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 $$\Delta {S}_{{P}_{0}\to P}^{{{\max }}}$$ Δ S P 0 → P max ∼71 J K−1 kg−1 around room temperature, associated with a structural phase transition. The phase transition temperature, Tt, varies dramatically with pressure at a rate of dTt/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 $$\left|\Delta {S}_{{P}_{0}\to P}^{{{\max }}}/\Delta P\right|$$ Δ S P 0 → P max / Δ P ∼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 Tt. This work is expected to advance the practical application of barocaloric refrigeration.