  Solid-state refrigeration based on caloric effects is an energetically
efficient and environmentally friendly technology, which is deemed as a
potential alternative to the conventional vapor-compression technology. One of
the greatest obstacles to the real application is the huge driving fields.
Here, we report a giant barocaloric effect in inorganic NH4I with maximum
entropy changes of {\Delta}S_BCE^max ~89 J K-1 kg-1 around room temperature,
associated with the orientationally order-disorder phase transition. The phase
transition temperature, Tt, varies dramatically with pressure in a rate of
dTt/dP ~0.81 K MPa-1, which leads to a very much small saturation driving
pressure of {\Delta}P ~20 MPa, an unprecedentedly large caloric strength of
|{\Delta}S_BCE^max/{\Delta}P| ~4.45 J K-1 kg-1 MPa-1, as well as a broad
temperature window of ~68 K under an 80 MPa driving pressure. Comprehensive
characterization of the crystal structure and dynamics by neutron scattering
measurements reveals a strong reorientation-vibration coupling that is
responsible for the large pressure sensitivity of Tt. This work is expected to
advance the practical application of barocaloric refrigeration.
