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The vibrational properties of double-walled carbon nanotubes (DWNTs) is investigated by high-pressure resonance Raman scattering up to 30 GPa in two different pressure-transmitting media (PTM): paraffin oil and NaCl. The protection effect on the outer tube during compression is verified .The collapse of DWNTs is experimentally observed for the first time, showing to be two-step: the onset of the outer 1.56 nm diameter tube collapse at ∼21 GPa is followed by the collapse of the inner 0.86 nm diameter tube at a higher pressure of ∼25 GPa. This observation is supported by calculations. We show that filling a tube with another tube leads to a pressure stabilization against collapse, in strong opposition to what is observed when filling a tube with fullerenes or iodine. The collapse pressure in DWNTs appears to follow a 1/d tav 3 law, where d tav is the average diameter from the inner and outer tubes, in agreement with predictions [ Yang X. Yang X. Appl. Phys. Lett.200689113101]. Contrary to SWNTs and peapods, for DWNTs, the observed collapse pressure is independent of the PTM nature. Those differences are discussed in terms of tube filling homogeneity and of the separate roles of inner and outer tubes: the outer tube offers chemical screening to the inner tube, whereas the inner tube guarantees mechanical support to the outer one. This leads to high collapse pressure independent of the DWNT environnment: a characteristic that makes DWNTs ideal fillers for composite nanomaterials for high load mechanical support.

Vibrational properties and the temperature-induced phase transition mechanism have been studied in [NH4][Zn(HCOO)3] and [ND4][Zn(DCOO)3] metal organic frameworks by variable-temperature dielectric, IR, and Raman measurements. DFT calculations allowed proposing the detailed assignment of vibrational modes to respective motions of atoms in the unit cell. Temperature-dependent studies reveal a very weak isotopic effect on the phase transition temperature and confirm that ordering of ammonium cations plays a major role in the mechanism of the phase transition. We also present high-pressure Raman scattering studies on [ND4][Zn(DCOO)3]. The results indicate the rigidity of the formate ions and strong compressibility of the ZnO6 octahedra. They also reveal the onset of a pressure-induced phase transition at about 1.1 GPa. This transition has strong first-order character, and it is associated with a large distortion of the metal formate framework. Our data indicate the presence of at least two nonequivalent formate ions in the high-pressure structure with very different C-D bonds. The decompression experiment shows that the transition is reversible.

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