Exploring Vibrational Dynamics in Barocaloric Quinuclidinium Salts Using Inelastic Neutron Scattering

In this study, we investigate the promising barocaloric behaviour in a series of quinuclidinium salts, in which the approximately spherical quinuclidinium (azabicyclooctane, C7H14N+, [Quin]+) cation packs with inorganic counteranions into structures analogous to the alkali halides. These materials have the NaCl or CsCl structures in a disordered high-T phase, freezing into ordered, less symmetrical phases at low T. Importantly for applications, these materials have solid-solid phase transitions at or near room temperature driven by the configurational disorder of the quinuclidinium ions, resulting in a large reversible entropy change. However due to the crystallographic disorder in the high-symmetry phases of these materials, traditional crystallographic methods are of limited use in characterising their behaviour. Therefore, it is as important to explore the dynamical disorder and its contribution to the entropy change.
Inelastic neutron scattering (INS) is a powerful technique for probing vibrational dynamics in materials. By measuring the energy transfer between incident neutrons and the phonon modes of a sample, INS provides direct insights into vibrational excitations over a wide energy range. It can detect both coherent and incoherent scattering, offering detailed information on collective lattice vibrations (phonons) as well as localized modes. Unlike IR and Raman spectroscopy, which depend on dipole moment or polarizability changes, INS interacts directly with atomic nuclei, making it highly sensitive to hydrogen. This sensitivity allows us to capture vibrational modes involving hydrogen, providing a more detailed picture of vibrational contributions than calorimetry, which only offers bulk thermodynamic data. While calorimetry measures total entropy changes, it lacks the molecular-level resolution needed to separate configurational and vibrational entropy.
We conducted two INS experiments on two different instruments at ISIS: TOSCA to measure the low T(~10 K), low-frequency phonon density of states of seven quinuclidinium salts with counterions Cl^–, Br^–, I^–, NO3^–, BF4^–, PF6^– and IO4^–; MAPS to measure the phonon density of states of three quinuclidinium salts, with counterions NO3^–, BF4^–, and PF6^–, in both ordered and disordered phases. DFT calculations were used to validate and interpret the TOSCA data. The MAPS data lets us directly compare the vibrational contribution across the phase transition and the effect of changing counterion. In addition, we have used quasi-elastic neutron scattering (QENS), a complementary technique that probes slower dynamics such as molecular rotations and translations to fully map out the dynamics of QuinPF6 . This will be presented by my colleague separately.