Phonon Kinetics at the Solid to Liquid Phase Transition

TL;DR

This study uses terahertz spectroscopy and computational methods to investigate phonon behavior and melting kinetics in fructose crystals, revealing specific phonon shifts and a 3D growth model with high activation energies.

Contribution

It combines THz TDS with density functional calculations to analyze phonon dynamics and melting kinetics in molecular crystals, providing new insights into structural changes before melting.

Findings

Low frequency phonons red shift with heating

Lowest energy phonon blue shifts at melting point

Melting follows a 3D growth model with high activation energies

Abstract

Terahertz time domain spectroscopy (THz TDS) is used to measure the melting kinetics of fructose molecular crystals. Combining single crystal anisotropy measurements with density functional calculations we assign the phonon frequencies and interrogate how specific phonons behave with melting. While nearly all the low frequency phonons continuously red shift with heating and melting, the lowest energy phonon polarized along the c-axis blue shifts at the melting temperature, suggesting an initial structural change immediately before melting. We find that the kinetics follow a 3D growth model with large activation energies consistent with previous differential scanning calorimetry (DSC) measurements. The large activation energies indicate multiple H-bonds must break collectively for the transition. The results suggest the generality of the kinetics for molecular crystals and that THz TDS with picosecond resolution could be used to measure ultra-fast kinetics.