Synthesis, crystal structures and nonlinear optical properties of polymorphism: α- and β-RbHgI3·H2O

Abstract

New noncentrosymmetric polymorphs, namely, α- and β-RbHgI3·H2O, have been synthesized by conventional solution reaction. The two titled compounds crystallize in the monoclinic space groups Cc (no. 9) and Pc (no. 7), respectively. In their crystal structures, [HgI4] tetrahedrons connect with each other via sharing corners into one-dimensional (1-D) [HgI3] chains, and then all the 1-D chains connect with [Rb(H2O)]+ complexes to form the three-dimensional (3-D) frameworks. The consistent packing style results in a strong macroscopic nonlinear optical effect due to the superimposition of the microscopic second-order susceptibilities of [HgI4] tetrahedra. Powder second-harmonic generation (SHG) measurements show that both α- and β-RbHgI3·H2O are phase-matchable (type I), and the measured SHG coefficients are about 15.0 and 14.0 times that of KH2PO4 (KDP). Both the compounds exhibit wide transparency from near UV to mid and far-infrared region, suggesting that α- and β-RbHgI3·H2O are potential infrared NLO crystals. Band structure and optical property were also calculated by DFT methods.