Understanding the motional dynamics of the ammonium ion in the mechanism of multiferroicity of Cr(V) peroxychromates: a 1 H NMR study.

Cr(5+)-based peroxychromates, M 3 Cr(O 2 ) 4 , with M = NH 4 or a mixed NH 4 -alkali metal are a new class of multiferroics for potential use in molecular memory devices, with the NH 4 + being a key element, but the underlying chemical mechanism is not fully understood. The NH 4 + ion occupies two different sites, but their specific roles are not known. We thus performed detailed 1 H NMR spin-relaxation ( T 1 ) measurements on (NH 4 ) 3 Cr(O 2 ) 4 over a wide temperature range (120-300 K) to probe the displacive as well as hindered rotational dynamics of the NH 4 + ions with the view of understanding their specific roles in the phase transitions. The NH 4 + dynamics is seen to consist of at least three different processes with varying activation energies. The sharp jump in the T 1 at around 250 K is assigned to the change in the displacive motion at one of the two sites, while a kink around 140 K is ascribed to motional slowing at the second site. Interestingly, the slowing down starts around 250 K, well above the structural phase transition at 140 K. Taken together, these results provide a clue to the role of the site and symmetry of the NH 4 + ion in the mechanism of solid-solid phase transitions.