Understanding the emergence of the boson peak in molecular glasses

20 April 2022, Version 2

Abstract

One of the common features of a glass is the so-called “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 the boson peak are controversial, albeit the emergence of locally ordered structures inhibiting crystallisation has been put forward as a possible culprit. Unfortunately, the terahertz spectral range is often congested with many other contributions complicating observation of the boson feature. Here, we show that depolarised Raman scattering—obtained using femtosecond optical Kerr-effect spectroscopy—in liquids consisting of highly symmetric molecules can be used to isolate the boson peak, allowing its detailed observation over a wide range of temperatures from the (supercooled) liquid into the glass. The boson peak in the vibrational spectrum matches the excess heat capacity observed in low-temperature heat capacity measurements. As the boson peak intensifies on cooling towards the glass transition, 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—reproducing both boson and pre peaks—indicate that these clusters are caused by over-coordinated molecules tending towards but prevented from attaining crystalline structural order. These findings represent an essential step toward our understanding of the physics of vitrification.

Keywords

supercooling
vitrification
glass transition
boson peak
Raman
optical Kerr effect
terahertz
WAXS
molecular dynamics
calorimetry
structure
ssNMR
FTIR
viscometry
DFT
Vogel-Fulcher-Tammann

Comments

Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.