On (not) deriving the entropy of barocaloric phase transitions from crystallography and neutron spectroscopy

Abstract

We consider well-known signatures of disorder in crystallographic and inelastic neutron scattering data. We show that these can arise from different types of disorder, corresponding to different values of the system entropy. Correlating the entropy of a material with its atomistic structure and dynamics is in general a difficult problem that requires correlating information between multiple experimental techniques including crystallography, spectroscopy, and calorimetry. These comments are illustrated with particular reference to barocalorics, but are relevant to a broad range of calorics and other disordered crystalline materials.

Orientational order/disorder and network flexibility in deuterated methylammonium lead iodide perovskite by neutron total scattering

“…In practice, this is unlikely to be true, so that vibrational contributions must also be included for an accurate assessment of the entropy change. 49 For instance, we recently showed that the entropy change in ammonium sulfate, despite being very close to 3R ln 2, is best interpreted in vibrational terms rather than twofold congurational disorder of the three molecular units. 50 5 Flexibility and fluctuations of the lead iodide octahedral network…”