Changes of vibrational modes upon melting mercury (II) halides

Abstract

Raman spectra of the “molecular” polycrystalline solids HgCl2 (D), HgBr2 (C) and of the yellow form of HgI2 (C) were measured from LN2 up to premelting temperatures. Polarized Raman spectra of molten HgX2 (X = Cl, Br, I) were measured at temperatures up to 950 K. The pure rotational band contour and the vibrational rotational contour of the HgX2 vapors were also measured. Upon melting the vibrational modes of the high temperature solid appear to be preserved. The spectra of the three liquids are similar and are best correlated to the spectra of the D (HgCl2) solid and the vibrational modes of the HgX2 molecules. Reduced Raman spectra of the liquids show splitting of the stretching mode to a doublet, ν1 (A1) and ν1 (B2g), which is attributed to strong intermolecular coupling. The ν2 and ν3 internal modes of the solid are also transferred into the melt. The predominant features of the low frequency reduced spectra is a band, VR, which is assigned to the rotatory modes of the HgX2 molecules and can be correlated to both the rotational band contour of the vapor and the librational modes of the solid. With increasing temperature the spectra of the liquid show softening of the rotatory modes and a blue shift of the ν1 mode, both implying weakening of the intermolecular coupling. An overall decrease in the Raman intensity of the liquid spectra with increasing temperature is attributed to the rapid molar volume increase.

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

“…The Raman spectrum of the obtained metastable β-HgI2 at room temperature is also shown in Fig 8, consistent with the reported results. [16][17][18][25][26][27] The DFT modeling reproduces well both the Raman and infrared spectra of the isolated monomer as well as the geometry of the linear HgI2 triatomic molecule, Fig. S2 and Table S2 (Supporting information).…”

“…New low-frequency features appear and grow with increasing x in the (HgI2)x(As2S3)1-x glasses. The most intense mode at 143 cm -1 corresponds to symmetric Hg-I stretching similar to the 1 (Σ + ) fundamental in yellow mercury (II) iodide (137 cm -1 at room temperature 26 ) and triatomic molecules in the vapor phase (157 cm -1 at 728 K 18 ). The Raman spectrum of the obtained metastable β-HgI2 at room temperature is also shown in Fig 8, consistent with the reported results.…”

“…[11][12][13] The mercury (II) iodide vapor consists of linear HgI2 triatomic molecules confirmed by gas-phase electron diffraction 14,15 and Raman spectroscopy. [16][17][18] Basically, the enhanced stability of the ∞ℎ symmetry for HgI2 and Group 12 dihalides is related to their electronic structure, relativistic effects and the -block (Zn, Cd) or lanthanide (Hg) contraction. [11][12][13] The HgY2 linear molecules, where Y = Cl, Br, I, were considered to be exceptionally rigid as a result of the large relativistic contraction of their orbitals.…”

Raman spectra and structure of molten fluorides: The binary mixtures MF2–AF (M=Zn,Pb; A=K,Cs) and the compounds K2ZnF4 and K4PbF6

“…The situation is similar for the glass forming ZnX 2 (X = Cl, Br) [5] and BeCl 2 [6] melts where the divalent cation preserves a predominant four-fold coordination at all compositions and forms a network-like structure of bridged ÔtetrahedraÕ. Somewhat different is the structural behavior of the HgX 2 (X = Cl, Br, I) melts where the pure components are molecular while in the mixtures with alkali metal halides both four-and three-coordinated ðHgX 2À 4 ; HgX À 3 Þ species are argued to exist [7].…”

The origin of disorder in CH 3 HgX (X = Cl, Br and I) crystals investigated by temperature dependent Raman spectroscopy

“…Whereas much is known in the literature about Hg 2 Cl 2 , Hg 2 Br 2 and Hg 2 I 2 where they are cited as examples of improper ferroelectrics [2][3][4] and also about HgCl 2 [5,6], HgBr 2 [5,7], and HgI 2 [5,8], there is less data on CH 3 HgX (X = Cl, Br and I) compounds.…”