In situ hydrothermal synthesis of polar second-order nonlinear optical selenate Na5(SeO4)(HSeO4)3(H2O)2

Abstract

The first alkali–metal selenate nonlinear optical (NLO) crystal, Na₅(SeO₄)(HSeO₄)₃(H₂O)₂, has been obtained by in situ hydrothermal synthesis. It crystallizes in the polar and noncentrosymmetric (NCS) space group Cc (no. 9). The structure of Na₅(SeO₄)(HSeO₄)₃(H₂O)₂ can be described as isolated [SeO₄]/[HSeO₄] tetrahedra connected by H-bonding interactions and Na–O bonds, forming a three-dimensional framework. Powder second-harmonic generation (SHG) measurements show that Na₅(SeO₄)(HSeO₄)₃(H₂O)₂ is phase-matching with a moderate SHG response of ca. 1.6 times that of benchmark KH₂PO₄. The UV-Vis-NIR spectrum shows that the title compound is a wide-band semiconductor with an optical band gap of 3.57 eV. The birefringence of Na₅(SeO₄)(HSeO₄)₃(H₂O)₂ was calculated to be 0.037 at 1064 nm. First-principles studies show that the [SeO₄]/[HSeO₄] tetrahedra are the main contributors to its NLO properties. This study indicates that pure selenate is a promising material system to afford NCS structures with SHG responses.