From CeF₂(SO₄)·H₂O to Ce(IO₃)₂(SO₄): Defluorinated Homovalent Substitution for Strong Second-Harmonic-Generation Effect and Sufficient Birefringence

Second-harmonic-generation (SHG) efficiency and birefringence are two crucial parameters for nonlinear optical (NLO) crystals. However, the simultaneous optimization of these two key parameters remains a great challenge due to their contrasting microstructure prerequisites. In this paper, the first example of tetravalent rare-earth iodate-sulfate Ce­(IO3)2(SO4) (CISO) was designed by a defluorinated homovalent substitution strategy based on the parent compound CeF2(SO4)·H2O (CFSO) from the centrosymmetric to the noncentrosymmetric structure, which shows a large SHG effect of 3.5 × KDP in metal sulfates and a significant birefringence (0.259 at 546 nm) in hetero-oxyanion tetrahedral-group-based optical crystals. In the structure of CISO, both [IO3] and [SO4] groups are in the cis-position to each other, while [CeO8] polyhedra are highly distorted due to the different coordination environments of two kinds of oxyanions in one structure, which displays optimized optical anisotropy and polarizability attributed to their synergistic effect, leading to the strong SHG efficiency and sufficient birefringence. First-principles simulations for CISO have been employed to rationalize the correlations between the molecular structure and optical properties.二次谐波产生（Second-harmonic generation, SHG）效率与双折射率是非线性光学（Nonlinear Optical, NLO）晶体的两项核心参数。然而，由于二者对微观结构的先决条件相悖，同时优化这两项关键参数仍是一项重大挑战。本文以中心对称结构的母体化合物CeF₂(SO₄)·H₂O（CFSO）为基础，通过脱氟同价取代策略设计出首例四价稀土碘酸盐硫酸盐Ce(IO₃)₂(SO₄)（CISO），实现了从中心对称结构到非中心对称结构的转变。该化合物在金属硫酸盐体系中展现出3.5倍于磷酸二氢钾（Potassium Dihydrogen Phosphate, KDP）的强SHG效应，同时在杂多氧阴离子四面体基光学晶体中具备优异的双折射率（546 nm处为0.259）。在CISO的晶体结构中，[IO₃]与[SO₄]基团均处于顺式相对位置，而由于结构中两类氧阴离子的配位环境存在差异，[CeO₈]多面体呈现高度畸变。这种结构通过协同效应优化了光学各向异性与极化率，进而带来了强劲的SHG效率与充足的双折射率。本文通过第一性原理模拟对CISO进行研究，阐明了其分子结构与光学性能之间的内在关联。