[SIT27-P02] High-pressure study of coronene: phase transitions, oligomerization, decomposition and thermal expansion
Keywords:aromatic hydrocarbons, carbon cycle, planetary interiors
Here we performed high-pressure experiments using multianvil apparatus and DAC. We observed phase transition (P21/a-P2/m) between 0 and 0.9 GPa. Compressibility parameters of coronene phase P2/m were defined in the pressure range of 0.9-8.1 GPa at 300 K as K0 = 13.0(3) GPa, K0’ = 7 at V0 = 795.5 Å3 using Vinet EOS (Vinet et al., 1987); the thermal expansion coefficient was found to be low at 2.0-7.5 GPa and 473-873 K (about 10-5 K-1). The same low thermal expansion coefficient at P > 3 GPa was defined previously for naphthalene C10H8 (Likhacheva et al., 2014).
Coronene decomposition was determined in the pressure range of 2.0-15.5 GPa between 900-1000 K. Coronene decomposition products consist of nanocrystalline graphite, amorphous carbon and diamond with trans-polyacetylene lying along the grain boundaries. At lower temperatures (500-773 K) we observed significant oligomerization of coronene by MALDI measurements. Coronene oligomer formation occurs via PAH dehydrogenation and successive fusion of the initial hydrocarbon molecules through C-C bond formation. Based on our results and previous experimental study at ambient pressure (Talyzin et al., 2011) we have identified PT diagram of coronene phase transitions, oligomerization and decomposition parameters to 16 GPa and 1000 K (Fig. 1). Defined coronene phase diagram is extremely important for understanding the planet accretion by carbonaceous chondrites.
Fugure 1. PT-diagram of coronene with phase transitions, oligomerization and decomposition parameters. Shaded area is a coronene oligomerization field.