The high-pressure behavior of polyiodides confined into the hollow core of single-walled carbon nanotubes organized into bundles has been studied by means of Raman spectroscopy. Several regimes of the structural properties are observed for the nanotubes and the polyiodides under pressure. Raman responses of both compounds exhibit correlations over the whole pressure range (0–17 GPa). Modifications, in particular, take place, respectively, between 1 and 2.3 GPa for polyiodides and between 7 and 9 GPa for nanotubes, depending on the experiment. Differences between one experiment to another are discussed in terms of nanotube filling homogeneity. These transitions can be presumably assigned to the tube ovalization pressure and to the tube collapse pressure. A nonreversibility of several polyiodide mode modifications is evidenced and interpreted in terms of a progressive linearization of the iodine polyanions and a reduction in the charged species on pressure release. Furthermore, the significant change in the mode intensities could be associated to an enhancement of lattice modes, suggesting the formation of a new structure inside the nanotube. Changes in the nanotube mode positions after pressure release point out a decrease in the charge transfer in the hybrid system consistent with the observed evolution of the charged species.

Similarly to preformed polymeric phases such as 1D-Pmnn [O] or 2D-Immm [T], and in contrast to monomeric FCC C60, dimeric C60 [D or R'] is expected to yield new polymerized phases of higher dimensions when subjected to high pressure. We have carried out an in-situ high-pressure powder diffraction experiment in a diamond anvil cell using MoKagr laboratory X-rays, on a pure dimeric sample previously carefully characterized at ambient pressure using synchrotron X-ray HR XRPD and Raman spectroscopy. A transition is indeed observed from 1.2 to 8.6 GPa. A robust analysis shows that the volume per C60 unit decreases from 689 Aring3 (0 GPa) to 550 Aring3 (8.6 GPa), resulting in a 20% effect. The new phase can no longer be described by a cubic (FCC) unit cell, whereas it can now be indexed and LeBail-refined using a trigonal/rhombohedral unit cell predicted by Dzyabchenko et al. in 1999.

“Geodesic Polyarenes” belong to a new class of carbon materials intermediate between carbon nanotube and graphene. As a matter of fact these molecules are “bowls shaped” polycyclic aromatic hydrocarbon with curved convex or concave surfaces, composed by a carbon skeleton and “the edge of the bowls” is terminated by hydrogen atoms. Different molecules present different curvatures related to the number and the distribution of five numbered and six membered rings. Like flat PAH geodesic polyarenes form crystalline materials exhibiting different structures and intermolecular interactions. The general aim of our work is to bring a better understanding of these new molecules, particularly focusing on the influence of the molecules curvature and size on their structural organization (for flat PAH, it has been demonstrated that the size and shape of the molecule is influencing the crystalline organization) and on their dynamics. Thus infra-red absorption, neutron inelastic scattering have been performed between 10 and 300K in order to understand the dynamics in these crystalline materials. High pressure Raman and X-ray diffraction have also been performed on these crystalline materials in order detect polymorphism associated to possible molecular reorientation.

Résumé (à complèter)

We demonstrate that while the metastable face-centered cubic (fcc) phase of Ge2Sb2Te5 becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. Upon higher compression, a body-centered cubic phase is obtained in both cases around 30 GPa. Upon decompression, the amorphous phase is retained for the starting fcc phase while the initial structure is recovered for the starting trigonal phase. We argue that the presence of vacancies and associated subsequent large atomic displacements lead to nanoscale phase separation and loss of initial structure memory in the fcc staring phase of Ge2Sb2Te5.