Exploration of new second-order nonlinear optical materials of the Cs–Hg–Br–I system

Abstract

A first example of a Cs–Hg–Br–I system, namely Cs₂Hg₂Br₂I₄·H₂O (1), has been synthesized by reaction in solution. It belongs to the polar monoclinic space group pc with a = 7.460(5) Å, b = 13.458(9) Å, c = 8.891(6) Å, and β = 92.448(9)°. The basic anionic group in the crystal is non-centrosymmetric [Hg₂Br₂I₄]²⁻, and the groups are aligned in the same direction in the crystal. As a result, 1 exhibits a phase-matchable second harmonic generation (SHG) response as strong as 6 times that of KH₂PO₄ (KDP). For the purpose of comparison, the known compound Cs₂Hg₃I₈·H₂O (2) was also synthesized and its SHG properties were studied for the first time. Band structure and optical property calculations for the two compounds based on DFT methods were also performed. The comprehensive properties of 1 and 2 as well as their analog, Cs₂HgCl₂I₂, have been compared and discussed.