Microstructural analysis of high-pressure compressed C 60
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M. Akaishi NIRIM, Namiki 1-1, Tsukuba, 305, Japan (Received 15 January 2001; accepted 11 April 2001)
The structure and hardness of C60 bulk specimens compressed under 5.5 GPa at room temperature to 600 °C are investigated by high-resolution transmission electron microscopy, x-ray diffraction, and micro-Vickers hardness tests. A strong accumulation of the [1 1¯ 0]tr orientation of high-pressure-treated C60 specimens was developed along the compression axis, and stacking faults and nano-sized deformation twins were introduced into the C60 specimens compressed at 450–600 °C. Curved lattice planes indicating a polymerization of C60 were observed by high resolution transmission electron microscopy (HRTEM). The polymerization of the high-pressure-compressed C60 is also supported by the computer simulation of HRTEM images.
I. INTRODUCTION
It has been reported that C60 molecules are polymerized by high-pressure and high-temperature treatment.1,2 Structural characterization of the polymerized C60 phase has been performed by x-ray diffraction (XRD), neutron diffraction, Raman spectrometry, FTIR, and so on, and rhombohedral, tetragonal, face-centered-cubic system (fcc), body centered cubic (bcc), and orthorhombic structures have been assigned for the high-pressure-treated C60.1– 4 Little study using HRTEM, however, has been reported for the structure of polymerized C60. As highresolution transmission electron microscopy (HRTEM) is a powerful tool to obtain detailed atomic scale information, it will be useful to understand the local structural characteristics of the polymerized C60 phase. The purpose of this paper is to show the microstructure of C60 phases compressed under high pressure by using HRTEM and to discuss the results in connection with the data obtained by XRD. Along with the structural analyses, their mechanical properties are also examined by hardness tests, because the high-pressure-polymerized C60 has been reported to be very hard.3,5
rate of 20 °C min−1 to 400 °C and kept for 2 h in the first experiment. In a second experiment, the temperature was increased in the same way as in the first experiment and then further heated to 600 °C and kept for 2 h. After removal of the Au capsules, disks of the compressed C60 specimens with a diameter of 6.5 mm and a height of 1.5 mm were obtained (Fig. 2). The surfaces of compressed C60 specimens were mirror polished with SiC papers and diamond powders. XRD measurements (MAC MXP-18, 40 kV, 200 mA, Cu K␣) were done for the mirror-polished surfaces, and the powders prepared by pulverizing the high-pressure-compressed C60 in an
II. EXPERIMENTAL PROCEDURES
Powder of as-received pristine C60 crystals (99.95%, MER Corp.) was put into cylindrical gold capsules and compacted by using a cylindrical mold at room temperature (RT). Next, these capsules were put into an anvil cell (Fig. 1) of a belt-type high-pressure apparatus which uses NaCl as a pressure-transmitting medium. The pressure was kept at 5.5 GPa. The temperature was increased at a 1960
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