Temperature induced changes on the structure and the dynamics of the “tetrahedral” glasses and melts of ZnCl2 and ZnBr2

Abstract

Glassy, supercooled, and molten ZnCl2 and ZnBr2 have been studied by Raman spectroscopy over the broad temperature range −196 to 800 °C in an effort to follow in detail the structural changes caused by temperature variation. A systematic study has also been undertaken for the corresponding crystalline polymorphs showing that each material exists in only one crystalline phase if water traces are not present. The reduced isotropic and anisotropic Raman spectra of the ZnCl2 and ZnBr2 glasses and melts are isomorphous. Unusually drastic changes of the relative intensities of particular bands occur with temperature in the reduced isotropic spectra. A comparison between the spectral features of crystals, glasses, and melts has revealed that the network structure of the glasses and melts consists of ZnX4/2 (X=Br,Cl) tetrahedra bound to each other by apex- and edge-bridged halides. The substructure of the glass/melt is formed by mixing a variety of tetrahedra participating in “open,” “cluster,” and “chain” networks which are bound to each other by bridged halides. The boundaries of the substructure involve neutral or charged terminal halide bonds with zinc of an average threefold coordination. Temperature rise breaks up the substructure to smaller fragments, increases the number of terminal bonds, and rearranges the apex- and edge-bridging networks. The good glass-forming ability of the ZnX2 melts is attributed to the existence and mixing of the three topologically different networks within the substructure. Our data of ZnCl2 are in qualitative agreement with molecular dynamics simulations as regards the frequency distribution of vibrational modes. However, simulations are not able to correctly predict polarization properties in the glass and the melt. The low-frequency Raman spectra reveal the presence of the Boson peak in both glasses, which interestingly persists, well resolved, also into the normal liquid state above the melting point. The spectra in the region of the Boson peak are also discussed in the framework of relevant theoretical models and empirical correlations.

Raman spectroscopy of ionic liquids derived from 1‐n‐butyl‐3‐methylimidazolium chloride and niobium chloride or zinc chloride mixtures

“…On the basis of these results Lecocq et al 27 have not presented the experimental EM spectra, therefore it was not possible for us to consider the presence of other Zn x Cl 2 2xC2 anionic species, as for instance, the Zn 4 Cl 10 2 that was already observed for ZnCl 2 melts. 42 As discussed above, and supported by literature data, 33,34,38,41,42 we conclude that for X ZnCl 2 < 0.3, the main ionic species present in the ILs are the BMI C cation and the ZnCl 4 2 anion.…”

“…Yannopoulos et al 42 have also observed a feature at 152 cm 1 for the ZnBr 2 melt that was assigned to a Zn 4 Br 10 2 species. These authors have predicted that the corresponding Zn 4 Cl 10 2 species would present a Raman feature at 265 cm 1 .…”

“…As reported above, Babushkina and Volkov 41 have assigned a Raman feature at 292 cm 1 to the Zn 2 Cl 6 2 ion for 0.3 < X ZnCl 2 < 0.7, with a maximum intensity for X ZnCl 2 D 0.5. However, Yannopoulos et al 42 investigating ZnCl 2 and ZnBr 2 melts have observed a Raman feature at 300 cm 1 in the ZnCl 2 melts that was assigned to a dimeric structure similar to the Zn 2 Cl 6 2 ion. In this sense, we attribute the Raman signal at 317 cm 1 to the dimer Zn 2 Cl 6 2 .…”

Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

“…Manifestations of edge-sharing tetrahedra in Raman spectra are common in tetrahedral glasses, see for example ZnX 2 (X: Cl, Br) [27] and GeS 2 [28]. In both cases, the frequency of the ν 1 (A 1 ) vibrational mode associated with edge-sharing configurations is higher than that of the corresponding mode of apex-bridged species, which indicates species with stronger intratetrahedral bonding in the former.…”

Raman spectroscopic studies of chemical speciation in calcium chloride melts

“…Raman spectroscopy is an inelastic light scattering technique that was previously used with success in studies on several molten salt systems [1][2][3][4][5][6][7] including MgCl 2 [7]. In the case of molten MgCl 2 , a clearly resolved band was observed at 205 cm -1 that was assigned to the symmetric stretch of the tetrahedral MgCl 4 2-species.…”