New Ultraviolet Nonlinear Optical Crystal Rb3Hg2(SO4)3Cl

Ultraviolet (UV) nonlinear optical (NLO) crystals play an irreplaceable role as the key materials to realize the frequency conversion for all solid state lasers. To date, it is still difficult to design UV NLO crystals with large second harmonic generation (SHG) coefficients, moderate birefringences and wide band gaps. Benefiting from the large band gap, sulfate has become an important research direction of UV NLO crystals. However, since SO4 is isotropic tetrahedral building units with nearly nonpolar Td symmetry, it exhibits small microscopic second order polarizability and polarizability anisotropy, which tends to result in a weak SHG effect and small birefringence. In this work, we introduced Hg2+ ions that are easy to form distorted polyhedrons into the sulfates, resulting in a new NLO material, Rb3Hg2(SO4)(3)Cl. It crystallizes in a monoclinic space group (P2(1)) with the lattice parameters a=0.78653(2) nm, b=0.97901(2) nm, c=1.00104(3) nm, and beta=110.95(3) (Z=2). Structure of Rb3Hg2(SO4)(3)Cl consists of [SO4] tetrahedra, [HgO5] and [HgO4Cl] polyhedral, which connected by a common corner to form a spatial 3D network. All the Rb atoms reside in the cavity of 3D network. The powder SHG measurement proposed by Kurtz and Perry indicates that Rb3Hg2(SO4)(3)Cl is a phase-matchable material in the visible region and exhibits a moderate SHG response about 1.5 times that of KH2PO4 (KDP). In addition, the UV-Vis-NIR diffuse reflectance spectral measurement indicates that Rb3Hg2( SO4)3Cl has a short UV cut-off edge of 251 nm, corresponding to the band gap of 4.94 eV. Its polarizing microscope measurement reveals that Rb3Hg2( SO4)(3)Cl has a moderate birefringence ( The birefringence of Rb3Hg2( SO4)(3)Cl crystal at 546.1 nm is 0.04). Moreover, first-principles calculations uncover that the distorted [ HgO5], [HgO4Cl] and [SO4] polyhedral are responsible for its SHG effect. Our study shows that Rb3Hg2(SO4)(3)Cl may have potential applications as a UV NLO crystal.