Probing Dynamics of Quinuclidinium Hexafluorophosphate by Quasi-Elastic Neutron Scattering

Barocaloric materials will have a critical role in the future of green refrigeration. One good source of barocaloric materials is the family of orientationally disordered ionic crystals, which have order-disorder phase transitions similar to those in molecular plastic crystals. Reorientation of the globular molecules and ions in the plastic crystalline phase gives a high reversible entropy change.

We have recently studied the salts of the quinuclidinium (Quin) ion, C₇H₁₄N⁺. The Quin ion has a quasi-spherical shape, which enables easy molecular reorientation, but has a relatively low (C_3v) symmetry. The combination of low symmetry and reorientations in the plastic phase creates a high configurational entropy. But the entropy change also contains strong dynamical contributions, which are more complicated to understand.

An excellent way to probe dynamical disorder is quasi-elastic neutron scattering (QENS): inelastic scattering with an energy transfer typically smaller than 1 meV. This technique provides information on molecular rotations and translational diffusion, revealing both the timescale and geometry of motion. QENS data complement structural techniques (e.g. crystallography) and dynamical techniques (e.g. vibrational spectroscopy) and give more detail than calorimetry.

Here we report studies of quinuclidinium hexafluorophosphate (QuinPF₆), C₇H₁₄NPF₆, which adopts the CsCl structure. It has an order-disorder phase transition at 298 K with a reversible entropy change of 151 J K^-1kg^-1 at 1000 bar, and barocaloric coefficient 14.0 K kbar^-1. We performed two quasi-elastic neutron scattering experiments to investigate the diffusion and rotation of molecules inside QuinPF6. Data were collected from two different instruments: OSIRIS (ISIS) is optimised for very fast dynamics (dynamic range ±400 µeV) with moderate resolution (25 µeV) while BASIS (ORNL) is optimised to consider slower motion (dynamic range ±100 µeV) with high resolution (3.5 µeV).
The BASIS experiment revealed translational ion diffusion across a range of temperatures and an increasing reorientation speed as the material changes to the plastic phase. In the OSIRIS experiment, by using a new pressure cell to collect data as a function of pressure as well as temperature, we not only confirmed those motions at ambient pressure, but also monitored the decrease in hopping frequency at higher pressures. Combining data from these two instruments allows us to consider both rotational and translational diffusion, producing the most detailed atomic picture so far of QuinPF₆’s dynamics. We also use these data to complement molecular dynamics simulations, which will be presented separately.