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

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.