a high-temperature solid-state reaction method. Both compounds crystallize in the monoclinic polar space group Pc (7), and their three-dimensional (3D) frameworks are assembled from [SnQ4] tetragonal pyramids and tetranuclear [Ga4Q11] units. The two crystals show large second harmonic generation (SHG) conversion efficiencies, high laser-induced damage thresholds (LIDTs), and wide transparent regions in the mid-infrared (MIR) zone. Compound 2 exhibits the largest SHG conversion efficiency among type-I phase-matchable chalcogenides of about 3.8 × AgGaS2 at 2.05 μm radiation and a high LIDT of 4.6 × AgGaS2. Theoretical studies elucidate that the stereochemically active lone-pair (SCALP) electrons of Sn2+ can significantly improve the polarity of the [SnQ4] unit. Large nonlinear optical (NLO) responses for 1 and 2 originate from the covalent interactions of Sn–Q and the cooperative effects of polarities between the units [SnQ4] and [GaQ4]. Additionally, their comparatively simple chemical composition and high chemical stability give them promise for practical applications.本研究通过高温固相反应法成功合成了两种新型三元非中心对称（noncentrosymmetric, NCS）同构化合物：SnGa4S7（1）与SnGa4Se7（2）。两种化合物均结晶于单斜极空间群Pc（7号空间群），其三维（3D）骨架由[SnQ4]四方锥与四核[Ga4Q11]结构单元构筑而成。这两种晶体均表现出优异的二次谐波产生（second harmonic generation, SHG）转换效率、较高的激光损伤阈值（laser-induced damage thresholds, LIDTs）以及较宽的中红外（mid-infrared, MIR）透光波段。在Ⅰ型相位匹配硫系化合物中，化合物2的SHG转换效率最高，在2.05 μm激光辐射下其效率约为硫镓银（AgGaS2）的3.8倍，激光损伤阈值可达AgGaS2的4.6倍。理论研究表明，Sn²+的立体化学活性孤对电子（stereochemically active lone-pair, SCALP）可显著提升[SnQ4]结构单元的极性。两种化合物的强非线性光学（nonlinear optical, NLO）响应源于Sn-Q间的共价相互作用，以及[SnQ4]与[GaQ4]结构单元间的极性协同效应。此外，两种化合物的化学组成相对简单且化学稳定性优异，具备良好的实际应用前景。