Origin of the large entropy change in the molecular caloric and ferroelectric ammonium sulfate

Abstract

The deceptively simple inorganic salt ammonium sulfate undergoes a ferroelectric phase transition associated with a very large entropy change and both electrocaloric and barocaloric behaviour. While the structural origins of the electrical polarisation are now well established, those of the entropy change are more controversial. To resolve this question, we have performed total (elastic), quasielastic, and inelastic neutron scattering, as functions of both temperature and pressure, and DFT simulation. We

Pressure dependence of atomic dynamics in barocaloric ammonium sulfate: II. Vibrations

“…Our results reveal that the subtle difference in structures between the Pna2 1 and Pnam phases of ammonium sulfate produces a distinct change in the phonon spectrum. We have previously reported our analysis of the entropy change calculated from these phonon calculations, showing that the lowered frequency of the ammonium librations across the phase transition -including the anharmonic potential associated with the harmonically unstable modes -is more than sufficient to account for the large entropy change observed experimentally, and hence for this material's very large caloric effects [18]. Our results here suggest a structural mechanism for this change.…”

“…Even more strikingly, these decrease in frequency -some modestly, others substantially -such that the lowest-frequency of these librations become unstable within the harmonic approximation. We have previously argued that the rather flat energy landscape of these modes is responsible for the large entropy change in this material [18].…”

“…In many potential barocalorics, crystallographic disorder suggests a configurational interpretation of the large entropy change [13][14][15][16], although even in these cases there will also be a vibrational contribution [17]. Unusually, in ammonium sulfate there is no crystallographic disorder, so that, as we have recently shown, the dynamics are the sole source of information about the molecular origins of the entropy of transition [18]. Understanding these dynamics is a challenging prerequisite for the rational design of new barocaloric materials.…”