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Brillouin and Raman scattering experiments have been performed on vitreous silica samples in order to check the validity of several models proposed for the interpretation of the low-frequency part of the Raman spectrum of glasses including the so-called ``boson peak.'' Samples of various origins, various fictive temperatures, and impurities contents as well as irradiated silica have been considered. It is demonstrated that the softening of acoustic modes responsible for the variation of the location of the boson peak maximum ${\mathrm{\ensuremath{\omega}}}_{\mathrm{max}}$ is not fully accounted by the variation of the sound velocity V. This indicates that the characteristic length l (related to V and ${\mathrm{\ensuremath{\omega}}}_{\mathrm{max}}$ in several models) is not a constant. Moreover, a clear quantitative correlation between the hypersound attenuation of the phonons and the low-frequency Raman intensity is evidenced. \textcopyright{} 1996 The American Physical Society.

Silicalite, a high-silica zeolite with MFI topology, undergoes pressure-induced amorphisation (PIA) when compressed with nonpenetrating pressure media. PIA is prevented by penetrating pressure media. In several zeolites, links have recently been found between pressure-induced phase transitions and the flexibility window, a geometric property of the framework identified by geometric simulation. We have analyzed structural data from compression experiments on silicalite, and we find that PIA occurs while the structure lies within its flexibility window. Penetrating media, which prevent PIA, push the structure outside the flexibility window. Thus framework flexibility is required for PIA to occur in silicalite. This link between amorphisation and flexibility is further evidence of the deep connections between framework geometry, flexibility, and the physical properties of zeolites.

In spite of intensive experimental and theoretical studies, no model at the atomic scale has been proposed to explain the large piezoelectric effect in ${\text{PbZr}}_{0.52}{\text{Ti}}_{0.48}{\text{O}}_{3}$ (PZT) compared to the low piezoelectric response in the simple end-member lead titanate ${\text{PbTiO}}_{3}$. X-ray absorption spectroscopy (XAS) appears as the technique of choice not only to clarify the role of Zr, but also to quantify the Zr displacement through the Ferroelectric-Paraelectric $(F\text{\ensuremath{-}}P)$ transition. We clearly show evidence of the polar character of the Zr-atoms in PZT with a Zr-shift which will produce a small polarization. Such an atomic configuration for one type of atoms leads to relatively easy switching, i.e., relatively low electric field to align the Zr-polar atoms, which will create a favorable energetic situation for the cooperative switching of the strongly polar Ti-O dipole and would therefore account for the large piezoelectric effect in PZT.

alpha-quartz-type Si1−xGexO2 solid solutions (x=0.06, 0.11, and 0.24) and pure SiO2 were investigated by Raman spectroscopy. Coupled and decoupled vibrational modes were identified at room temperature as a function of composition. Tetrahedral tilting librational modes involved in the displacive alpha beta-quartz phase transition are decoupled. The wave number of the coupled A1 mode located at 464 cm−1 for pure alpha-quartz was found to vary quite linearly with germanium content. Raman spectra were recorded up to 1473 K. Substitution of germanium in the quartz lattice clearly improves the thermal stability of the alpha phase. The alpha beta-quartz phase transition temperature increases from 846±1 K for x=0 to about 1300±50 K for x=0.24. At the same time, the dynamic disorder observed in quartz well below the transition is reduced in the solid solutions.

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., accepted in Physical Review B