Understanding the Complex Phase Space of [N_111CN][PF₆] for Solid State Refrigeration

Refrigeration systems account for 17% of the global electricity usage and this value will continue to increase with the ongoing problem of climate change. ^[1] The current vapour-compression technology releases greenhouse gases which has led to the demand for solid-state replacements. ^[1] Through the application of hydrostatic pressure on solids, changes in isothermal entropy (ΔS_iso) and adiabatic temperature (ΔT_ad) can be induced. This so-called barocaloric effect is increasingly probed by thermoanalytical techniques such as high-pressure differential scanning calorimetry (HP-DSC) in a wide range of materials.^[2] A deep insight into the structural changes across these transitions is often overlooked in lieu of their physical properties.
In this work, we present results based on in-situ combined variable temperature and high-pressure crystallography using both neutron and x-ray diffraction to investigate the complex phase behaviour of the organic ionic plastic crystal (OIPC) (cyanomethyl)trimethylammonium hexafluorophosphate ([N_111CN][PF₆]). This provides an understanding towards the hysteresis and structural reversibility of the caloric effect.
OIPCs are a class of materials which exhibit at least one solid-solid phase transition upon varying the temperature of the system. As in molecular plastic crystals, upon cooling these structures, the mobility of the ions becomes limited, increasing the order within the structures.^[3] By varying the ions present within the system, changes in the interionic interactions and stable packing of the crystal structure will change. Due to this, it is thought that OIPCs may exhibit a wider range of degrees of freedom compared to their molecular counterparts, and thus, higher entropy changes across their transitions. However, in order to exploit these tunable properties, it is crucial to understand their structural origins.
[N_111CN][PF₆] exhibits a four-step disorder-order phase transition upon cooling. At room temperature, the material is in a high symmetry, disordered, hexagonal phase (I) (P6₃/mmc) and upon cooling to 253 K the structure transitions to an ordered, monoclinic phase (IV) (P2₁) via two lower symmetry intermediate, metastable states (II and III) (Pnab and P2₁/c). Upon heating, the system transitions directly from phases IV-I, at 283 K, showcasing the importance of understanding the underlying structural properties of the material.
Upon application of 1000 bar of hydrostatic pressure, [N_111CN][PF₆] transforms from phases I-II. At 1200 bar, the system exhibits a short region of phase coexistence of phases II, III, and IV, requiring further compression to 1800 bar, to transform fully to phase IV, highlighting the high-pressures needed for reversible transitions.
Analogous to heating, upon decompression the system transitions directly from phases IV-I at 500 bar, showcasing the hysteresis present in the material.
From isobaric variable temperature measurements, the transition temperature was found to be sensitive to hydrostatic pressure with δT_t/δP = 30.5 K kbar⁻¹ with a colossal ΔS_iso(IV-I) of 138 J K^-1 kg^-1, reaching the same order of magnitude as that of current gas refrigerants.
Only by investigating the structural complexity of the phase space of [N_111CN][PF₆] can we understand the origin of the hysteresis seen across the IV-I phase transition. Therefore, studies like this pave the way to understand how hysteresis and reversibility within the caloric effect can be mitigated.