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

Shurong Yuan; Bernet E. Meijer; Guanqun CaiRichard J. C. DixeyFranz DemmelMartin T. DoveJiaxun LiuHelen Y. PlayfordHelen C. WalkerAnthony E. Phillips

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

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

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

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《Advanced functional materials》 |2022年第45期|2207717.1-2207717.12|共12页
作者
Shurong Yuan; Bernet E. Meijer; Guanqun CaiRichard J. C. DixeyFranz DemmelMartin T. DoveJiaxun LiuHelen Y. PlayfordHelen C. WalkerAnthony E. Phillips;
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作者单位

School of Physical and Chemical Sciences Queen Mary University of London Mile End Rd, London E1 4NS, UK;

ISIS Neutron and Muon Source Rutherford Appleton Laboratory Didcot OX11 0QX, UK;

School of Physical and Chemical Sciences Queen Mary University of London Mile End Rd, London E1 4NS, UK,School of Computer Sciences Sichuan University No 24 South Section 1, Yihuan Road, Chengdu 610065, China,Department of Physics Wuhan University of Tech;

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正文语种英语
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anharmonicity; barocalorics; disorder; entropy;

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

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