Capturing Transitory States of Reconstructive Phase Transitions in Shock-recovery Experiments
O. Tschauner1,2, S. N. Luo3, P. D. Asimow2, T. J. Ahrens4, and D. C. Swift3
1High Pressure Science
and Engineering Center, Department of Physics, University of Nevada Las
Vegas
2Division of Geological and Planetary Sciences,
California Institute of Technology
3Plasma Physics (P-24) and
Earth and Environmental Sciences (EES-11), Los Alamos National Laboratory
4Lindhurst Laboratory of Experimental Giophysics, Seismological
Laboratory, California Institute of Technology
Upon compression many materials undergo major reconstruction of their structure and bonding involving an increase in coordination of constituting atoms and change in bonding-character. While transforming, the materials pass through intermediate states, which are usually to fugitive to be captured and examined. Shock experiments allow in many cases for quenching such intermediates structural states. We review recent results on intermediate states for carbon phases. First we discuss an initial state of C60 polymerisation, then an example of extreme compaction of a C60-like cage structure. C60 forms intermolecular bonds by 2+2 cycloaddition.This process can be induced by irradiation with light of suitable energy at ambient pressure leading to formation of dimers and oligomers. Cycloadditon also occurs at elevated pressures and temperatures where it yields itinerant polymers. Pressure induced polymerisation is selective with respect to rotational orientation of the C60 molecules. At ambient pressure molecular C60 exhibits partial rotational ordering at 300 K. We performed X-ray diffraction studies on C60 retrieved from laser driven hypervelocity shock experiments and find rotational ordering enhanced even at 300 K. Ordering occurs in the (111) plane and is accompanied by slight rhombohedral distortion of the cubic metric. Further we observe diffuse scattering at Q-values between the (220) and (311) reflections which is interpreted as result of random polymerisation of C60 molecules in the (111) plane. In consequence, the remarkably high degree of rotational ordering in shocked molecular C60 is interpreted as result of enhanced lattice strain in the (111) plane induced by random polymerisation. The present findings provide insights into the mechanism of pressure-induced polymerisation of C60 and their relation to rotational ordering.
At much higher pressures and temperatures, C60 polymers collapse into disordered networks. Ultrafast dynamic compression and decompression allows for quenching a crystalline dense polymer at the brink of this density-driven breakdown: We describe an anisotropic 3D polymer cage structure equivalent to an extremely compact C60 polymer where random displacements of carbon atoms maximize bond distances. This material has been synthesized by laser-driven hypervelocity shock out of graphite. Thus, it represents a structural crossing point between graphite interlayer bridging and C60 polymerization as two paths of diamond formation out of low-density carbon phases.
