Bent HgI₂ Molecules in the Melt and Sulfide Glasses: Implications for Nonlinear Optics

Abstract

Non-linear 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 temporary 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 non-centrosymmetric 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. There is a gap in the molecular evolution; direct structural measurements of the liquid HgI2 phase are missing. Using high-energy X-ray scattering, pulsed neutron diffraction and Raman spectroscopy supported by structural and vibrational modeling, we show that the mercury (II) iodide melt and HgI2-containing sulfide glasses are built-up by bent HgI2 monomers (the bond angle ∠I-Hg-I = 156±2° in the melt). The non-centrosymmetric entities imply intrinsic optical non-linearity of the second order, confirmed by a strong SHG response.

Intrinsic second‐order nonlinearity in chalcogenide glasses containing HgI2

“…Similar results were obtained using DFT calculations (Figure ). The DFT modeling shows that isolated HgI 2 molecule appears to be linear and extremely rigid in accordance with previous DFT simulations . As expected, the DFT Raman spectrum consists of a single Hg–I symmetric stretching mode at 148 cm −1 (), while the DFT IR spectrum exhibits the Hg–I asymmetric stretching at 220 cm −1 () and degenerated bending at 49 cm −1 (), Figure A,B.…”

“…In contrast, the monoclinic yellow HT form consists of bent HgI 2 molecules (∠I–Hg–I = 160°) of the symmetry and exhibits a SHG signal . Our recent high‐energy X‐ray diffraction studies have revealed that liquid mercury (II) iodide is also formed by bent HgI 2 monomers (∠I–Hg–I = 156 ± 2°) . A concomitant evidence is given by the X‐ray total correlation function , obtained by the Fourier transform of the structure factor , where is the scattering vector, the scattering angle and the wavelength of the incident X‐ray beam.…”

“…Simple semiempirical Modified Neglect of Differential Overlap (MNDO) calculations with the Hg‐I interatomic distance of 2.6 Å, similar to that in yellow β‐HgI 2 (2.623 Å) or liquid mercury (II) iodide (2.62 Å), and the I–Hg–I bond angle of 160° yield the following spectral positions for the vibrational fundamentals: = 61 cm −1 (the I–Hg–I bending), = 137 cm −1 (the Hg–I symmetric stretching), and = 198 cm −1 (the Hg‐I asymmetric stretching).…”

“…The low intensity of the mode is directly related to unsubstantial intermolecular interactions within the (HgI 2 ) 2 dimer and insufficient bending of the weakly interacting monomers. The HgI 2 entities embedded in a glass network will be going through much stronger contacts with the nearest neighbors implying more significant distortion and enhanced microscopic second‐order susceptibility, observed for HgI 2 –As 2 S 3 glasses …”

“…28 Our recent high-energy X-ray diffraction studies have revealed that liquid mercury (II) iodide is also formed by bent HgI 2 monomers (∠I-Hg-I = 156 ± 2°). 29 A concomitant evidence is given by the X-ray total correlation function T X (r), obtained by the Fourier transform of the structure factor S X (Q), where Q = 4 sin ∕ is the scattering vector, the scattering angle and the wavelength of the incident X-ray beam. The positions of the Hg-I first neighbors at 2.62 Å and I-I second neighbors at ≈5.15 Å directly indicate that mercury (II) iodide monomers are bent; the bond angle ∠I-Hg-I ≈ 159°.…”