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Lattice Vibrations and Molecular Conformations in a Plastic Crystal: A Path to Solid-State Cooling Materials

Ares Sanuy1,2, Carlos Escorihuela-Sayalero1,2, Pol Lloveras1,2, Josep-Lluís Tamarit1,2, Luis Carlos Pardo1,2,*, and Claudio Cazorla1,2,†

  • *Contact author: luis.carlos.pardo@upc.edu
  • †Contact author: claudio.cazorla@upc.edu
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PRX Energy 4, 033019 – Published 23 September, 2025

DOI: https://doi.org/10.1103/bknr-9rnn

Abstract

In recent years, orientationally disordered crystals, or plastic crystals, have transformed the field of solid-state cooling due to the significant latent heat and entropy changes associated with their temperature-induced molecular order-disorder phase transition, which can produce colossal caloric effects under external field stimuli. However, the molecular mechanisms underlying these huge caloric effects remain inadequately understood and general principles for enhancing the performance of caloric plastic crystals are lacking. Previous studies have predominantly focused on molecular rotations, overlooking other potentially critical factors, such as lattice vibrations and molecular conformations. In this study, we employ classical molecular dynamics (MD) simulations to both replicate and elucidate the microscopic origins of the experimentally observed colossal barocaloric (BC) effects—those driven by hydrostatic pressure—in the archetypal plastic crystal neopentyl glycol (NPG). Our MD simulations demonstrate that in NPG, the combined BC response and phase-transition entropy changes arising from lattice vibrations and molecular conformations are nearly equal to those from molecular reorientations. These findings suggest that alongside hydrogen bonding—which directly impacts molecular rotational dynamics—lattice vibrational and molecular structural features, often overlooked, must be integrated into the rational design and modeling of advanced caloric plastic crystals. These insights are not only of significant fundamental interest but also essential for driving the development of next-generation solid-state refrigeration technologies.

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Popular Summary

How can we design greener and more efficient refrigeration systems without relying on harmful greenhouse gases? One promising answer lies in “plastic crystals,” solids whose molecules can change orientation and structure in response to temperature and pressure. These materials can produce exceptionally large thermal changes, making them attractive for next-generation solid-state cooling. Until now, scientists believed that such effects were driven mostly by molecular rotations. In this study, researchers used molecular simulations of neopentyl glycol, a widely studied plastic crystal, to show that lattice vibrations and changes in molecular shape contribute just as much to the cooling effect as rotations. The work, which closely matches experimental measurements, highlights that these previously overlooked factors are key to understanding and improving cooling materials. By broadening the design principles beyond rotations, this research opens new directions for developing efficient, eco-friendly refrigeration technologies.

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PRX Energy 4, 033019– Published 23 September, 2025
  • Received 22 March 2025
  • Revised 18 July 2025
  • Accepted 29 August 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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