Origin of the Large Entropy Change in the Molecular Caloric and Ferroelectric Ammonium Sulfate

The deceptively simple inorganic salt ammonium sulfate undergoes a ferroelectric phase transition associated with very large entropy change and both electrocaloric barocaloric functionality. While the structural origins of electrical polarisation are now well established, those have been controversial for over 50 years. This question is resolved here using combination density-functional theory phonon calculations inelastic neutron scattering under variable temperature pressure, supported by complementary total quasielastic experiments. A model in which each molecular ion disordered across mirror plane high symmetry phase, although widely used literature, proves to be untenable. Instead, arises from low-frequency librations ions this harmonic terms that small or even negative. These results suggest that, search materials functionality derived changes, vibrational arising broad energy minima likely just as important configurational crystallographic disorder.