Dynamics of Density and Orientation Fluctuations in Supercooled Zinc Halides

Why the Brillouin Light Scattering Relaxation Times of Molten Zinc Chloride Are Shorter and Weaker in Temperature Dependence than the Structural Relaxation Times from Depolarized Light and Neutron Spin Echo Spectroscopy

A longstanding problem in the Brillouin light scattering (BLS) study of polymers is the relaxation times τ_BLS(T) being more than an order of magnitude shorter than the α-relaxation times τ_α(T) determined by dielectric, depolarized light scattering (DLS), and molecular dynamics simulations. In tackling the problem, τ_BLS(T) was identified with the relaxation time τ₀(T) of the primitive relaxation in the coupling model, which can be calculated from τ_α(T) and the stretch exponent β_K of the Kohlrausch correlation function for the α-relaxation.. The problem was solved by finding that indeed τ₀(T) is in good agreement with τ_BLS(T). A recent work performed the neutron spin echo study of the structural α-relaxation of the network ionic liquid ZnCl₂ and found the same anomaly as polymers. The α-relaxation time τ_NSE(T) from neutron spin echo (NSE) as well as the α-relaxation time τ_DLS(T) from DLS of ZnCl₂ are much longer than τ_BLS(T) from BLS obtained before by several research groups. The finding of the same anomaly in ZnCl₂ and polymers with very different chemical and physical structures offers an opportunity to critically test the explanation given before. The test was carried out by calculating the primitive relaxation times τ_0,DLS(T) and τ_0,NSE(T) from τ_DLS(T) and τ_NSE(T), respectively, in zinc chloride. Good agreements of τ_BLS(T) from BLS with τ_0,DLS(T) and τ_0,NSE(T) were found and thus the explanation given for polymers remains valid for ZnCl₂. The test was extended to glycerol by comparing τ_BLS(T) with τ_0,ICNS(T) and τ_0,CNS(T) calculated from the α-relaxation time τ_ICNS(T) and τ_CNS(T) from incoherent and coherent neutron scattering, respectively. There is good agreement between τ_BLS(T) and τ_0,ICNS(T) in glycerol. There is also semiquantitative agreement of τ_BLS(T) with τ_0,DS(T) from dielectric spectroscopy as well as τ_0,CNS(T). Thus, the explanation for polymers is verified in the two very different glass formers, ZnCl₂ and glycerol, and it is an advancement in the application of BLS to study the dynamics of glass formers.

Neutron Spin-Echo Studies of the Structural Relaxation of Network Liquid ZnCl2 at the Structure Factor Primary Peak and Prepeak

Using neutron spin-echo spectroscopy, we studied the microscopic structural relaxation of a prototypical network ionic liquid ZnCl₂ at the structure factor primary peak and prepeak. The results show that the relaxation at the primary peak is faster than the prepeak and that the activation energy is ∼33% higher. A stretched exponential relaxation is observed even at temperatures well-above the melting point T_m. Surprisingly, the stretching exponent shows a rapid increase upon cooling, especially at the primary peak, where it changes from a stretched exponential to a simple exponential on approaching the T_m. These results suggest that the appearance of glassy dynamics typical of the supercooled state even in the equilibrium liquid state of ZnCl₂ as well as the difference of activation energy at the two investigated length scales are related to the formation of a network structure on cooling.

The progression of thermodynamic anomalies in MX2 networks with local tetrahedral geometries

Key thermodynamic anomalies in density and compressibility, as well as the related stability limits, are determined using an ionic model for BeF₂ which includes many-body polarization terms. BeF₂ is chosen as an example of an archetypal network-forming system whose structure can be rationalised in terms of connected local tetrahedral coordination polyhedra. The anion dipole polarizability (which effectively controls the bond angles linking neighbouring tetrahedra) is used as a single free parameter in order to help rationalise the changes in the anomaly locations in phase space, whilst all other potential parameters remain fixed. The anomalies and stability limits systematically shift to lower temperature and higher pressure as the anion polarizability is increased. At high dipole polarizabilities the temperature of maximum density anomaly locus becomes suppressed into the supercooled regime of the phase space. The movements of the anomaly loci are analysed in terms of the network structure and the correlation with the inter-tetrahedral bond angles is considered. The high sensitivity of the anomalies to the details of the potential models applied is discussed with reference to previous works on related systems. The relationship to analogous studies on Stillinger-Weber liquids is discussed.

Structure and dynamics in network-forming materials

The study of the structure and dynamics of network-forming materials is reviewed. Experimental techniques used to extract key structural information are briefly considered. Strategies for building simulation models, based on both targeting key (experimentally-accessible) materials and on systematically controlling key model parameters, are discussed. As an example of the first class of materials, a key target system, SiO₂, is used to highlight how the changing structure with applied pressure can be effectively modelled (in three dimensions) and used to link to both experimental results and simple structural models. As an example of the second class the topology of networks of tetrahedra in the MX₂ stoichiometry are controlled using a single model parameter linked to the M-X-M bond angles. The evolution of ordering on multiple length-scales is observed as are the links between the static structure and key dynamical properties. The isomorphous relationship between the structures of amorphous Si and SiO₂ is discussed as are the similarities and differences in the phase diagrams, the latter linked to potential polyamorphic and 'anomalous' (e.g. density maxima) behaviour. Links to both two-dimensional structures for C, Si and Ge and near-two-dimensional bilayers of SiO₂ are discussed. Emerging low-dimensional structures in low temperature molten carbonates are also uncovered.

