Lanthanide Contraction Eliminates Disorder while Holding Robust Second Harmonic Generation in a Series of Polyiodates

Abstract

For nonlinear optical crystals, the existence of atomic disorder within the structure may lead to deterioration or instability in optical performances. Herein, the lanthanide contraction effect was implemented to modulate the structure, atomic disorder, and optical properties of a series of novel lanthanide-based polyiodates, namely, REI₅O₁₄ (RE = Eu, Er, Tm, Yb). Among them, EuI₅O₁₄ crystallizes to an acentric space group (Cm) and exhibits a strong second harmonic generation (SHG) response (15 × KH₂PO₄@1064 nm) but contains severe atomic disorder. When the central lanthanide comes into contact with Er³⁺, Tm³⁺, and Yb³⁺, the smaller ionic radius changes the space group into Pmn2₁ and eliminates the atomic disorder. For the first time, it is discovered that the lanthanide contraction effect possesses the function of eliminating atomic disorder. Simultaneously, REI₅O₁₄ (RE = Er, Tm, Yb) preserve the privileged layered framework and the large nonlinear coefficient of EuI₅O₁₄, with an SHG response of 15 × KH₂PO₄@1064 nm for TmI₅O₁₄ and 9.5 × KH₂PO₄@1064 nm for YbI₅O₁₄. Systematical property comparison reveals that their stability in air and water is inextricably linked to their structural evolution. Our work provides a feasible route to gain new nonlinear optical materials with great application potential without atomic disorder.