High-performance infrared (IR) nonlinear optical (NLO) materials with large NLO response and suitable birefringence are urgently needed for various applications. A Hg-based IR NLO material KHg₄Ga₅Se₁₂ with such desirable properties has been newly discovered. In the structure, obviously distorted HgSe₄ and GaSe₄ tetrahedra are connected to each other by vertex-sharing to form a three-dimensional framework with the counterion K⁺ residing in the cavities. Remarkably, all the NLO-active building units in the title compound are arranged in a completely parallel manner. Such a topological structure and the large susceptibility of the Hg–Se bonds enable the material to achieve good phase-matchability with a tremendous powder second harmonic generation (SHG) response at 2.09 μm that is about 20-times that of the benchmark material AgGaS₂ (one of the largest responses among all the phase-matchable IR NLO chalcogenides reported to date). The optical band gap of KHg₄Ga₅Se₁₂ was determined as 1.61 eV. Moreover, on the basis of the electronic band structure, the real-space atom-cutting analysis, the SHG-weighted electronic densities, and the local dipole moments calculations, the origin of the superior linear and nonlinear optical properties of the title compound is ascribed to the (Hg/Ga)Se₄ group. The calculated values for the maximum coefficient d₃₃ and birefringence (Δn) at 2.09 μm are −65.257 pm/V and 0.072, respectively. Such values agree well with experimental observations. Our study demonstrates that Hg-based metal chalcogenides are a class of IR NLO material with competitive features (good phase-matchability, very large SHG efficiency, wide transparency) desirable for practical applications.

中文翻译：

---

基于汞的红外非线性光学材料KHg ₄ Ga ₅ Se _12具有良好的相匹配性和优异的二次谐波产生响应

迫切需要具有大NLO响应和适当双折射的高性能红外（IR）非线性光学（NLO）材料。新发现了具有这种期望特性的基于Hg的IR NLO材料KHg ₄ Ga ₅ Se ₁₂。在该结构中，明显变形的HgSe ₄和GaSe ₄四面体通过顶点共享相互连接，从而形成了带有抗衡离子K ^+的三维骨架居住在洞中。值得注意的是，标题集中的所有NLO主动建筑单元均以完全平行的方式排列。这种拓扑结构和Hg-Se键的高磁化率使该材料具有良好的相匹配性，并且在2.09μm处具有巨大的粉末二次谐波生成（SHG）响应，约为基准材料AgGaS _2的20倍。（迄今为止报道的所有相匹配的IR NLO硫族化物中最大的响应之一）。KHg ₄ Ga ₅ Se ₁₂的光学带隙被确定为1.61 eV。此外，根据电子能带结构，实空间原子切割分析，SHG加权电子密度和局部偶极矩计算，可以归因于标题化合物优异的线性和非线性光学性质的起源。 （Hg / Ga）Se ₄组。最大系数d ₃₃和2.09μm处的双折射（Δn）的计算值分别为-65.257 pm / V和0.072。这样的值与实验观察非常吻合。我们的研究表明，基于汞的金属硫属化物是一类具有实用特性的竞争特性（良好的相匹配性，非常高的SHG效率，宽的透明度）的IR NLO材料。