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

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.