RbNa(HC3N3O3)·2H2O exhibiting a strong second harmonic generation response and large birefringence as a new potential UV nonlinear optical material

Abstract

A new mixed alkali hydro-isocyanurate, RbNa(HC₃N₃O₃)·2H₂O, with the noncentrosymmetric space group Pna2₁ (No. 33), has been successfully grown by a hydrothermal method. The structure can be described as 2D wavy [Na(HC₃N₃O₃)O₄]_∞ layers separated by Rb atoms. The UV-vis diffuse reflection measurement revealed that RbNa(HC₃N₃O₃)·2H₂O has a large band gap of 5.10 eV, corresponding to the UV cutoff edge of 241 nm. The polarizing microscope measurement of a micron-sized single crystal shows that RbNa(HC₃N₃O₃)·2H₂O has a large birefringence of 0.194. The powder SHG measurement indicated that RbNa(HC₃N₃O₃)·2H₂O exhibits a strong SHG effect that is 5.3 times that of KH₂PO₄ (KDP) and can realize phase-matching in the visible and UV region. Moreover, the differences in the SHG responses between RbNa(HC₃N₃O₃)·2H₂O and two isostructural compounds have been investigated, which depend heavily on the structural modulation of the (HC₃N₃O₃)²⁻ group by cations. Meanwhile, the electronic structures and optical properties were well analyzed using DFT methods.