Solid-state cooling using barocaloric materials is a promising avenue for ecofriendly, inexpensive, and highly efficient cooling. To design barocaloric compounds ready for deployment, it is essential to understand their thermodynamic behavior under working conditions. To this end, we have studied the rotational dynamics in the molecular-ionic crystal ammonium sulfate under pressure, providing detailed insight into the origin of its large barocaloric effect. Using quasielastic neutron scattering experiments, we show that rotation of the ammonium cations is facilitated by pressure in the low-entropy phase, with the rotational “hopping” motion increasing in frequency as the pressure-induced phase transition is approached. We explain this unusual behavior in terms of the competing hydrogen-bond networks represented by the two phases. This work includes the first results of a recently developed low-background, high-pressure gas cell for neutron scattering, showcasing its power in obtaining high-precision measurements of molecular dynamics under pressure.

中文翻译：

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气压硫酸铵中旋转动力学的压力依赖性

使用气压热材料的固态冷却是一种有前途的环保、廉价和高效冷却的途径。要设计可部署的气压热化合物，必须了解它们在工作条件下的热力学行为。为此，我们研究了分子离子晶体硫酸铵在压力下的旋转动力学，详细了解了其大气压热效应的起源。使用准弹性中子散射实验，我们表明铵阳离子的旋转是由低熵相中的压力促进的，随着接近压力引起的相变，旋转“跳跃”运动的频率增加。我们根据两个阶段所代表的竞争性氢键网络来解释这种不寻常的行为。