Abstract
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.
Highlights
In the last few years, the giant caloric effects within ferroic materials were a subject of intense study in material science as the possible new environmental-friendly and efficient cooling devices [1,2,3]
The considered phase transition mechanism in ammonium sulfate (AS) is of a mixed displacive − order-disorder nature, where the anion precession disrupts the balance between two minima in the double-well potential corresponding to the PE and FE orientations of the NH4+(I)
The relaxation of NH4+(I) hindered rotations is connected with the coupling between NH4+(I) librations and SO42− v2 modes, which is induced by the anion precession
Summary
In the last few years, the giant caloric effects within ferroic materials were a subject of intense study in material science as the possible new environmental-friendly and efficient cooling devices [1,2,3]. The nature of EC and inverse BC effects was ascribed to the order-disorder phase transition type [8,9] This explanation, in contrast to the X-ray and neutron diffraction studies [12,27,39], assumes the disorder of all ions in the PE phase. Such an approach perfectly fits in the long-lasting dissonance between the descriptions of AS structure and PT based on the structural and spectroscopic data.
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