In situ measurements of density fluctuations and compressibility in silica glass as a function of temperature and thermal history

In this paper, small-angle X-ray scattering measurements are used to determine the different compressibility contributions, as well as the isothermal compressibility, in thermal equilibrium in silica glasses having different thermal histories. Using two different methods of analysis, in the supercooled liquid and in the glassy state, we obtain respectively the temperature and fictive temperature dependences of the isotheraml compressibility. The values obtained in the glass and supercooled liquid states are very close to each other. They agree with previous determinations of the literature. The compressibility in the glass state slightly decreases with increasing fictive temperature. The relaxational part of the compressibility is also calculated and compared to previous determinations. We discussed the small differences between the different determinations

Influence of thermal history on the structure and properties of silicate glasses

We studied a set of float glass samples prepared with different fictive temperature by previous annealing around the glass transition temperature. We compared the results to previous measurements on a series of amorphous silica samples, also prepared with different fictive temperature. We showed that the modifications on the structure at a local scale are very small, the changes of physical properties are moderate but the changes on density fluctuations at a nanometer scale are rather large: 12 and 20% in float glass and silica, for relative changes of fictive temperature equal to 13 and 25% respectively. Local order and mechanical properties of silica vary in the opposite way compared to float glass (anomalous behavior) but the density fluctuations in both glasses increase with temperature and fictive temperature

Dynamic sound attenuation at hypersonic frequencies in silica glass

In order to clarify the origin of the dominant processes responsible for the acoustic attenuation of phonons, which is a much debatted topic, we present Bril louin scattering experiments in various silica glasses of different OH impurities content. A large temperature range, from 5 to 1500 K is investigated, up to the glass transition temperature. Comparison of the hypersonic wave attenuation in various samples allows to identify two different processes. The first one induce s a low temperature peak related to relaxational processes; it is strongly sensitive to the extrinsic defects. The second, dominant in the hig h temperature range, is weakly dependent on the impurities and can be ascribed to anharmonic interactions

Characterization of the glass transition in vitreous silica by temperature scanning small-angle X-ray scattering

The temperature dependence of the x-ray scattering in the region below the first sharp diffraction peak was measured for silica glasses with low and high OH content (GE-124 and Corning 7980). Data were obtained upon scanning the temperature at 10, 40 and 80 K/min between 400 K and 1820 K. The measurements resolve, for the first time, the hysteresis between heating and cooling through the glass transition for silica glass, and the data have a better signal to noise ratio than previous light scattering and differential thermal analysis data. For the glass with the higher hydroxyl concentration the glass transition is broader and at a lower temperature. Fits of the data to the Adam-Gibbs-Fulcher equation provide updated kinetic parameters for this very strong glass. The temperature derivative of the observed X-ray scattering matches that of light scattering to within 14%.Comment: EurophysicsLetters, in pres

Minimal Surfaces, Screw Dislocations and Twist Grain Boundaries

Large twist-angle grain boundaries in layered structures are often described by Scherk's first surface whereas small twist-angle grain boundaries are usually described in terms of an array of screw dislocations. We show that there is no essential distinction between these two descriptions and that, in particular, their comparative energetics depends crucially on the core structure of their screw-dislocation topological defects.Comment: 10 pages, harvmac, 1 included postscript figure, final versio

Universal behavior of internal friction in glasses below T : anharmonicity vs relaxation

Comparison of the internal friction at hypersonic frequencies between a few K and the glass transition temperature Tg for various glasses brings out general features. At low temperature, internal friction is only weakly dependent on the material. At high temperature but still below Tg the internal friction for strong glasses shows a T-independent plateau in a very wide domain of temperature; in contrast, for fragile glass, a nearly linear variation of internal friction with T is observed. Anharmonicity appears dominant over thermally activated relaxational processes at high temperature.Comment: accepted in Physical Review

Chiral and herringbone symmetry breaking in water-surface monolayers

We report the observation from monolayers of eicosanoic acid in the L′2 phase of three distinct out-of-plane first-order diffraction peaks, indicating molecular tilt in a nonsymmetry direction and hence the absence of mirror symmetry. At lower pressures the molecules tilt in the direction of their nearest neighbors. In this region we find a structural transition, which we tentatively identify as the rotator-herringbone transition L2d−L2h

Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"

We consider a system of coupled classical harmonic oscillators with spatially fluctuating nearest-neighbor force constants on a simple cubic lattice. The model is solved both by numerically diagonalizing the Hamiltonian and by applying the single-bond coherent potential approximation. The results for the density of states $g(\omega)$ are in excellent agreement with each other. As the degree of disorder is increased the system becomes unstable due to the presence of negative force constants. If the system is near the borderline of stability a low-frequency peak appears in the reduced density of states $g(\omega)/\omega^2$ as a precursor of the instability. We argue that this peak is the analogon of the "boson peak", observed in structural glasses. By means of the level distance statistics we show that the peak is not associated with localized states

Evidence of tilted monolayer cybotactic groups in a partially bilayer SA phase

A study using X-ray diffraction for 4 nonanoate 4' cyanobiphenyl (8 COO CB) shows that the SA phase has a partially bilayer structure with layer thickness d = 1.4 L (L : molecular length). It also shows monolayer fluctuations inside this phase. Moreover, it points out that there exist skewed cybotactic groups of single molecules in the partially bilayer S A phase in the supercooled range of the SA phase. The size of these groups is about 500 molecules, the tilt angle is about 35°. A study using dielectric relaxation for this compound has also been performed. It shows that the activation energy for reorientational movement of the molecule has unusual behaviour. With decreasing temperature, this energy decreases to the N-SA transition but it increases in smectic supercooled range. Variations obtained are in agreement with information obtained from X-ray experiments.Une étude en diffraction de rayons X du 4 nonanoate 4' cyanobiphényle (8 COO CB) montre que la phase SA est de type partiellement bicouche avec une épaisseur de couche valant 1,4 L (L : longueur moléculaire). Elle met aussi en évidence des fluctuations monocouches dans la structure partiellement bicouche. De plus, elle montre que dans la gamme de température correspondant à la phase smectique en surfusion, des groupes cybotactiques monomoléculaires de structure SC apparaissent dans la phase SA. L'étendue des groupes est de l'ordre de 500 molécules et l'angle d'inclinaison des molécules est de 35° environ. Une étude en relaxation diélectrique sur ce composé a été faite. Elle indique que l'énergie d'activation du mouvement de réorientation de la molécule varie de façon inattendue. Lorsque la température décroît, l'énergie diminue à la transition N-SA mais elle augmente pour les températures correspondant à la phase SA en surfusion. Les variations relevées sont en accord avec les renseignements obtenus à partir des expériences de rayons X

Effect of the electric field on the elastic and dielectric properties of a lithium-potassium tantalate crystal

We have measured in a K$_{1-x}$Li$_{x}$TaO$_3$ crystal with x = 1.7 % the elastic constant with longitudinal acoustic waves of frequency between 30 and 330 MHz and the dielectric constant between 10 Hz and 1 MHz in the temperature range from 4 to 200 K under an electric field applied upon cooling. The main effect of the applied field on the dielectric constant is attributed to space charges. The real part of the elastic constant (the sound velocity) was found to increase when the field was applied perpendicular to the acoustic wave vector and to decrease when it was applied parallel to the wave vector. The effect of the cooling rate was also studied. These results are explained by the occurrence of clusters of oriented dipoles borne by the Li$^+$ ions, favoured by the field