We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous-to-crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed being compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

The polymer chain conformation under confinement and the morphology of the nanohybrid materials are investigated in hydrophilic polymer/layered silicate nanocomposites. A series of poly(ethylene oxide)/sodium montmorillonite hybrids were synthesized utilizing melt intercalation with compositions covering the complete range from pure polymer to pure clay. Intercalated nanocomposites with mono- and bilayers of PEO chains are obtained in all cases. The intercalated chains as well as the ones adsorbed on the outer surface of the clay particles remain purely amorphous; their conformation, however, exhibits different characteristics from those of the amorphous neat polymer melt. It is only for compositions where a large amount of excess polymer exists outside the completely full galleries that the polymer crystallinity is recovered.

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For the case example of GeTe, we demonstrate that the use of techniques that probe the average structure may lead to misleading conclusions regarding the nature of phase change. GeTe is a narrow band-gap semiconductor and a ferroelectric with the simplest conceivable structure that-according to previous studiesundergoes a ferroelectric-to-paraelectric displacive phase transition at ϳ705 K. In this work, we provide direct experimental evidence that, contrary to the existing paradigm, the local distortion remains essentially unchanged with temperature and argue that the previous conclusion about the displacive nature of the ferroelectric-to-paraelectric transition was due to misinterpretation of Bragg diffraction, a technique that is only sensitive to the average structure and does not "see" random local distortions. The reported results have far-reaching implications for other materials exhibiting displacive phase transitions where conclusions have been reached based on results obtained using averaging techniques.

The pressure response of double-wall carbon nanotubes has been investigated by means of Raman spectroscopy up to 10 GPa. The intensity of the radial breathing modes of the outer tubes decreases rapidly but remain observable up to 9 GPa, exhibiting a behavior similar (but less pronounced) to that of single-wall carbon nanotubes, which undergo a shape distortion at higher pressures. In addition, the tangential band of the external tubes broadens and decreases in amplitude. The corresponding Raman features of the internal tubes appear to be considerably less sensitive to pressure. All findings lead to the conclusion that the outer tubes act as a protection shield for the inner tubes whereas the latter increase the structural stability of the outer tubes upon pressure application.

We report on a detailed, temperature-dependent, off-resonant Raman scattering study of glassy and supercooled selenium. Raman spectra in the frequency regime of the first-order scattering (5-450 cm(-1)) have been recorded over a wide temperature range, i.e., 143-353 K. To facilitate the analysis, the spectra have intuitively been divided in three spectral regions. The analysis of the high frequency region (bond-stretching vibrational modes) yielded information on the rings-chains equilibrium. In particular, the polymer content was found to amount to more than 85% around the glass transition temperature, exhibiting a weak temperature dependence, which extrapolates nicely to the high-temperature dissolution data. The intermediate frequency range (representative of the medium-range structural order) was treated together with the low frequency regime (where low-energy excitations, i.e., the quasielastic line and the Boson peak are the dominant contributions) owing to their strong overlap. The study of the bond-bending regime revealed information which made it possible to clarify the role of ringlike and chainlike fragments incorporated in polymeric molecules. The temperature evolution of the Boson peak and the frequency dependence of the Raman coupling coefficient Comega were also determined. An attempt to decompose the partial contribution of the pure Boson peak to Comega revealed valuable information concerning the limiting (omega-->0) behavior of the coupling coefficient.

In this paper we demonstrate the ability of inelastic (Raman) light scattering to probe polymerization transitions. We show that after proper treatment—that is, separating isotropic and anisotropic contributions and employing the reduced representation which removes the thermal population effect of vibrational energy levels—the Raman data can be used as an accurate quantitative indicator of monomer↔polymer transitions. In particular, we have applied this method to study the thermoreversible polymerization transition of liquid sulfur up to 300 °C. Raman spectra obtained from rapidly quenched samples over a broad low-temperature range, from −180 °C to ambient temperature, revealed the fact that the equilibrium between monomers and polymers in the solid amorphous state is precarious; thus quench-and-dissolution methods employed to determine the polymer content of the liquid are not accurate. Our data are compared with existing data obtained via quench-and-dissolution techniques showing considerable dissimilarities above 250 °C. Our experimental data seem to conform better to the equilibrium polymerization transition theories than previously reported data especially in view of the lack of a high-temperature plateau value in the extent of polymerization, although complete agreement between experiments and theory is still missing. Finally, the width of the Raman peak assigned to polymeric sulfur vibrations has been used as a rough indicator of the size distribution of sulfur chains.

Present-day multimedia strongly relies on re-writable phase-change optical memories. We find that, different from current consensus Ge 2 Sb 2 Te 5 (GST), the material of choice in digital versatile discs-random access memory (DVD-RAM), possesses a structure similar to ferroelectric GeTe, namely that Ge and Sb atoms are located off-center giving rise to a net dipole moment. Amorphisation of both GeTe and GST results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in the short-range order. We demonstrate that the order-disorder transition in GeTe and GST is primarily due to a flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions cycles.