Dynamical studies of an aqueous electrolyte in the supercooled liquid and glassy states

Some interesting aspects of the dynamical behavior of aqueous electrolyte solutions have been summarized. They are based on the ability of these systems to form glasses providing ‘ideal’ conditions to study the glass transition phenomena and to characterize the different thermodynamical states in connection with the non-linear behavior of the static properties. Structural relaxation analysis and collective vibrational dynamics are discussed pointing out their connection with the state of the system. The existence of two different scaling behaviors of the relaxation time connected to the change in the multiplicity of microscopic states is given in addition, with some information on the intramolecular stretching vibrations at the glass transition.     

Relaxational features of supercooled and glassy m-toluidine

Recent investigations performed by dielectric and mechanical spectroscopy have provided detailed information about the dynamical response of m-toluidine in the supercooled and glassy states. In the present work we analyze the properties of the α-relaxation process on the basis of dielectric and shear mechanical data, discussing the unusual behavior of this fragile glass-former. The nature of the secondary relaxation is elucidated by high resolution dielectric measurements and discussed in terms of the Coupling Model and literature results on other small molecule glass-forming liquids.     

Molecular dynamics simulation of viscosity in supercooled liquid and glassy AgCu alloy

The viscosity of the model AgCu alloy is simulated by several methods using (i) correlation functions through the Green–Kubo formalism, (ii) a non-equilibrium molecular dynamics approach and (iii) creep tests under constant stress. Temperature dependences of the shear viscosity and the diffusion coefficient show the breakdown of the Stokes–Einstein relation well above the glass-transition temperature T_g. This observation is interpreted as a manifestation of the development of heterogeneities in the supercooled liquid approaching T_g. Based on a generalized Einstein formula for the viscosity of liquid, a temperature dependence of heterogeneity degree of supercooled liquid is estimated. Using the dependence of the deformation rate on external stress, the activation volume is evaluated to be four atomic volumes in liquid state. However, below the mode-coupling temperature T_c the activation volume increases by several times.     

Mössbauer studies of glasses and supercooled liquids

Several examples of Mössbauer spectra taken from amorphous materials are presented. The purpose is to help assess whether this technique is likely to be useful in the determination of structure for non-crystalline solids. One conclusion is that, except for iron, the resolution is poor and unique interpretations are unlikely. A second conclusion is that in a glass formed from FeCl₂·9H₂O the microenvironment is very uniform, as might be expected from a continuous-random-network model.     

Devitrification of Ni-based glassy alloys containing noble metals in relation with the supercooled liquid region

The formation of the supercooled liquid region and devitrification behavior of Ni-based glassy alloys were studied by using X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and isothermal calorimetry. oC68 Ni₁₀Zr₇-type phase is primarily formed in the studied alloys in the initial stage of the devitrification process by nucleation and three-dimensional diffusion controlled growth. The replacement of Cu by Ni in Cu₅₅Zr₃₀Ti₁₀Pd₅ glassy alloy induces precipitation of oC68 Ni₁₀Zr₇ phase directly from the glassy phase. The reasons for such a behavior are discussed taking into account mixing enthalpy in a liquid state and the interval of the supercooled liquid region.     

Radial distribution studies of glassy tellurium-silicon alloys

X-ray investigations of glassy tellurium-silicon alloys (Si content 10–40 at%) are described. Alloys with 13 to 27 at% Si were obtained in bulk form, non-crystalline samples with 10, 30, 33, 36 and 40 at% Si were obtained by rapid quenching from the melt by the splat-cooling technique. The interference functions and the radial distribution functions of all alloys show a great similarity. However, the positions of the maxima of the pair distribution function decrease continuously with growing Si content and lead to values which are similar to atomic distances in crystalline Te and Si₂Te₃. The areas of the first two peaks of the RDF, F₁ (rougnly 2) and F₂ (roughly 12), are compared with calculated areas derived from different structural models which are based on the short-range order in crystalline Te with a chain structure and in Si₂Te₃ with a tetrahedron configuration. An interstitial model with Si atoms in holes of Te chains and a random substitution model, where the Te atoms and the Si atoms have the same short-range order, both show a concentration dependence of the areas F₁ contrary to the experimental values. A structural model which assumes the tetrahedron configuration of Si₂Te₃ for Si atoms (coordination number N_Si = 4) and a coordination number N_Te = 2 for Te atoms can describe the short-range order of glassy TeSi alloys in the whole investigated concentration range with 10–40 at% Si.     

