Summary of 2004 activity (only related to HPHT
of C60).
At high pressure and high temperature conditions (HPHT) C60 is known to undergo polymerisation. Below 9 GPa and 900 K several kinds of one- and two-dimensional polymeric phases have been obtained: orthorhombic (linear chains), tetragonal (planar with four connections per molecule) and rhombohedral (with six connections per molecule). One- and two-dimensional polymeric phases are well characterized by different techniques including Raman spectroscopy and XRD. Three-dimensionally polymerized superhard fullerites have been claimed to exist at pressures above 12-13 GPa and temperatures above 800K.
Most of the previous studies were performed ex situ after cooling the sample and release of pressure. Using externally heated diamond anvil cell designed by L.Dubrovinsky, we performed during last years extensive studies of the P-T diagram of C60 using in situ Raman spectroscopy and XRD. Very recently it was also proposed that pre-synthesized polymeric phases of C60 (tetragonal and rhombohedral) can be used as a precursor for synthesis of three dimensional polymers. First, we pressurized tetragonal and rhombohedral polymers at room temperature up to ~30 GPa. These experiments has not confirmed literature report about direct transformation of tetragonal phase into a new three-dimensionally polymerized fullerite [1]. In the next experiments [2], the pressure region where 3D polymerization is expected (about 13 GPa and 800K) was approached using unusual P-T path. Typically, 3D polymeric phases were attempted for synthesis using first pressure increase and heating on the second step. In our studies we used instead isothermal pressurizing at 800K. At these conditions C60 is already transformed into a two-dimensionally polymerized phases. The aim of experiments was to check if these two dimensional polymers (tetragonal in one experiment and rhombohedral in second) can be transformed into a tree-dimensional phase by linking polymeric planes. It could be that such synthesis would result in much better ordering of 3D polymers, which were always observed to be almost amorphous and very difficult for characterization.
Our experiments revealed very strong difference in behavior of two-dimensional polymers upon isothermal compression at 800K. As it can be seen in Figure, tetragonal phase exhibited unusual stability in the P-T region where 3D polymerization is believed to occur. This could be explained by relative orientation of C60 molecules in tetragonal polymers which prevent them from formation of inter-planar links.
1. Talyzin
A.V., Dubrovinsky L.S., "Pressure-induced phase
transformations
in tetragonal and
rhombohedral C60 polymers.", High Temperatures-High
Pressures, 35/36, 47 -53,
(2003/2004).
2. A V
Talyzin and L S Dubrovinsky, "
In situ Raman study of C60
polymerization during
isothermal pressurizing at 800 K",
J. Phys.:
Condens. Matter, 16, 757-772, (2004).
3. Talyzin A.V., Dubrovinsky
L.S.
“In situ Raman study of p
Isothermal compression up to 32 GPa
Figure 1 Phases observed during in situ isothermal pressurizing experiments with tetragonal (black squares) and rhombohedral (open triangles) phases as starting materials. Dashed lines mark P-T diagram known from experiments performed using isobaric heating at different pressures.