作为实现全固态激光器频率转换功能的关键材料, 紫外非线性光学晶体发挥着不可替代的作用。设计兼具大的非线性光学系数、合适的双折射和宽带隙的紫外非线性光学晶体仍然是该领域亟待攻克的一个难题。由于具有宽的带隙, 硫酸盐已成为紫外非线性光学晶体领域的一个重要研究方向。SO₄四面体基团具有接近非极性的Td对称性, 使其极化率各向异性和二阶极化率较小, 因而对晶体的非线性系数和双折射贡献很小。通常引入畸变程度高的阳离子多面体可以增加晶体的非线性效应和双折射。本工作将易于形成畸变多面体的Hg²⁺离子引入到硫酸盐体系中, 采用高温熔体法合成出新型非线性光学晶体材料Rb₃Hg₂(SO₄)₃Cl。该晶体属于单斜晶系, 空间群为P2₁, 晶胞参数为a=0.78653(2) nm, b=0.97901(2) nm, c=1.00104(3) nm, β=110.95(3), Z=2。其晶体结构由[SO₄]四面体、[HgO₅]和[HgO₄Cl]多面体以角共享的方式连接形成空间网状结构, 而Rb⁺填充在孔洞中。Rb₃Hg₂(SO₄)₃Cl晶体的粉末倍频效应为1.5倍KDP, 且能够在可见光区实现相位匹配。紫外漫反射光谱测试表明, 紫外截止边为251 nm, 对应光学带隙为4.94 eV。利用偏光显微镜确定该晶体在546.1 nm处的双折射为0.04。此外, 第一性原理计算表明, 晶体的非线性系数主要来源于扭曲的[HgO₅]、[HgO₄Cl]和[SO₄]多面体。上述结果表明, Rb₃Hg₂(SO₄)₃Cl是具有潜在应用前景的紫外非线性光学晶体材料。

Ultraviolet (UV) nonlinear optical (NLO) crystals play an irreplaceable role as the key materials to realize the frequency conversion for all solid state lasers. To date, it is still difficult to design UV NLO crystals with large second harmonic generation (SHG) coefficients, moderate birefringences and wide band gaps. Benefiting from the large band gap, sulfate has become an important research direction of UV NLO crystals. However, since SO₄ is isotropic tetrahedral building units with nearly nonpolar T_d symmetry, it exhibits small microscopic second order polarizability and polarizability anisotropy, which tends to result in a weak SHG effect and small birefringence. In this work, we introduced Hg²⁺ ions that are easy to form distorted polyhedrons into the sulfates, resulting in a new NLO material, Rb₃Hg₂(SO₄)₃Cl. It crystallizes in a monoclinic space group (P2₁) with the lattice parameters a=0.78653(2) nm, b=0.97901(2) nm, c=1.00104(3) nm, and β=110.95(3) (Z=2). Structure of Rb₃Hg₂(SO₄)₃Cl consists of [SO₄] tetrahedra, [HgO₅] and [HgO₄Cl] polyhedral, which connected by a common corner to form a spatial 3D network. All the Rb atoms reside in the cavity of 3D network. The powder SHG measurement proposed by Kurtz and Perry indicates that Rb₃Hg₂(SO₄)₃Cl is a phase-matchable material in the visible region and exhibits a moderate SHG response about 1.5 times that of KH₂PO₄ (KDP). In addition, the UV-Vis-NIR diffuse reflectance spectral measurement indicates that Rb₃Hg₂(SO₄)₃Cl has a short UV cut-off edge of 251 nm, corresponding to the band gap of 4.94 eV. Its polarizing microscope measurement reveals that Rb₃Hg₂(SO₄)₃Cl has a moderate birefringence (The birefringence of Rb₃Hg₂(SO₄)₃Cl crystal at 546.1 nm is 0.04). Moreover, first-principles calculations uncover that the distorted [HgO₅], [HgO₄Cl] and [SO₄] polyhedral are responsible for its SHG effect. Our study shows that Rb₃Hg₂(SO₄)₃Cl may have potential applications as a UV NLO crystal.