Brillouin scattering experiments have been performed from 5 to 1600 K in vycor, a porous silica glass. The acoustic velocity and attenuation at hypersonic frequencies are compared to those of bulk silica and others porous silica samples. The experimental evidence for the influence of porosity on the scattering by acoustic waves is compared to calculations. The correlation between internal friction and thermal conductivity at low temperature is discussed.

The structure of glasses is characterized by the existence of density and composition fluctuations on the nanometre scale. We present three examples of the use of small-angle X-ray scattering to get information about these density fluctuations. The thermal history and OH content were observed to have a huge influence. The static compressibility decreases when the OH content or fictive temperature increase. We showed that temperature scanning small-angle X-ray scattering can provide an accurate description of the position, width and shape of the glass transition.

Pressure dependence of the orthorhombic polymeric phase of C60 has been studied by angle-dispersive, X-ray powder diffraction in a diamond anvil cell. Transformation to a new phase through the simultaneous action of pressure and X-rays was observed between 0.2 and 1.66 GPa. A possible structure of this new phase of C60 is discussed. Further increase in pressure leads gradually to the irreversible formation of a disordered phase.

The high-pressure stability of agr-quartz-type materials is of considerable interest for materials (piezoelectrics etc.) and Earth science. Representative compositions of two agr-quartz type solid solutions SiO2-PON and AlPO4-GaPO4 were studied at high pressure by X-ray diffraction. These materials exhibit distinct high pressure behaviour as the diffraction lines of agr-quartz-type Si0.56P0.44O1.56N0.44 were found to decrease in intensity above 23 GPa with the pattern becoming completely featureless above 40 GPa, whereas Al0.30Ga0.70PO4 transforms to a poorly-crystallised phase above 12 GPa. This difference in behaviour can be linked to the initial degree of positional disorder in the two solid solutions. A highly disordered, possibly amorphous, phase is favoured in the SiO2-PON system, in which positional disorder is present on all cation and anion sites. In contrast, disorder affects only frac12 of the cation sites in the AlPO4-GaPO4 system. A high degree of disorder, particularly on the site occupied by phosphorus, plays a role in inhibiting the formation of ordered phases with increased cation coordination.

he objective of this work is to compare density and density fluctuations of silica as a function of temperature in the anomalous domain between 950 and 1480°C via Archimedes' macroscopic measurements and light scattering. Parallel qualitative behaviour is also recorded between density fluctuations and macroscopic density. Density fluctuations of silica show a minimum as a function of temperature, as with macroscopic density. This correlation indicates a possible common origin of both observations.

The fictive temperature of vitreous silica containing approximately 900 wt ppm of hydroxyl groups was monitored with small angle x-ray scattering. The measurements were carried out during annealing and while scanning the temperature, with annealing temperatures ranging between 930 and 1330 K. Fitting the data to the Adam-Gibbs-Fulcher equation by using the Tool-Narayanaswamy method yields a particularly simple thermorheological behavior for type-III vitreous silica. Unlike the general case for glass kinetics, including vitreous silica with low hydroxyl content, the relaxation time constant is nearly decoupled from the fictive temperature. This high degree of decoupling of the state of the glass and the relaxation rate agrees with the results of viscosity measurements. By improving the data analysis procedure, we have significantly increased the precision of the results, and it was possible to resolve changes of the activation energy of the relaxation processes to within 0.5%. This has made sample aging effects that had previously been undetectable visible.