非线性光学材料可以通过二阶倍频等非线性光学效应拓展激光的光谱范围，对于激光技术的应用和发展至关重要。目前已商业化的非线性光学晶体一般以磷酸盐或硼酸盐等含氧酸盐为主，但这些含氧酸盐在中远红外波段会有光谱吸收。相比之下，硫属化合物一般具有很宽的中远红外透过区，因此有望成为中远红外光学材料。目前已经被商业化的硫属化合物非线性光学晶体还很稀少，只有AgGaS₂和AgGaSe₂两种，虽然它们在中远红外波段透光性良好，非线性光学系数大，但是由于光学带隙小，存在双光子吸收，激光损伤阈值低等缺点，这两种晶体的实际应用受到了很大的限制。因此，开发同时具备性能均衡（激光损伤阈值大并且非线性光学系数也大）的红外非线性光学硫属晶体具有重大的意义。

本论文的工作主要是通过将第14族金属Ge或 Sn引入到硫化物体系中与S配位形成非中心对称的结构单元，再引入电正性的碱金属或碱土金属增加化合物的带隙，从而得到一些结构新颖并且具有良好非线性光学性质的硫化物。我们通过高温固相反应探索合成出这些硫化物，并利用单晶X-射线衍射实验来确定这些未知化合物的晶体结构。通过改进的Kurtz粉末法测试化合物的二阶倍频响应（SHG）。我们还利用密度泛函理论计算相关化合物的电子结构，并对晶体的非线性光学性质进行理论研究。本文的工作主要分为以下四个部分：

1.通过替换已知化合物Ba₆Cd₂Sn₄S₁₆中的结构单元，我们成功设计出了7个具有立方非心结构的同构化合物Ba₆Li_2.67Sn_4.33S₁₆， Sr₆Li_2.67M^IV_4.33S₁₆ (M^IV = Ge，Sn)及Sr₆Li₂M^IIM^IV₄S₁₆ (M^II = Cd，Zn；M^IV = Ge，Sn)，并通过高温固相法合成出了这些化合物。研究表明，Sr₆Li₂M^IIM^IV₄S₁₆(M^II = Cd，Zn；M^IV = Ge，Sn)具有良好的二阶非线性光学性质（SHG在2.1 μm处可达标准样品AgGaS₂的0.36到1.76倍，激光损伤阈值（LIDT）可以达到AgGaS₂的10-14倍）。

2.非公度调制结构可以使一些固体材料具有铁电或电荷密度波等有趣的物理性质，但是目前，具有非公度调制结构的非线性光学材料还非常罕见，很少有科学研究将结构调制与晶体的二阶非线性光学性质联系起来。我们利用高温固相反应，通过多硫化钾助熔剂法，得到了两个含有S₂^3-基团的非心多硫化物A₂SnS₅(A = Sr，Ba)。这两个化合物具有非公度调制结构，是首个被发现的具有非公度调制结构红外非线性光学晶体。这两个化合物具有良好的非线性光学性质，在1.57 μm处的SHG可以达到AgGaS₂的1.1-0.3倍，激光损伤阈值可以达到AgGaS₂的7-8倍。通过利用(3+1)D超空间群理论，A₂SnS₅(A = Sr，Ba)的非公度调制结构得以确定。两个化合物的结构调制主要是由平均结构中一维带状[Sn₂S₇]_∞结构引发的。两个结构的主要差异是：在Sr₂SnS₅中，Sn是长程有序排列的；而Ba₂SnS₅中的Sn是无序裂分的。Ba₂SnS₅中的非公度调制使得结构含有畸变更严重的Sn/S多面体，使得偶极矩急剧增加,进一步造成Ba₂SnS₅的二阶倍频响应相对于结构相近的周期结构硒化物α-Ba₂SnSe₅显著增加。这项研究表明，合理的非公度调制结构有利于平衡非线性光学材料的非线性光学系数和激光损伤阈值。

