Structural and phonon properties of bundled single- and double-wall   carbon nanotubes under pressure
  In this work, we report a theoretical coupled study of the structural and phonons properties of bundled single- and double-walled carbon nanotubes (DWNTs), under hydrostatic compression. Our results confirm drastic changes in volume of SWNTs in high-pressure regime as assigned by a phase transition from circular to collapsed phase which are strictly dependent on the tube diameter. For the DWNTs, those results show first a transformation to a polygonized shape of the outer tube and subsequently the simultaneous collapse of the outter and inner tube, at the onset of the inner tube ovalization. Before the DWNT collapse, phonon calculations reproduce the experimentally observed screening effect on the inner tube pressure induced blue shift both for RBM and tangential G$_z$ modes . Furthermore, the collapse of CNTs bundles induces a sudden redshift of tangential component in agreement with experimental studies. The G$_z$ band analysis of the SWNT collapsed tubes shows that the flattened regions of the tubes are at the origin of their G-band signal. This explains the observed graphite type pressure evolution of the G band in the collapsed phase and provides in addition a mean for the identification of collapsed tubes 
Acrisio. L. Aguiar, Rodrigo B. Capaz, A. G. Souza Filho, Alfonso   San-Miguel
cond-mat.mes-hall cond-mat.mtrl-sci