SUPERHARD PHASES OF C₆₀- AND C₇₀ - FULLERITES

V.D. Blank ^1,2, N.R. Serebryanaya ^1,2, G.A. Dubitsky ², S.G. Buga ^1,2, S.N. Sulyanov ³.

¹ Institute of Spectroscopy of RAS, 142092 Troitsk, Moscow region, Russia.
² Research Centre for Superhard Materials of Ministry for Science and Technologies of Russia,7-a Centralnaya Str., Troitsk, Moscow region, 142092 Russia.
³ Shubnikov Institute of Crystallography of RAS, Leninsky pr. 59, Moscow, 117333 Russia.

Keywords: C₆₀ - and C₇₀ -fullerites, high pressure, high temperature, phase transitions, hardness.

Superhard phases of C₆₀ have been found in the material synthesized in the toroid-type apparatus at high pressures (up to 13 GPa) and high temperatures (up to 2100 K) [1]. It has been revealed that new carbon states on basis of C₆₀ have polymerised structures and exhibit very interesting physical properties such as hardness eceeding diamond one, semiconductor properties and some others. A search of new superhard fullerene derivates forces to pay attention to C₇₀ as the closest to C₆₀. The comparison of structures of the C₆₀ and C₇₀ hard phases is represented.

The X-ray powder diffraction method and diffractometer Kard-6 with a flat proportional chamber on the fast delay lines are applied for the investigation of crystal structures of these phases. The Cu Ka-radiation and graphite monochromator are applied.

Two states (amorphous and crystalline) have been found in samples of the both kinds of fullerites. Crystalline states of the both fullerites are most alike at low pressures up to 8 GPa. The crystal structures are rhombohedral (R3 m) with a small rhombohedral distortion (a~56⁰) for the both fullerites (Fig. 1). The diffraction patterns are poor, with few broad peaks, it is a two-dimensional (2D)-polymerised structure. The hardness is equal to that of glass. At the low temperatures up to 720 K the rhombohedral structure of C₇₀ has no changes within the limits of precision at any pressure. The rhombohedral structure of C₆₀ exists up to 8 GPa only.

Fig. 1. Diffraction patterns of C₆₀- and C₇₀-samples after treatement at 8 GPa and different temperatures.

At P>=9.5 GPa and T>=750 K there are different structures for C₆₀ and C₇₀: quazy-face-centred cubic (with orthorhombic distortion) and tetragonal respectively (Fig.2). Diffraction peaks are very broad, particularly for C₇₀-phases (FWHM>0.9^o). There are three-dimensional (3D)-polymerised structures for the both fullerites, the shortest distanse between molecular centers attaines the value 1.6-1.5 Å. AT P>12.5 GPa, the unit-cell cubic C₆₀- parameter is equal to 12.1 Å, and the analysis of intensities of this structure of C₆₀ supposes the formation of the second fullerite-like cluster in the octahedral cavities of fcc - structure. The unit-cell parameters of all crystalline phases of both fullerites are represented. There are hard phases: the hardness is equal to hardness of a (100) face of diamond.

Crystalline state remaines up to 900-1000 K for all the pressure interval, then fullerite cages deform producting different 3D-polymerised disordered structures. The cross-linked layered disordered structure is observed for the both fulllerites up to 9.5GPa and above 900 K. These diffraction patterns are similar to that of graphite, but the (002)-reflection moves to large 2-angles and its FWHM is approximately equal to 1.5^o, the (100) and (101) merge into one halo. The hardness is higher than that of graphite, its value is in the range between sapphire and cubic BN.

At P>=12.5 GPa the (002)-displacement increases and interplanar space relation becomes a cubic one with a~6.0 Å (Fig 2). The hardness is equal to that of the (100) face of diamond. At 13 GPa only one halo (d =2.13-2.18 Å) remains in the observed interval up to 50⁰ 2q, it is the hardest phase of C₆₀ with hardness exceeds that of the (111) face of diamond. The P-T phase diagram of C₆₀ is represented.

Fig.2. Diffraction patterns of C₆₀- and C₇₀-samples after treatement at 12 GPa and different temperatures.

The main difference between diffraction patterns of new phases of C₆₀ and C₇₀ is grade of disorder. Diffraction patterns of C₇₀ have more broad peaks than those of C₆₀ and they become amorphous at lower temperature than that for C₆₀. The hardest crystalline C₆₀ has the highest symmetry (Fm3m) and such C₇₀ is a tetragonal.

1. Blank V.D., Buga S.G., Serebryanaya N.R., et al. Phys.Lett., 1996, A220, 149.