Atomistic mechanisms of phase transitions in all-temperature barocaloric material KPF$_6$

TL;DR

This study uncovers the atomistic mechanisms behind the wide-temperature-range barocaloric effect in KPF$_6$, revealing phase transitions driven by pressure and FOD dynamics, with implications for designing advanced caloric materials.

Contribution

The paper combines first-principles and machine-learning simulations to elucidate phase transition mechanisms in KPF$_6$, a material with exceptional all-temperature barocaloric effects, which was not previously understood.

Findings

Identified four distinct phases with different FOD and structural properties.

Pressure induces phase transitions that cause significant entropy changes.

All phases are wide-bandgap insulators, stable across temperature ranges.

Abstract

Conventional barocaloric materials typically exhibit limited operating temperature ranges. In contrast, KPF$_6$ has recently been reported to achieve an exceptional all-temperature barocaloric effect (BCE) via pressure-driven phase transitions. Here, we elucidate the atomistic mechanisms underlying the phase transitions through first-principles calculations and machine-learning potential accelerated molecular dynamics simulations. We identify four distinct phases: the room-temperature cubic (C) plastic crystal characterized by strong fluorine orientational disorder (FOD) and anharmonicity, the intermediate-temperature monoclinic (M-II) phase with decreasing FOD, the low-temperature monoclinic (M-I) phase with suppressed FOD, and the fully ordered rhombohedral (R) phase under pressure. Phonon calculations confirm the dynamic stability of the M-II, M-I, and R phases at 0 K, whereas the C phase requires thermal fluctuations for stabilization. Under pressure, all the C, M-II, and M-I phases transform to the R phase, which are driven by cooperative PF$_6$ octahedral rotations coupled with lattice modulations. These pressure-induced phase transitions result in persistent isothermal entropy changes across a wide temperature range, thereby explaining the experimentally observed all-temperature BCE in this material. Hybrid functional calculations reveal wide-bandgap insulating behavior across all phases. This work deciphers the interplay between FOD, anharmonicity, and phase transitions in KPF$_6$, providing important insights for the design of BCE materials with broad operational temperature spans.