Quantitative Evaluation of the Effects of Positional versus Orientational Disorder on the Scattering of Acoustic Phonons in Disordered Matter

TL;DR

This study investigates how positional and orientational disorder affect phonon scattering in ethanol's solid phases, revealing that orientational disorder significantly increases phonon scattering, making the crystal behave similarly to amorphous solids.

Contribution

It provides a detailed comparison of phonon scattering in ordered and disordered ethanol phases using experimental and simulation methods, highlighting the impact of orientational disorder.

Findings

Orientational disorder leads to phonon scattering similar to amorphous solids.

Thermal conductivity in orientationally disordered crystals shows a temperature dependence akin to amorphous materials.

Simulations agree with experimental data, contrasting previous inelastic X-ray analyses.

Abstract

The phonon scattering processes on the three solid phases of ethanol are investigated by means of thermal conductivity, light and neutron scattering measurements as well as molecular dynamics simulations on single-crystalline models for the two crystalline modifications (fully ordered monooclinic and orientationally disordered bcc phases). The orientationally disordered crystal is found to exhibit a temperature dependence of the thermal conductivity that is remarkably close to that found for the structurally amorphous solid, specially at low temperatures. The results, together with measurements of the Brillouin linewidths as derived from light scattering measurements emphasize the role of orientational disorder on phonon scattering. The experimental results obtained on polycrystal samples are then discussed with the aid of computer simulations on single-crystalline models of both bcc and monoclinic crystals. Our findings are in good agreement with the wealth of thermodynamic and dynamic data vailable so far [1], but are in stark contrast with inferences made from the analysis of inelastic X-ray data on polycrystalline samples [2], where a common nature for the excitations in all phases is postulated.