Dynamics in the disordered phases of a paradigmatic molecular ferroelectric and barocaloric

Orientationally disordered ionic crystals are currently of great interest because of their diverse functionality, including ionic conduction; ferro-, piezo- and pyroelectricity; and heat transport and storage. However, the structural and dynamic changes across their order-disorder phase transitions are far more complex than their simple chemistry might suggest. Here we will investigate the paradigmatic material tetramethylammonium tetrachloroferrate, formed between two ions that are ubiquitous in "plastic ionic" crystals. This has no fewer than seven phases at ambient pressure between 150 and 400 K. Structural characterisation of these has so far been limited to their average crystallography, but other experimental evidence (for instance, from electron spin resonance) intriguingly suggests substantial local correlations and variations in dynamics between the phases. This is the second of two related proposals in which we aim to reveal for the first time how the local structure and dynamics change through the series of phase transitions. Here, we will collect quasielastic neutron scattering data from the tetramethylammonium ion motion in each phase. This will complement neutron pair distribution function data collected in our first proposal. These data will give unprecedented insight into the complex energy landscape in this material, and more generally into how orientational disorder can be designed to achieve specific functionality.