Browsing by Author "Marshall, WG"

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    Carbon molecules in space: a thermal equation of state study of solid hexamethylenetetramine
    (Australian Institute of Physics, 2020-02-04) Novelli, G; McIntyre, GJ; Maynard-Casely, HE; Marshall, WG; Kamenev, KV; Parsons, S
    Properties such as compressibility, thermo-elasticity and the energy landscape remain unknown for many organic compounds under conditions encountered on extraterrestrial planets and moons and in space. In this study, a thermal Equation of State (EoS) for the crystalline solid hexamethylenetetramine was determined by neutron powder diffraction in the temperature and pressure ranges of 113-480 K and 0-5 GPa, respectively. The material was chosen as a molecular model for its high symmetry and its property of remaining in the same phase throughout the experimental conditions selected to simulate the planetary environments. Equations of States (EoSs) show how the thermodynamic variables of temperature (T), pressure (P) and volume (V) are inter-related. The ideal gas law, PV = nRT, is an example of an EoS which is used as a simple but effective model to explain the properties of gases. More complex EoSs, where the assumption of ideality is relaxed, can be applied to solids in order to describe how the geometry and energy transform when they experience dramatic changes in their environment. Such information acquires enormous importance in planetary materials science, where scientists are trying to understand the fate of carbon, the fourth most abundant element in our galaxy, in the context of the origin of life and planetary environments. Despite the large heterogeneity of galactic and interstellar regions, the organic chemistry of the universe seems to follow common pathways. Molecules of high astrobiological and astrophysical relevance such as amino acids, polyaromatic hydrocarbons, and N-heterocycles have been identified across the solar system, but how they behave under such varied conditions is a question yet to be answered. Key to our approach was the determination of how the internal energy (U), entropy (S) and the Gibbs free energy (G) vary with pressure not only computationally, but also, and for the first time, experimentally. A new method has been developed, able to transform directly variable-PT crystallographic data into thermodynamic information. Although it is quite common to model thermal expansion at ambient pressure with a VTEoS, and compression at ambient temperature using a PV-EoS, determinations of PVT-EoSs are much less common, particularly for organic materials. This paucity of PTV-EoSs reflects the difficulty of varying pressure and temperature simultaneously in crystallographic experiments, especially at reduced temperatures. The task was addressed in this study by the variable-temperature insert for the Paris-Edinburgh press available on the PEARL instrument at the ISIS Neutron Spallation Source (UK). The results were successfully combined with periodic DFT (Figure 1) and other semiempirical calculations, where pressure and temperature can be included at little time cost, enabling the stability profile of the material to be understood, right down to the level of individual intermolecular interactions.
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    Investigating carbon molecules with pressure-volume-temperature equations of state
    (Australian Nuclear Science and Technology Organisation, 2019-09-03) Novelli, G; McIntyre, GJ; Maynard-Casely, HE; Funnell, NP; Marshall, WG; Kamenev, KV; Parsons, S
    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.
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    Pressure-induced intersite Bi--M (M=Ru, Ir) valence transitions in hexagonal perovskite
    (Wiley Online Library, 2014-02-24) Huang, Z; Auckett, JE; Blanchard, PER; Kennedy, BJ; Miller, W; Zhou, Q; Avdeev, M; Johnson, MR; Zbiri, M; Garbarino, G; Marshall, WG; Gu, QF; Ling, CD
    Pressure-induced charge transfer from Bi to Ir/Ru is observed in the hexagonal perovskites Ba3+nBiM2+nO9+3n (n=0,1; M=Ir,Ru). These compounds show first-order, circa 1 % volume contractions at room temperature above 5 GPa, which are due to the large reduction in the effective ionic radius of Bi when the 6s shell is emptied on oxidation, compared to the relatively negligible effect of reduction on the radii of Ir or Ru. They are the first such transitions involving 4d and 5d compounds, and they double the total number of cases known. Ab initio calculations suggest that magnetic interactions through very short (ca. 2.6 Å) M[BOND]M bonds contribute to the finely balanced nature of their electronic states. © 2014 Wiley‐VCH.