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.