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.