Understanding the emergence of the boson peak in molecular glasses

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.

Double glass transitions in single-component homogeneous liquids due to intramolecular vitrification

“…Rheology shows that, on cooling from room temperature, the shear viscosity increases in a VFT-like fashion down to Tg2 and then continues to rise in a different VFT-like fashion down to Tg1. Although double VFT behaviour has been observed previously 26 here it seen in the trimeric alkoxide liquids but not in monomeric (e.g., silicon) 8 or dimeric (niobium) alkoxides. The latter also do not exhibit the double calorimetric glass transition.…”

“…Short chain silicon alkoxides can crystallise while longer chain ones (e.g., silicon tetrabutoxide) only vitrify into a glass and were recently studied to understand the emergence of the boson peak in molecular glasses. 8 Pentaalkoxides based on niobium and tantalum are typically six-coordinated, dimeric in both liquid and crystalline phases, and bioctahedral.…”

“…To model this, the spectra were fitted with two Brownian-oscillator functions-one for the alkoxide librations and one for the vibrations-while the diffusive mode was modelled with a Cole-Cole function (for fit parameters see Supplementary Table 3 to Supplementary Table 5). 8 The inset of Figure 3(a) shows the amplitude of the diffusive mode and the frequency of the alkoxide libration, both of which show a large jump at 230 K.…”

Structure and properties of naphthalene-diimide N-functionalized with stilbene

“…, ref. 62), which is not present in the spectra of powder indicating the crystallinity of the latter. In the disordered regions of the film, the torsional angles between the stilbene moieties and the NDI core are expected to be altered because of the very shallow torsion potential well up to ∼30° (see Fig.…”

Boson and two-dimensional cluster model composed of quenching disorder eigenvalues and eigenvectors in the glass transition

“…(v) CI-boson can occur at a relaxation time, ranging from femtoseconds to any time period within a macroscopic time scale, i.e., terahertz 43 to 100 seconds low frequency 44 . This is because CI-bosons come from clustered collisions between two HSMs, and their collision frequency is higher than Debye vibration frequency.…”