Pressure dependence of atomic dynamics in barocaloric ammonium sulfate: II. Vibrations

TL;DR

This study investigates how atomic vibrations in ammonium sulfate change with pressure and temperature, revealing key vibrational modes linked to its barocaloric effect, aiding the design of new cooling materials.

Contribution

It combines inelastic neutron scattering and density functional theory to analyze phonons in ammonium sulfate under working conditions, identifying vibrational modes crucial for entropy change.

Findings

Ammonium librational modes have negative Grüneisen parameters.

Experimental and theoretical phonon results are in excellent agreement.

Structural differences across phase transition relate to atomic dynamics.

Abstract

Ammonium sulfate is a giant inverse barocaloric material that is cheaply and commercially available. Exploiting its potential for cooling applications requires an understanding of the mechanism driving the entropy change. Here we report an investigation by inelastic neutron scattering and density functional theory of the phonons under working conditions of temperature and pressure. We find excellent agreement between the experimental and calculated results. The ammonium librational modes that are crucial to the entropy change are identifiable by their negative Gr\"{u}neisen parameter. Our results connect the differences in structure across the phase transition to those in the atomic dynamics, suggesting a route towards designing new caloric materials.