H NMR in μc-Si:H

NMR, IR, ESR, Raman scattering and X-ray diffraction measurements were performed in μc-Si:H prepared by various methods. Results of H NMR in some films are qualitatively similar to those i n a-Si:H, but the NMR lines exhibit a motional narrowing. Other films which exhibit sharp IR peaks exhibit H NMR signal shape different from that in a-Si:H.     

Preparation and NMR studies of cesium hydroselenite

This work is a part of the systematic study of the structure and the physical properties of alkaline hydroselenite crystals with the general formula MeHSeO₃, where Me is an alkali ion. The characteristic feature of this family is a net of hydrogen bonds between SeO₃ groups forming closed [HSeO ₃ ^? ]₂ dimers.     

EXAFS studies of glassy and liquid ZnCl2: A comparison with vitreous GeO2

The Zn-EXAFS (extended X-ray absorption fine structure) above its K-absorption edge has been measured for glassy ZnCl₂ at low temperature, through T_g (375 K), and into the supercooled and normal liquid state (mp = 593 K) at 673 K. By Fourier filtering and fitting the normalized glass spectra using α-ZnCl₂ as a model compound, the Zn²⁺Cl^? distance was determined to be (2.34 ± 0.01) Å and the average coordination number about the Zn²⁺ was found to be 5.1 ± 0.8. The latter value agrees with the value of 4.7 nearest neighbors obtained by the molecular dynamics computer simulation study of Woodcock et al., for liquid ZnCl₂ just above its melting point. The agreement is supportive of the notion that short-range order in the glass is reflective of that of the corresponding liquid from which it was quenched. Spectral measurement as a function of temperature indicates that there is considerable dynamic decoupling of the Zn²⁺ from its nearest Cl^? neighbor even below T_g. This is contrasted with similar data in glassy GeO₂ which show that the motion of Ge is strongly coupled with its four oxygen neighbors all the way to T_g. This difference in dynamic disorder substantiates the notion that glassy ZnCl₂, as well as vitreous BeF₂, provides a further weakened structural analog for glassy GeO₂ and SiO₂.     

Ion transport studies in CuI-doped silver borovanadate glassy system

In the present report, ionic transport properties and microstructural investigations of superionic materials in a cost-effective glassy system xCuI–(100 ? x)[2Ag₂O–0.7V₂O₅–0.3B₂O₃], where x = 30, 40, 45, 50 and 60, have been described. The temperature dependent electrical conductivity studies were carried out by ac impedance analysis. The microstructure of the materials studied by SEM indicated the presence of dispersed CuO and AgI micro-crystals in the silver oxysalt glass matrix. High room temperature electrical conductivity of 3.58 × 10^?3 S cm^?1 and low activation energy of 0.24 eV were obtained for the best conducting composition. The ac impedance data were analyzed using impedance and modulus formalisms. These materials show extremely high t_i value of 0.999 and the ionic conductivity is apparently due to Ag⁺ ions only. The use of two glass formers helped to form materials with higher T_g, higher thermal stability and better ionic transport properties.     

A low-frequency Raman study of glassy,supercooled and molten silica and the preservation of the Boson peak in the equilibrium liquid state

A temperature dependent Raman spectroscopic study of the Boson peak in silica is being presented. For the first time experimental data are obtained not only above the glass transition temperature but also above the melting point. The intriguing temperature dependencies exhibited by the Boson peak frequency and intensity are examined in detail. A comparison of the above features between the frequency-reduced and the true excess density of vibrational states revealed that the spectrum of the latter does not exhibit the anomalous trend of the Boson peak as regards the temperature dependence of its frequency, and further the number of the excess modes decreases with increasing temperature. The observability of the Boson peak in the normal liquid state as a well-resolved peak is also discussed in the context of recent theoretical and simulation works that relate the Boson peak with the details of the potential energy landscape of the supercooled liquid.     

