Mohammad Kassem, Maria Bokova, Andrey S. Tverjanovich, Daniele Fontanari, David Le Coq, Anton Sokolov, Pascal Masselin, Shinji Kohara, Takeshi Usuki, Alex C. Hannon, Chris J. Benmore and Eugene Bychkov*, Bent HgI2 Molecules in the Melt and Sulfide Glasses: Implications for Nonlinear Optics, Chem. Mater. 2019. DOI: 10.1021/acs.chemmater.9b00860
Nonlinear optical (NLO) crystals are widely used in advanced photonic technologies for second harmonic and difference frequency generation (SHG and DFG, respectively), producing coherent light at frequencies where existing lasers are unavailable. Isotropic glasses do not exhibit SHG or DFG, except temporarily induced anisotropy under external stimuli. However, recent reports on glasses with chiral structural motifs show promising permanent NLO properties. We propose an alternative solution: hybrid molecular/network glasses with noncentrosymmetric HgI2 monomers. Mercury(II) iodide consists of linear HgI2 triatomic molecules in the vapor phase and in the yellow orthorhombic polymorph stable above 400 K. At lower temperatures, the tetragonal red form is composed of corner-sharing HgI4/2 tetrahedra forming a layered extended framework...