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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 present a Raman study of the pressure induced structural phase transitions in the tungstates Caw04 and SrWO4. Raman spectra up to 24 GPa at room temperature reveal reversible pressure induced phase transitions at 10 and 11.5 GPa for Caw04 and SrW04, respectively. The lowest B, mode, associated with the wo4-wo4 vibration along the c-axis, exhibits a softening in the scheelite structure, while in the high-pressure phase its slope becomes positive. Above the phase transition the modes originating from the antisymmetric v3(E,) and vg(Bg) stretching modes of the W 0 4 group exhibit a frequency jump to higher and lower frequencies. In the high pressure phase the modes originating from v3 show an unusual, strongly nonlinear softening in a small pressure region before becoming almost pressure independent.

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