CMSP Seminar (Atomistic Simulation Seminar Series) 16 October, 4:00PM, by L.A.V. Naden Robinson

[CMSP Seminars Secretariat]

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CMSP Seminar (Atomistic Simulation Seminar Series)
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*Wednesday, 16 October 2024, 4:00PM*
*/Budinich Lecture Hall (Leonardo Building)/*/
/

Speaker:*  Lawrence Alexander Victor **NADEN ROBINSON*** (The University 
of Edinburgh)

Title: *Phase transitions in molecules, metals under pressure, and a new 
phase of ice *

Abstract:
High pressure research has led to many exciting discoveries, such as the 
highest critical temperature superconductors currently found [1], 
although not without recent controversy. Many other unexpected phenomena 
have been observed and predicted at high pressure, and here I will go 
through some of them. Molecular mixtures at planetary conditions, such 
as water+ammonia and water+methane, are expected to be found in many 
celestial bodies such as Uranus and Neptune. Here we find new ionic 
phases and mixtures of ammonia and water were predicted up to 500 GPa, 
and now recently measured [2], using crystal structure searching 
methods. These solid mixtures were then heated to reveal the expected 
superionic transition before melting, but also found a plastic regime at 
lower temperatures associated with various forms of molecular movements. 
At lower pressures, liquid methane and water have been observed to mix 
and here we perform long AIMD simulations, as well as many body 
decomposition (ML) potentials, between 0 and 3.0 GPa combined with 
neutron scattering data to examine the structure and increased 
solubility of methane in water. The second half of this talk examines 
the effect of high pressure on the previously considered simple alkali 
metals. These elements enter complex phases, such as host-guest and 
electride structures, following the typical bcc and fcc once compressed. 
Here we investigated the ideal host-guest atom ratio and found that 
these phases have moderate electride character, shown by a build of 
electron localization in non-nuclear sites. These host-guests structures 
also show an order-disorder transition, known as "chain-melting", and 
here we trained a machine-learned interatomic potential to reveal how 
these guest chains disorder with temperature [3]. Making further use of 
ML potentials and AIMD for liquids, we show that alkali metals localize 
electrons on non-nuclear sites as "pseudoanions" under sufficient 
pressure during a continuous liquid-liquid transition[4]. Finally, I may 
discuss recent work on mixing water+hydrogen at extreme conditions, the 
oxidation state of early earth from a material science perspective, and 
recent observations of a post ice X phase of ice above 230 GPa.

[1] Lilia, Boeri, et al. "The 2021 room-temperature superconductivity 
roadmap." Journal of Physics: Condensed Matter 34.18 (2022): 18300.
[2] Phys. Rev. Lett. *126*, 015702 (2021).
[3] Naden Robinson, Victor, et al. "On the chain-melted phase of 
matter." Proceedings of the National Academy of Sciences 116.21 (2019): 
10297-10302.
[4] Zong, Hongxiang, et al. "Free electron to electride transition in 
dense liquid potassium." Nature Physics 17.8 (2021): 955-960.


http://www.ictp.it/research/cmsp.aspx

The Abdus Salam International Centre for Theoretical Physics
https://www.ictp.it/

    
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