Symmetry Breaking Induced by Hydration-Number Tuning Enables Second-Order Nonlinear Optical Responses in Lanthanum Sulfate Hydrates

Abstract

The tetrahedral [SO4] unit is an attractive building block for ultraviolet (UV) nonlinear optical (NLO) materials because its wide energy gap supports deep-UV (DUV) transparency. However, its high symmetry often favors centrosymmetric (CS) arrangements that preclude second-order NLO responses. To facilitate the formation of noncentrosymmetric (NCS) lanthanide sulfates, we express a hydration-number-tuned symmetry-breaking strategy here. On the basis of the synthetic route for La2(SO4)3·9H2O, 18-crown-6 was utilized as a coordination regulator to modulate the coordination environment of La3+ cations and the connectivity of structural units. As a result, La2(SO4)3·8H2O, the lanthanum sulfate hydrate with an NCS crystal structure, was synthesized. It crystallizes in space group Pn and exhibits an absorption edge below 200 nm. Moreover, due to the effective superposition of the dipole moments of both the [LaO5(H2O)4] polyhedron and [SO4] tetrahedron in the structure, it presents a moderate second harmonic generation (SHG) response of approximately 0.51 times that of KH2PO4 (KDP). These results demonstrate that hydration-number regulation can be used to modulate the structural symmetry and NLO properties of lanthanide sulfate hydrates and may provide an exercisable design strategy for sulfate-based DUV NLO materials.