Proton NMR studies of smectic-C phases

We present proton NMR second moments studies of some compounds exhibiting smectic-C phases. From the comparison of experimental and theoretical angular dependences of the second moments the tilt angles δ are derived. The ratio f of the second moments of samples with statistical distribution of domain orientations and of perfectly aligned samples is calculated and compared with experimental data. For certain substances with constant tilt here are significant deviations from the theoretically expected values suggesting that the underlying theoretical model has still to be modified.     

NMR study of μc-Si:H

The behavior of hydrogen in glow-discharge (GD) μc-Si:H has been characterized by ¹H NMR. The ¹H spectra consist of two components with different linewidths. The linewidth (FWHM) of the narrow component is about 0.5 kHz at ν₀ = 90 MHz, being much narrower than has been observed in GD a-Si:H deposited under a conventional low RF-power condition. It has been demonstrated that the 0.5-kHz FWHM component originates from the hydrogens in motional narrowing state, and such fast-moving hydrogens are incorporated both in μc-Si:H and high-power deposited a-Si:H.     

X-Ray diffraction and IR-spectroscopic studies of UO2(n-C3H7COO)2(H2O)2 and Mg(H2O)6[UO2(n-C3H7COO)3]2

Single crystals of UO₂(n-C₃H₇COO)₂(H₂O)₂ (I) and Mg(H₂O)₆[UO₂(n-C₃H₇COO)₃]₂ (II) are synthesized. Their IR-spectroscopic and X-ray diffraction studies are performed. Crystals I are monoclinic, a = 9.8124(7) Å, b = 19.2394(14) Å, c = 12.9251(11) Å, β = 122.423(1)°, space group P2₁/c, Z = 6, and R = 0.0268. Crystals II are cubic, a = 15.6935(6) Å, space group $Pa\bar 3$ , Z = 4, and R = 0.0173. The main structural units of I and II are [UO₂(C₃H₇COO)₂(H₂O)₂] molecules and [UO₂(C₃H₇COO)₃]^? anionic complexes, respectively, which belong to AB ₂ ⁰¹ M ₂ ¹ (I) and AB ₃ ⁰¹ (II) crystal chemical groups of uranyl complexes (A = UO ₂ ²⁺ , B ⁰¹ = C₃H₇COO^?, and M ¹ = H₂O). A crystal chemical analysis of UO₂ L ₂ · nH₂O compounds, where L is a carboxylate ion, is performed.     

SAXS and DSC studies on the structural characteristics of siliconized s-triazine glassy hybrid materials

Three hybrid materials composed of planar s-triazine rings and polyhedral silica (SiO₂), phenylsilsesquioxane (PhSiO_1.5) and diphenylsiloxane (Ph₂SiO) building blocks were investigated by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) techniques. These measurements revealed that the geometrically dissimilar components were fully integrated into intact glassy hybrid structures. Their DSC thermograms showed that these hybrids are thermally stable below 350 °C with moderate glass transition temperatures (T_g) of 56–110 °C consistent with the increasing structural connectivity of the silicone component. The SAXS data was analyzed to obtain different structural information using Porod, Guinier and Kratky approximations. The general features of each of the SAXS profiles of these hybrids are very similar to those of polyphenylsilsesquioxane (PPhSQ). The SAXS profiles reveal that these hybrids can be described as nano-scale primary particles that are self-organized in macromolecular ensembles to form extended unfolded textures of varying scattering lengths (91–168 Å). The obtained hybrid particles adopt either 3-D bulk fractals with open structures or 2-D surface fractals with dense cores. The short interfacial thickness (< 3 Å) and the low thermal fluctuation parameters strongly suggest that these particles are held together by substantial cohesion forces.     

10B NMR studies of lithium borate glasses

¹⁰B NMR studies have been carried out on lithium borate glasses over the entire glass-forming region of the system. Using the ideas of Krogh-Moe, the relative abundances of the various structural groupings are inferred from the data by fitting computer-simulated lineshapes to the experimental spectra. Structural model are proposed which are consistent with the data: for 0.4 R < 1.0, where R = mol% Li₂O/mol% B₂O₃, the model states that diborate and tetraborate units are proportionately destroyed to form metaborate units and loose BO₄ units; for 1.0 < R < 1.86, the model states that metaborate units and loose BO₄ units are destroyed linearly but not proportionately to form pyroborate and orthoborate units. These results are compared with earlier ¹¹B NMR studies from this laboratory.