Network topology and the fragility of tetrahedral glass-forming liquids

Network-forming liquids comprising tetrahedral motifs are investigated by large-scale molecular dynamics computer simulations within the framework of an ionic interaction model. The network topology is controlled by varying the anion polarizability, which governs the intertetrahedral bond angle, for different system densities. A coupling is found between the growth in magnitude and range of extended range oscillations and the appearance of ordering on an intermediate length scale. The interrelation between the system fragility and the structural arrangements on these two different length scales shows the trends that are observed for glass-forming systems. In particular, the fragility increases with number of edge-sharing motifs.

Dynamic light scattering study on phase separation of a protein-water mixture: application on cold cataract development in the ocular lens

We present a detailed dynamic light scattering study of the phase separation in the ocular lens emerging during cold cataract development. Cold cataract is a phase separation effect that proceeds via spinodal decomposition of the lens cytoplasm with cooling. The intensity autocorrelation functions of the lens protein content are analyzed with the aid of two methods, providing information on the populations and dynamics of the scattering elements associated with cold cataract. It is found that the temperature dependence of many measurable parameters changes appreciably at the characteristic temperature approximately 16+/-1 degrees C which is associated with the onset of cold cataract. By extending the temperature range of this work to previously inaccessible regimes, i.e., well below the phase separation or coexistence curve at Tcc, we have been able to accurately determine the temperature dependence of the collective and self-diffusion coefficients of proteins near the spinodal. The analysis showed that the dynamics of proteins bears some resemblance to the dynamics of structural glasses, where the apparent activation energy for particle diffusion increases below Tcc, indicating a highly cooperative motion. Application of ideas developed for studying the critical dynamics of binary protein-solvent mixtures, as well as the use of a modified Arrhenius equation, enabled us to estimate the spinodal temperature Tsp of the lens nucleus. The applicability of dynamic light scattering as a noninvasive, early-diagnostic tool for ocular diseases is also demonstrated in light of the findings of the present paper.

Temperature-induced structural changes in glassy, supercooled, and molten silica from 77 to 2150 K

In situ polarized and depolarized Raman spectra of glassy, supercooled, and molten SiO2 have been measured over the broad temperature range 77-2150 K in an effort to examine possible structural changes caused by temperature variation. A new experimental setup using a CO2 laser for heating the sample has been designed allowing measurement with controllable blackbody radiation background at temperatures up to 2200 K. Careful and systematic relative intensity measurements and the use of the isotropic and anisotropic Raman representation of the spectra revealed hidden bands in the bending mode region and resolved bands in the stretching region of the spectra. Overall the spectra behavior shows similarities with the spectra of the recently studied tetrahedral glasses/melts of ZnCl2 and ZnBr2. Increasing temperature causes subtle changes of the relative intensities within the silicon-oxygen stretching region at approximately 750-850 cm(-1) and gives rise to a new band at approximately 930 cm(-1). The spectral behavior is interpreted to indicate that the "SiO42" tetrahedra are bound to each other to form the network by apex-bridging and partly by edge-bridging oxygens. The network structure of the glass/melt is formed by mixing a variety of tetrahedra participating in "open" (cristobalitelike), "cluster" (supertetrahedra), and "chain" edge-bridged substructures bound to each other by bridging oxygens. A weak in intensity but strongly polarized composite band is resolved at approximately 1400 cm(-1) and is assigned to Si[Double Bond]O terminal bond frequency. Temperature rise increases the concentration of the terminal bonds by breaking up the network. These structural changes are reminiscent of the polyamorphic transformations occurring in silica as has recently been predicted by computer simulations. At low frequencies the Raman spectra reveal the presence of the Boson peak at approximately 60 cm(-1) which is well resolved even above melting temperature up to 2150 K.

Dynamic light scattering study of the liquid <--> glass transition for the GdCl3-3AlCl3 glass-forming mixture

The dynamics of density and orientation fluctuations for a member of a family of inorganic glass-forming molten salts based on rare-earth halide-aluminum halide mixtures, namely, the GdCl3-3AlCl3, has been investigated in the supercooled state by means of dynamic light scattering. The temperature dependence of the structural relaxation time near the glass transition temperature was found to be rather strong, indicating the "fragile" character of this liquid. The microscopic origin of the structural relaxation has been rationalized by employing structural information that has been independently extracted from other spectroscopic techniques.

Brillouin scattering study of molten zinc chloride

Polarized and depolarized Brillouin scattering experiments on molten ZnCl2 were performed between 300 and 600 degrees C in different geometries. VV spectra measured in backscattering and small angle scattering were analyzed with conventional viscoelastic theory using either a Debye or a Cole-Davidson model for the memory function. We also analyzed in the same way the temperature dependence of the transverse Brillouin lines detected in a 90 degrees VH geometry. We show that the Cole-Davidson memory function yields a consistent interpretation of all the spectra. The resulting shear and longitudinal relaxation times are equal within their error bars, and are about 2.5 times smaller than the alpha relaxation time previously determined. The static shear viscosity values deduced from the analysis of the propagating transverse waves agree, at all temperatures, with the measured viscosity values.