Local atomic structures and lattice dynamics of inverse colossal barocaloric ammonium thiocyanate

Zhang, Z.*; Gong, W.   ; Kawakita, Yukinobu  ; 9 of others*

NHSCN exhibits successive phase transitions from monoclinic to orthorhombic, and then to tetragonal. The former phase transition is accompanied by an inverse colossal barocaloric effect and a huge negative thermal expansion. This transition is complicated by the possibly coexisting orientational disorder of NH and SCN, and the mechanism is still unclear. Here, NHSCN is investigated using high-resolution neutron powder diffraction, pair distribution function analysis, and Raman scattering. Both the average and local structural analysis indicate that only NH is orientationally disordered in the orthorhombic phase and SCN becomes subsequently disordered in the tetragonal phase. Both temperature-induced and pressure-induced hardening behaviors of SCN related vibrations are observed, which leads to the breakage of hydrogen bonds-linked NH with SCN and further facilitates phase transition. Our findings provide a solid standing point of atomic structures and dynamics, on which the scenario of the phase transition and the associated physical properties is established.