Dynamics in the ordered and disordered phases of barocaloric adamantane

TL;DR

This study demonstrates a significant barocaloric effect in adamantane, a simple molecular crystal, with low hysteresis and a large entropy change driven by vibrational and configurational effects, analyzed through neutron spectroscopy and lattice dynamics.

Contribution

It reveals the vibrational entropy contribution to the barocaloric effect in adamantane and shows how supercell lattice dynamics can model disorder effects in molecular crystals.

Findings

Colossal isothermal entropy change of 106 J K-1 kg-1 in adamantane

Low hysteresis allows operation at pressures below 200 bar

Vibrational entropy change is mainly due to softening of acoustic modes

Abstract

High-entropy order-disorder phase transitions can be used for efficient and eco-friendly barocaloric solid-state cooling. Here the barocaloric effect is reported in an archetypal plastic crystal, adamantane. Adamantane has a colossal isothermally reversible entropy change of 106 J K-1 kg-1 . Extremely low hysteresis means that this can be accessed at pressure differences less than 200 bar. Configurational entropy can only account for about 40% of the total entropy change; the remainder is due to vibrational effects. Using neutron spectroscopy and supercell lattice dynamics calculations, it is found that this vibrational entropy change is mainly caused by softening in the high-entropy phase of acoustic modes that correspond to molecular rotations. We attribute this behaviour to the contrast between an 'interlocked' state in the low-entropy phase and sphere-like behaviour in the high-entropy phase. Although adamantane is a simple van der Waals solid with near-spherical molecules, this approach can be leveraged for the design of more complex barocaloric molecular crystals. Moreover, this study shows that supercell lattice dynamics calculations can accurately map the effect of orientational disorder on the phonon spectrum, paving the way for studying the vibrational entropy, thermal conductivity, and other thermodynamic effects in more complex materials.