Displacive or Order-Disorder Phase Transition? The H-bond Dynamics in Multicaloric Ammonium Sulfate

Recently, ferroic materials with giant caloric responses emerged as a possible environmental-friendly alternative for the currently used cooling devices. In our work, we have performed the Born-Oppenheimer molecular dynamics calculations for both para- and ferroelectric phases of multicaloric (NH₄)₂SO₄. The simulations were performed in the NVT ensemble with several conditions applied for three different supercell sizes. Time and space correlations between the ion motions were analyzed using various strategies to study the interaction changes along the obtained trajectories. The investigation of thermally induced evolution of complicated H-bond system in ammonium sulfate structure was performed using calculated power spectra. The results of simulations collated with the obtained X-ray diffraction data enabled us to describe the mechanism of (NH₄)₂SO₄ phase transition as the one of a mixed displacive and order-disorder nature. According to the origin of such structural transformation, the giant inverse barocaloric effect in ammonium sulfate is caused by the reverse H-bond system reorganization induced by hydrostatic pressure in the vicinity of the critical temperature. The spontaneous polarization observed in the ferroelectric phase is a secondary effect of symmetry change and it partially results from the disorder relaxation of both distorted NH₄⁺ cations in low temperatures. The proposed investigation scheme should be useful in the studies of other ferrocaloric materials and H-bonded ferroelectrics.