Lanthanide\nContraction Eliminates Disorder while Holding\nRobust Second Harmonic Generation in a Series of Polyiodates

For nonlinear optical crystals, the existence of atomic\ndisorder\nwithin the structure may lead to deterioration or instability in optical\nperformances. Herein, the lanthanide contraction effect was implemented\nto modulate the structure, atomic disorder, and optical properties\nof a series of novel lanthanide-based polyiodates, namely, REI<sub>5</sub>O<sub>14</sub> (RE = Eu, Er, Tm, Yb). Among them, EuI<sub>5</sub>O<sub>14</sub> crystallizes to an acentric space group (<i>Cm</i>) and exhibits a strong second harmonic generation (SHG)\nresponse (15 × KH<sub>2</sub>PO<sub>4</sub>@1064 nm) but contains\nsevere atomic disorder. When the central lanthanide comes into contact\nwith Er<sup>3+</sup>, Tm<sup>3+</sup>, and Yb<sup>3+</sup>, the smaller\nionic radius changes the space group into <i>Pmn</i>2<sub>1</sub> and eliminates the atomic disorder. For the first time, it\nis discovered that the lanthanide contraction effect possesses the\nfunction of eliminating atomic disorder. Simultaneously, REI<sub>5</sub>O<sub>14</sub> (RE = Er, Tm, Yb) preserve the privileged layered\nframework and the large nonlinear coefficient of EuI<sub>5</sub>O<sub>14</sub>, with an SHG response of 15 × KH<sub>2</sub>PO<sub>4</sub>@1064 nm for TmI<sub>5</sub>O<sub>14</sub> and 9.5 ×\nKH<sub>2</sub>PO<sub>4</sub>@1064 nm for YbI<sub>5</sub>O<sub>14</sub>. Systematical property comparison reveals that their stability in\nair and water is inextricably linked to their structural evolution.\nOur work provides a feasible route to gain new nonlinear optical materials\nwith great application potential without atomic disorder.