3.利用多硫化钾作助熔剂，通过高温固相反应又成功合成出了三个多硫化物，它们分别是Sr₆Sb₆S₁₇，Sr₂HgS₅ 和Ba₃SnS_5.75。三个化合物都具有0维的晶体结构，它们的结构中分别含有[HgS₂]^2-， [Sb₃S₇]^5-，以及[SnS₄]⁴⁺基团。其中，Sr₂HgS₅，Sr₆Sb₆S₁₇含有 V字型S₃^2-基团，而Ba₃SnS_5.75含有 S₂^2-基团。其中Sr₆Sb₆S₁₇具有二阶非线性光学性质（SHG：0.15×AgGaS₂@1570nm）。本章的研究说明多硫化钾助熔剂法对于合成含碱土金属的三元多硫化物有一定的适用性，运用此方法有助于合成更多的新型多硫化物。

4.利用红外非线性光学硫化物β-Cu₂SrSnS₄为设计模板，通过将结构中的SnS₄四面体替换为Sn₂S₇二聚体，并引入Na以中和电性，我们成功设计了五元非中心对称化合物Cu₂Na₂ASn₂S₇ (A = Sr，Ba)，并用高温固相法成功合成了两个化合物的纯相。由于碱土金属元素的不同，引发两个化合物的结构不同，将Ba用Sr取代，结构从六方晶系转变为正交晶系，CuS₄及SnS₄由朝向非一致转变为朝向一致。这两个化合物的带隙都在2.1 eV左右，相比四元化合物β-Cu₂SrSnS₄，Cu₂Na₂ASn₂S₇（A=Sr，Ba）具有更的熔点（626-628℃），这样低的熔点更有利于晶体生长。另外，Cu₂Na₂ASn₂S₇（A=Sr，Ba）表现出中等的非线性光学性质（在1570 nm处，SHG：AgGaS₂标样的0.3-0.2倍），这说明它们有望在红外非线性光学方面得到应用。通过理论计算，化合物的电子结构和光学性质得以研究。此外，通过高温固相反应还发现了五元硒化物Na₂Cu₂BaSn₂Se₇，结构与Cu₂Na₂BaSn₂S₇相似。同时我们将Cu₂Na₂BaSn₂S₇中的Cu元素替换为等量的Na/Ag，又得到了结构新颖的系列非心化合物Ag_xNa_4-xBaSnS₇（0.4<x<1.3），它们的结构属于C2手性空间群，结构中存在[Ag_xNa_2-xSn₂S₇]三维网络结构，Na⁺离子与Ba²⁺分别填入结构中的不同孔道中。

Nonlinear optical (NLO) materials can expand the range of laser spectra by NLO effects such as second harmonic generation (SHG), making them very important for application and development of laser technology. To date, many oxides including borates and phosphates have been commercially applied as NLO materials. However, many of them can cause light absorption in the mid- and far-infrared region. Contrarily, chalcogenides usually have wide mid- and far-infrared transparent regions, which makes them promising IR optical materials. Right now, only a few chalcogenides including AgGaS₂ and AgGaSe₂ have been commercially applied as IR NLO crystals. Even though they have good transparency in the IR region, large NLO coefficients, their band gaps are relatively small, which causes two-photon absorption and low laser induced damage threshold (LIDT), hindering their application. It is in great urgency to developing novel NLO chalcogenides, which can reach balance between NLO coefficients and LIDTs.

In this dissertation, our research mainly focus on developing new sulfides with excellent NLO properties and unique structures by introducing distorted Sn/S (Ge/S) noncentrosymmetric (NCS) building units and alkali/ alkali earth metals (helping enlarge band gaps) into sulfide systems. High-temperature solid-state method was used to synthesize these compounds. Second-order harmonic generations (SHG) were measured by using modified Kurtz powder method. Furthermore, density function theory was also applied to study the electronic structures and nonlinear optical properties of the materials. This work includes four parts as follows:

1. By replacing the building units in the structure of Ba₆Cd₂Sn₄S₁₆, seven new isostructural NLO compounds with NCS cubic structures: Ba₆Li_2.67Sn_4.33S₁₆, Sr₆Li_2.67M^IV_4.33S₁₆(M^IV = Ge，Sn), and Sr₆Li₂M^IIM^IV₄S₁₆ (N = Cd，Zn；M = Ge，Sn) were successfully designed and then synthesized by high-temperature solid-state method. Among them, Sr₆Li₂M^IIM^IV₄S₁₆ (M^II = Cd，Zn；M^IV = Ge，Sn) possess large second harmonic generations (0.36-1.76×AgGaS₂@2100nm), and large LIDTs(10-14×AgGaS₂), which means they have potential to be applied as IR NLO materials.

