Machine learning assisted investigation of the barocaloric performance in ammonium iodide

Abstract

Using the ab initio-based training database, we trained the potential function for ammonium iodide (NH4I) based on a deep neural network-based model. On the basis of this potential function, we simulated the temperature-driven β ⇒ α-phase transition of NH4I with isobaric isothermal ensemble via molecular dynamics simulations, the results of which are in good agreement with recent experimental results. As it increases near the phase transition temperature, a quarter of ionic bonds of NH4+-I− break so that NH4+ starts to rotate randomly in a disorderly manner, being able to store thermal energy without a temperature rise. It is found that NH4I possesses a giant isothermal entropy change (∼93 J K−1 kg−1) and adiabatic temperature (∼27 K) at low driving pressure (∼10 MPa). In addition, through partial substitution of I by Br in NH4I, it is found that the thermal conductivity can be remarkably improved, ascribed to the enhancement of lifetime of low frequency phonons contributed by bromine and iodine. The present work provides a method and important guidance for the future exploration and design of barocaloric material for practical applications.

On (not) deriving the entropy of barocaloric phase transitions from crystallography and neutron spectroscopy

“…Furthermore, there is still great potential in applying simple rules of thumb, such as combining 'quasi-spherical' molecules or molecular ions to make orientationally disordered crystals: an approach that has had recent success in producing molecular ferroelectrics [64]. On the simulation side, cheap and accurate methods such as machine-learned interatomic potentials [65] will make exploration of potential energy surfaces more widely accessible. Finally, characterisation by the methods described here will benefit not only from advances in experimental technology-including sample environments that, for instance allow high-pressure phases to be probed directly-but also from advances in simulation that allow models of the molecular dynamics to be compared directly with experimental results.…”

Chemical pressure as an effective tool for tuning the structural disordering and barocaloric efficiency of complex fluorides (NH4)3MF7 (M: Sn, Ti, Ge, Si)

“…This is due to the low pressure required to realize the maximum value of ∆S BCE , which is equal to the entropy of the phase transition if the contribution of the crystal lattice thermal expansion to BCE is neglected [16]. Not surprisingly, the most successful recent searches are related to materials containing organic cations/anions in structure, which are often highly disordered in the initial high-temperature phase and ordered as a result of structural phase transitions, which leads to a large change in entropy [9,11,14,[17][18][19].…”

Structure and thermal property relationships in the thermomaterial di-n-butylammonium tetrafluoroborate for multipurpose cooling and cold-storage

“…In the recent years, barocaloric materials have been demonstrated to exhibit very large thermal changes, some of them as large as DS $ 100 J K −1 kg −1 (the same order of magnitude as refrigerant gases) under the application of low/ moderate pressures of p = 70-1000 bar (while refrigerant gases normally operate at p # 150 bar). [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] These barocaloric materials belong to many different families of compounds, such as ammonium or phosphate salts, [35][36][37][38][39][40][41][42][43] superionic conductors, 44,45 spin crossover materials, [46][47][48][49][50][51][52] n-alkanes, 53 hybrid organic-inorganic materials, [54][55][56][57][58][59][60][61] organic plastic crystals [62][6...…”

“…In the recent years, barocaloric materials have been demonstrated to exhibit very large thermal changes, some of them as large as Δ S ≥ 100 J K −1 kg −1 (the same order of magnitude as refrigerant gases) under the application of low/moderate pressures of p = 70–1000 bar (while refrigerant gases normally operate at p ≤ 150 bar). 17–34 These barocaloric materials belong to many different families of compounds, such as ammonium or phosphate salts, 35–43 superionic conductors, 44,45 spin crossover materials, 46–52 n -alkanes, 53 hybrid organic–inorganic materials, 54–61 organic plastic crystals 62–69 and polymers. 70–72 Even more recently, barocaloric effects have been combined with gas adsorption/desorption processes in solid-to-solid breathing-transitions in MOFs, giving rise to larger thermal changes of Δ S ∼ 300 J K −1 kg −1 under pressures as small as p ≤ 16 bar.…”