Ammonium ion dynamics in NH4I at high pressure

Ultrasensitive barocaloric material for room-temperature solid-state refrigeration

One of the greatest obstacles to the real application of solid-state refrigeration is the huge driving fields. Here, we report a giant barocaloric effect in inorganic NH₄I with reversible entropy changes of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {S}_{{P}_{0}\to P}^{{{\max }}}$$\end{document}ΔSP0→Pmax ∼71 J K⁻¹ kg⁻¹ around room temperature, associated with a structural phase transition. The phase transition temperature, T_t, varies dramatically with pressure at a rate of dT_t/dP ∼0.79 K MPa⁻¹, which leads to a very small saturation driving pressure of ΔP ∼40 MPa, an extremely large barocaloric strength of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left|\Delta {S}_{{P}_{0}\to P}^{{{\max }}}/\Delta P\right|$$\end{document}ΔSP0→Pmax/ΔP ∼1.78 J K⁻¹ kg⁻¹ MPa⁻¹, as well as a broad temperature span of ∼41 K under 80 MPa. Comprehensive characterizations of the crystal structures and atomic dynamics by neutron scattering reveal that a strong reorientation-vibration coupling is responsible for the large pressure sensitivity of T_t. This work is expected to advance the practical application of barocaloric refrigeration.

A small driving pressure is desirable for practical application of barocaloric materials. Here, the authors demonstrate a sensitive barocaloric effect in NH₄I due to strong reorientation-vibration coupling.