2. Incommensurate modulations can trigger many interesting physical properties such as ferroelectricity, or charge density waves. Though, incommensurate modulations are very rare among the NLO materials, and reseach about the connections between incommensurate modulations and nonlinear optical properties are even scarcer. Here, by using high-temperature solid state method with the aid of potassium polysulfide flux, A₂SnS₅ (A= Sr，Ba) were successfully synthesized. Both compounds contain S₂^3- groups. The two compounds have unusual incommensurately modulated structures. It is also the first time to report IR NLO materials with incommensurately modulated structures. The two compounds behave excellent NLO properties, exhibiting SHG about 1.1-0.3 times that of AgGaS₂ @ 1570 nm and LIDTs about 7-8 times that of AgGaS₂ @ 1064 nm. The incommensurately modulated structures of A₂SnS₅ (A = Sr，Ba) were successfully determined by using (3+1) superspace theory. The incommensurate modulations mainly rise from different [Sn₂S₇]_∞ belts of the average structures. We found that the two structures are different. For Sr₂SnS₅, Sn is long-range ordered, while the Sn sites in Ba₂SnS₅ are disordered. Such modulations generate Sn/S polyhedra which are more distorted in Ba₂SnS₅, causing the intense increase of dipole moments. This is the reason why SHG of Ba₂SnS₅ is much larger than that of periodic α-Ba₂SnSe₅. This work proves that appropriate incommensurate modulations can help NLO materials reach the balance between SHG and LIDTS.

3. Three ternary polysulfides including Sr₆Sb₆S₁₇, Sr₂HgS₅ and Ba₃SnS_5.75 were successfully synthesized by potassium polysulfide flux method through high-temperature solid-state reactions. 0-dimensional crystal structures of the three compounds contain [HgS₂]^2-, [Sb₃S₇]^5- and [SnS₄]^4- groups, respectively. Furthermore, Sr₂HgS₅ and Sr₆Sb₆S₁₇ contain S₃^2- groups, while Ba₃SnS_5.75 contain S₂^2-. Sr₆Sb₆S₁₇ possess NLO properties (SHG: 0.15×AgGaS₂@1570 nm). This work proved that potassium-polysulfide-flux method is suitable to help synthesize ternary polysulfides, which contain alkaline-earth metals, and may be able to help develop other new compounds.

4. By using IR NLO compound β-Cu₂SrSnS₄ as a design template, replacing SnS₄ with Sn₂S₇ dimers and inducing Na into the structures to balance the charge, Cu₂Na₂ASn₂S₇ (A=Sr，Ba) were successfully designed. The pure samples of two compounds were synthesized by high-temperature solid-state method. The diverse of earth alkali metals causes structure transformation between the two compounds. When Sr was substituted by Ba, the structure transform from orthorhombic hexagonal，the orientations of CuS₄ and SnS₄ change from conformity to inconformity. The band gaps of Cu₂Na₂ASn₂S₇ (A = Sr, Ba) are about 2.1 eV. Compared with β-Cu₂SrSnS₄, Cu₂Na₂ASn₂S₇ (A = Sr, Ba) have lower melting points (626-628 ℃)，which is more beneficial to the crystal growth. Cu₂Na₂ASn₂S₇(A = Sr，Ba) perform moderate NLO properties (SHG：0.3-0.2×AgGaS₂ @1570 nm).The electronic structures and optical properties of related compounds were also studied through theoretical calculations according to Density Functional Theory. Besides, by using high-temperature solid-state method, Na₂Cu₂BaSn₂Se₇, whose structure is similar to that of Cu₂Na₂BaSn₂S₇, was also found. Furthermore, by replacing all the Cu atoms in Cu₂Na₂BaSn₂S₇ with Ag/Na, novel NCS compounds Ag_xNa_4-xBaSnS₇ (0.4<x<1.3) with different crystal structures were also successfully designed and then synthesized. Ag_xNa_4-xBaSnS₇ (0.4<x<1.3) crystalize in C2 space group. In their crystal structures, Na⁺ and Ba²⁺ fill in different holes of [Ag_xNa_2-xSn₂S₇] 3D networks.