Dynamics in the ordered and disordered phases of barocaloric adamantane

Self Cite

We consider well-known signatures of disorder in crystallographic and inelastic neutron scattering data. We show that these can arise from different types of disorder, corresponding to different values of the system entropy. Correlating the entropy of a material with its atomistic structure and dynamics is in general a difficult problem that requires correlating information between multiple experimental techniques including crystallography, spectroscopy, and calorimetry. These comments are illustrated with particular reference to barocalorics, but are relevant to a broad range of calorics and other disordered crystalline materials.

Section: Spectroscopymentioning

confidence: 92%

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

Phillips,

Walker

2023

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“…A significant effort has been undertaken in recent years to synthesise and understand a variety of promising BC PCs [17][18][19][20][21][22]. NPG is perhaps the best studied material, because of its colossal BC effect.…”

Section: Introductionmentioning

confidence: 99%

Understanding variations of thermal hysteresis in barocaloric plastic crystal neopentyl glycol using correlative microscopy and calorimetry

Rendell-Bhatti,

Boldrin,

Dilshad

et al. 2024

Plastic crystals (PCs) exhibit solid-solid order-disorder first-order phase transitions that are accompanied by large correlated thermal and volume changes. These characteristics make PCs promising barocaloric solid-state working bodies for heating and cooling applications. However, understanding the variation of transition temperatures and thermal hysteresis in PCs with cycling is critical if these materials are to replace traditional gaseous refrigerants. Here, for the archetypal barocaloric PC neopentyl glycol (NPG), we correlate microstructure obtained from scanning electron microscopy with local and total thermal changes at the phase transition from infra-red imaging and calorimetry, respectively. We outline an evolution in microstructure as NPG recrystallises during repeated thermal cycling through its solid-solid phase transition. The observed microstructural changes are correlated with spatially inhomogeneous heat transfer, yielding direct insight into the kinetics of the phase transition. Our results suggest that the interplay of these processes affects the undesirable thermal hysteresis and the nature of the kinetic steady-state microstructures that are stabilised during cycling between the ordered and disordered phases. These observations have implications for using NPG and other PCs as technologically relevant barocaloric materials and suggest ways in which the hysteresis in these types of materials may be modified.

“…These colossal BC effects are comparable to the pressure-driven thermal changes seen in hydrofluorocarbons/hydrocarbons used in vapor-compression refrigeration 4 . PCs exhibiting colossal BC effects include neopentane derivatives [7][8][9][10], adamantane [11], adamantane derivatives [12,13], carboranes [14], and alkali hydrido-closoborates [15]. In addition, orientationally disordered solids which are not as mechanically soft as PCs, can also display large [16,17] and colossal [18] BC effects.…”

Section: Introductionmentioning

confidence: 99%

“…The discovery of colossal BC effects in PCs has sparked theoretical and computational interest primarily using density functional theory [24], molecular dynamics methods [15,[25][26][27], supercell lattice dynamics calculations [11], and mean-field microscopic modelling [28], aimed at understanding plastic-to-crystal transitions and predicting their associated BC response. However, a macroscopic description based on Landau theory is missing.…”

Section: Introductionmentioning

confidence: 99%

Landau theory of barocaloric plastic crystals

Marín-Delgado,

Moya,

Guzmán-Verri

2024

We present a minimal Landau theory of plastic-to-crystal phase transitions in which the key components are a multipole-moment order parameter that describes the orientational ordering of the constituent molecules, coupling between such order parameter and elastic strains, and thermal expansion. We illustrate the theory with the simplest non-trivial model in which the orientational ordering is described by a quadrupole moment, and use such model to calculate barocaloric effects in plastic crystals that are driven by hydrostatic pressure. The model captures characteristic features of plastic-to-crystal phase transitions, namely large changes in volume and entropy at the transition, as well as the linear dependence of the transition temperature with pressure. We identify temperature regions in the barocaloric response associated with the individual plastic and crystal phases, and those involving the phase transition. Our model is in overall agreement with previous experiments in powdered samples of fullerite C60, and predicts peak isothermal entropy changes of ∼ 90 J K − 1 kg − 1 and peak adiabatic temperature changes of ∼ 35 K under 0.60 GPa at 265 K in fullerite single crystals.