Understanding the emergence of the boson peak in molecular glasses

Mario González-Jiménez, Trent Barnard, Ben A. Russell, Nikita V. Tukachev, Uroš Javornik, Laure-Anne Hayes, Andrew J. Farrell, Sarah Guinane, Hans M. Senn, Andrew J. Smith, Martin Wilding, Gregor Mali, Motohiro Nakano, Yuji Miyazaki, Paul McMillan, Gabriele C. Sosso () and Klaas Wynne ()
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Mario González-Jiménez: University of Glasgow
Trent Barnard: University of Warwick
Ben A. Russell: University of Glasgow
Nikita V. Tukachev: University of Glasgow
Uroš Javornik: National Institute of Chemistry
Laure-Anne Hayes: University of Glasgow
Andrew J. Farrell: University of Glasgow
Sarah Guinane: University of Glasgow
Hans M. Senn: University of Glasgow
Andrew J. Smith: Harwell Science and Innovation Campus
Martin Wilding: University of Cardiff
Gregor Mali: National Institute of Chemistry
Motohiro Nakano: Osaka University
Yuji Miyazaki: Osaka University
Paul McMillan: University College London
Gabriele C. Sosso: University of Warwick
Klaas Wynne: University of Glasgow

Nature Communications, 2023, vol. 14, issue 1, 1-11

Abstract: Abstract A common feature of glasses is the “boson peak”, observed as an excess in the heat capacity over the crystal or as an additional peak in the terahertz vibrational spectrum. The microscopic origins of this peak are not well understood; the emergence of locally ordered structures has been put forward as a possible candidate. Here, we show that depolarised Raman scattering in liquids consisting of highly symmetric molecules can be used to isolate the boson peak, allowing its detailed observation from the liquid into the glass. The boson peak in the vibrational spectrum matches the excess heat capacity. As the boson peak intensifies on cooling, wide-angle x-ray scattering shows the simultaneous appearance of a pre-peak due to molecular clusters consisting of circa 20 molecules. Atomistic molecular dynamics simulations indicate that these are caused by over-coordinated molecules. These findings represent an essential step toward our understanding of the physics of vitrification.

Date: 2023
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DOI: 10.1038/s41467-023-35878-6

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