Understanding the Colossal Barocaloric Response in Choline Based Plastic Crystals from Ab Initio Molecular Dynamics

Solid-state refrigeration technologies based on barocaloric effects offer a cleaner eco-friendly solution to conventional refrigeration technologies based on vapor compression of greenhouse gases. Thus, materials with large solid-state caloric effects induced by external field (mechanical, electric or magnetic field) need further investigation and development to realise their full potential. The refrigeration capacity is associated with a large isothermal entropy change and/or with a large temperature change, induced by external stimuli such as electric field (electrocaloric effect) or magnetic field (magnetocaloric effect) or mechanical pressure (barocaloric effect), effects that are enhanced near phase transition [1]. To this end, ionic plastic crystals offer new interesting opportunities in the field of barocalorics due to their orientationally disordered phases and the phase transitions accompanied by huge enthalpy and entropy changes [2]. In the present work, we explore the potential of choline-based plastic crystals, (C₅H₁₄NO)₂MX₄, where M= Co, Zn, Cu, and X= Cl, Br, I, as promising barocaloric materials due to a large entropy change (∼100 JK^-1kg^-1) associated with the symmetry-breaking disorder-order phase transition. Using ab initio molecular dynamics as implemented in CP2K, we provide insights into the structural dynamics and hydrogen bonding between the choline and MCl₄^2- that characterise the two phases. From our calculation of vibrational density of states, we estimate the entropy change across the phase transition. The microscopic mechanism for the colossal entropy change in (C₅H₁₄NO)₂MCl₄ is attributed to the dynamic hydrogen bonding between the choline and MCl₄^2- in the two phases. The effect of different metallates (i.e. MX₄^2- where M= Co, Zn, Cu, and X= Cl, Br, I) on the colossal entropy change is explored, providing a rational design for these choline based plastic crystals with tailored barocaloric properties. Our model is validated with our experimental study, thus providing a complete understanding of the interplay between dynamic and disordering effects in choline based plastic crystals.

References
1. X. Moya, S. Kar-Narayan and N. D. Mathur. Caloric materials near ferroic phase transitions. Nature Materials, 2014, 13, 439-450.
2. B. Li, Y. Kawakita, S. Ohira-Kawamura, T. Sugahara et al. Colossal barocaloric effects in plastic crystals. Nature, 2019, 567, 506-510.