Observation by neutron incoherent inelastic spectroscopy of split higher harmonics of the ammonium ion librational mode in the cubic phase of NH4Br

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.

The high resolution inelastic neutron scattering spectrum of ammonium fluoride

The measured high resolution (deltaE/E approximately 2-3%) incoherent inelastic neutron scattering spectrum of ammonium fluoride is presented and discussed with reference to the available optical spectra. In addition, a full set of dispersion curves have been obtained from a new ab initio lattice dynamics calculation and these have been used to produce a rigorous interpretation of the spectrum. The librational modes of the ammonium ion occur at 560 cm(-1) and the anharmonicity in these modes is estimated to be 4%, about half that observed in the other ammonium halides. The reduction in anharmonicity is attributed to stronger hydrogen bonding and a deeper potential well. The calculations agree well with the observed spectrum apart from the librational modes which are shifted upwards by around 40 cm(-1) from the measured values. Dispersion and LO-TO splitting are important in this system with modes changing frequency by up to 135 cm(-1). The nature of the calculated LO-TO splitting in the optic mode region is indicative of a pseudo-cubic system confirming that the site symmetry of the ammonium ion is very close to T(d). Because of LO-TO splitting the ammonium ion asymmetric stretch, nu3, has components calculated to be at higher frequencies than those of the symmetric stretch, nu1, which contradicts the assignment scheme produced from optical data.