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

Abstract

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.