Understanding the emergence of the boson peak in molecular glasses

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.