Phase transformations in amorphous fullerite C 60 under high pressure and high temperature P.A. Borisova a , M.S. Blanter b,n , V.V. Brazhkin c , V.A. Somenkov a , V.P. Filonenko c a National Research Centre “Kurchatov Institute”, Moscow, Russia b Moscow State University of Instrumental Engineering and Information Science, Moscow, Russia c Institute for High Pressure Physics, Troitsk, Moscow Region, Russia article info Article history: Received 24 October 2014 Received in revised form 26 March 2015 Accepted 5 April 2015 Available online 6 April 2015 Keywords: Amorphous materials Fullerenes High pressure Neutron scattering Phase transitions abstract First phase transformations of amorphous fullerite C 60 at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crys- talline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С 60 molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous- like or crystalline graphite were formed at lower temperatures than in crystalline fullerite. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Fullerites, being metastable, undergo a great number of phase transformations at heating under pressure, forming polymerized crystalline phases, amorphous or crystalline graphite and diamond [1–17]. Carbon phases obtained by this way possess hardness and elasticity, which are essentially different from properties of phases obtained using different methods [13]. The transformations in crystalline fullerite С 60 were sufficiently well studied by ex-situ methods, and a kinetic transformation diagram in pressure-tem- perature coordinates was constructed [2,5,13]. Of interest would be studies of irreversible phase transforma- tions in amorphous fullerites at high temperature and at high pressure. These data are practically absent, except for a few works [15–18] limited to low pressures ( r0.07 GPa) and to elevated pressures at ambient temperature. It was shown [15] that the amorphous fullerite С 60 does not crystallize when it is heated without application of pressure, but transforms into intermediate amorphous phase or into amorphous-like (sp 2 modification) gra- phite. Application of pressure prevents the formation of this in- termediate phase [15,17]. In this work phase transformations in amorphous fullerite С 60 and the aforementioned intermediate amorphous phase [15] under pressures up to 8 GPa were studied using neutron diffrac- tion and Raman spectroscopy. In some samples these studies were supplemented by laboratory and synchrotron X-rays diffraction measurements. We constructed the kinetic diagram of phase transformations in temperature–pressure coordinates and com- pared the features of the transformations in amorphous fullerite and crystalline fullerite. 2. Experimental methods Our initial samples of С 60 fullerenes of 99.5% purity produced by “NeoTechProduct” were obtained by the high-temperature treatment of graphite, followed by isolation with organic solvents and subsequent chromatographic separation. The neutron dif- fraction patterns of our initial crystal fullerites corresponded to the fcc lattice with a period а ¼ 1.416 nm. Amorphous fullerites were prepared by mechanical grinding of weighed crystal С 60 fullerite (  1–1.5 g) in a Fritsch mill at low rates over long periods of time (up to 100 h) until the structural changes observed with neutron diffraction ceased. Stepwise annealing of amorphous С 60 fullerite powder was performed in a high-temperature vacuum furnace up to 1773 K for 5 min in each cycle, followed by cooling to room temperature. Vacuum heating under low pressure (0.07 GPa) was performed by electric pulse plasma sintering in a graphite mold at temperatures ranging from 1273 to 1623 K using the SPS_625 spark plasma Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jpcs Journal of Physics and Chemistry of Solids http://dx.doi.org/10.1016/j.jpcs.2015.04.001 0022-3697/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: mike.blanter@gmail.com (M.S. Blanter). Journal of Physics and Chemistry of Solids 83 (2015) 104–108