Pressure-induced phase transitions in iron-ﬁlled carbon nanotubes: X-ray diffraction studies Sukanta Karmakar, 1 Surinder M. Sharma, 1 P. V. Teredesai, 2 and A. K. Sood 2,3 1 Synchrotron Radiation Section, Bhabha Atomic Research Centre, Mumbai 400085, India 2 Department of Physics, Indian Institute of Science, Bangalore 560012, India 3 Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientiﬁc Research Centre, Jakkur Campus, Jakkur, Bangalore 560 064, India High-pressure x-ray-diffraction studies have been carried out upto 20 GPa on iron-ﬁlled multiwalled carbon nanotubes MWNTs. The pressure dependence of the intertubular spacing d 0 of the ﬁlled MWNTs shows a sharp change at 9 GPa which is not observed in pristine MWNTs. The iron present as nanowires inside the MWNT is in the form of -Fe and Fe 3 C. Both of these phases show higher compressibility than their bulk form. Most interestingly, the structural modiﬁcation of MWNTs at 9 GPa coincides with an iso-structural phase transition in the encapsulated Fe 3 C, in sharp contrast to the absence of a transition in the bulk Fe 3 C upto 70 GPa. INTRODUCTION Carbon nanotubes are amongst the most exciting new ma- terials being investigated because of their potential uses in new technologies and devices exploiting their unusual me- chanical and electrical properties. 1,2 In particular, multiwall carbon nanotubes MWNTs are of interest to the growing microﬂuidic and nanoﬂuidic industry. 3 These MWNTs are composed of several concentric cylindrical graphene tubules, with an intertube separation d 0 of 3.4 to 3.9 Å, which increases with decreasing radii. 4 Recently the synthesis of various metal ﬁlled carbon nanotubes has also been achieved successfully. 5–8 There have been a number of theoretical and experimental studies related to elasticity, strength, and tough- ness of MWNTs. 9–14 High-pressure x-ray-diffraction experi- ments on pristine MWNTs show that these nanotubes be- come partly amorphous when compressed above 8 GPa. 15 In addition, recent Raman scattering investigations on MWNTs show a small change in slope of the high-frequency tangen- tial modes at 1 GPa, which has been attributed to the re- versible ﬂattening of the nanotubes. 16 While the metal ﬁlling does not signiﬁcantly change shape and size of the nano- tubes, it can affect the mechanical properties signiﬁcantly. Molecular dynamics simulations 17 have shown that the buck- ling force of single-walled carbon nanotubes SWNTs is in- creased when ﬁlled with C 60 , CH 4 , and Ne. However, there is no high-pressure experimental study so far to understand the effect of ﬁlling on the elastic properties and stability of single-walled as well as multiwalled carbon nanotubes. In addition, such experiments will also help to understand the high pressure behavior of the nanocrystalline metallic wires or particles which are formed inside the nanotubes. With a view to understand the effect of metal ﬁlling on MWNTs, we have carried out high-pressure angle dispersive x-ray- diffraction experiments on pure and Fe-ﬁlled MWNT. A sharp change is seen in the intertubular distance d 0 in Fe- ﬁlled MWNTs at 9 GPa, in sharp difference to the pristine MWNTs. Encapsulated iron in the nanotubes is in the form of -Fe and Fe 3 C. 7,18 The pressure behavior of these nanocrystalline forms of -Fe and Fe 3 C are investigated and are shown to be very different from their bulk counter parts. EXPERIMENTAL DETAILS Fe-ﬁlled multiwall carbon nanotubes prepared by pyroly- sis of ferocene along with acetylene using a two-stage fur- nace are same as described in Ref. 7. TEM studies show the presence of nanowires encapsulated inside carbon nanotubes. 7 High-resolution electron microscope image Fig. 2 of Ref. 7 shows that there is no free space between the metal nanowire and the carbon nanotube. The nanowires show a distribution in their diameter and length, the diameter being in the range of 10–20 nm and the length in the 200– 800 nm range. In addition to the nanowires, a small portion of iron nanoparticles, with 20– 40 nm diameter, covered with graphite layer were also found. For the purpose of high pressure experiments Fe-ﬁlled MWNTs along with a few specs of gold were loaded in a hole of 120 m diameter drilled in a preindented 70 micron steel gasket of a Mao-Bell kind diamond-anvil cell DAC. Methanol:ethanol:water16:3:1 mixture was used as pressure transmitting medium which provides hydrostatic pressure environment until 15 GPa. The pressure was de- termined from the known equation of state of gold. 19 High- pressure angle dispersive x-ray-diffraction experiments, were carried out up to 20 GPa, at the 5.2 R beamline of Elettra Synchrotron source with monochromatized x rays (  =1.0 Å). The diffraction patterns were recorded using MAR345 imaging plate detector kept at a distance of 21 cm from the sample. Two-dimensional imaging plate records were transformed to one-dimensional diffraction proﬁles by the radial integration of diffraction rings using the FIT2D software. 20 Experiments on pure MWNTs were carried out using a laboratory x-ray-diffraction source Mo K  ) along with an imaging plate and ruby pressure marker. RESULTS AND DISCUSSION Figure 1 shows the x-ray-diffraction proﬁle of the iron ﬁlled multiwall carbon nanotubes at 0.3 GPa. The diffraction