Investigating carbon molecules with pressure-volume-temperature equations of state

We are interested in intermolecular interactions which determine thermodynamic stability in crystalline solids and their response to changes in the external conditions. In no area is this information of more importance than in planetary materials science, where scientists are trying to understand the fate of carbon in the context of the origin of life and/or the varied planetary surfaces observed. Molecules of high astrobiological and astrophysical relevance, such as amino acids (1), polyaromatic hydrocarbons, and N-heterocycles (2), have been identified across the Universe but how they behave under such varying conditions is a question yet to be answered. Key to our approach is the determination of the internal-energy, entropy and the Gibbs free energy - not only computationally but also, and for the first time, experimentally. We have developed a new method that transforms variable-pressure (P)-temperature (T) crystallographic data into thermodynamic information. Equations of State (EoSs) are the models of choice to fit these data, describing how pressure, temperature, and volume (V) are inter related in solid phases. Although it is quite common to model thermal expansion at ambient pressure with a VT equation of state (EoS), and compression at ambient temperature using a PV-EoS, determinations of PVT-EoSs are much less common, particularly for molecular materials (3). The paucity of PVT-EoSs reflects the difficulty in varying pressure and temperature simultaneously in crystallographic experiments, especially at reduced temperatures. These difficulties are addressed by the variable temperature insert for the Paris-Edinburgh press available on the PEARL instrument at the ISIS Neutron Spallation Source (4) and by the cryofurnace for the Merrill-Bassett cell available on the KOALA instrument at the ANSTO OPAL reactor (5). The results can then be combined with Periodic DFT and other semi-empirical calculations, where pressure and temperature can be included at little time cost, enabling the stability profile of a material to be understood, right down to the level of individual intermolecular interactions. Many classes of structure-directing intermolecular interactions involve hydrogen atoms: hydrogen bonds are an obvious example, but hydrogens can also be involved in dispersion and electrostatic interactions. The responses of different kind of crystalline organics containing these interactions, such as hexamethylenetetramine, naphthalene, histidine, and alanine are to be studied using powder and single-crystal neutron diffraction up to 5 GPa and between 105-480 K. We are specifically using neutron diffraction for the experiments because of its sensitivity to locate hydrogen atoms. Additionally, the penetrating nature of neutron radiation means that complete, high-quality data can be obtained for samples in elaborate extreme-conditions environments.

Keywords

Thermodynamic properties, Astrophysics, Carbon, Hydrocarbons, Equations of state, Thermal expansion, Monocrystals, Neutron diffraction, Naphthalene, Histidine, Urotropin, Alanines

Citation

Novelli, G., McIntyre, G. J., Maynard-Casely, H. E., Funnell, N. P., Marshall, W. G., Kamenev, K., & Parsons, S. (2019). Investigating carbon molecules with pressure-volume-temperature equations of state. Paper presented at ANSTO Young Researchers' Conference, Lucas Heights, NSW, Australia, Tuesday 03 September 2019. Retrieved from https://events01.synchrotron.org.au/event/98/book-of-abstracts.pdf

URI

https://events01.synchrotron.org.au/event/98/book-of-abstracts.pdf
https://apo.ansto.gov.au/dspace/handle/10238/12528

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