Origin of the entropy in molecular perovskite barocalorics

Many members of the well-known molecular perovskite family are barocalorics: they can be switched between high- and low-entropy states by applying pressure, which allows them to be used for environmentally-friendly solid-state cooling. Much of the entropy in the high-entropy phases comes from the atomic dynamics, especially of the guest A-site cation. However, it remains very difficult to predict the entropy from the structure alone. Our target materials for this experiment are two closely related materials in this family where the larger, apparently more flexible unit cell gives rise to a smaller entropy change!

In principle, QENS is the ideal method for probing these dynamics, because the A-site cations have many H atoms and give a strong incoherent signal. However, in practice these experiments are very difficult to perform on perovskites with moderately large unit cells (> 10 Å), because the Bragg scattering both dominates the incoherent structure factors and substantially increases multiple scattering.

Polarisation analysis allows us to resolve both of these problems. Here we propose to use PA-LET to measure clean incoherent structure factors out to 5 Å-1, allowing us to fit far more complex models to these structure factors than would otherwise be possible. Our results will shed new light on the structural features responsible for large entropy changes in barocaloric framework materials, paving the way to identifying and even designing new materials.