The dynamics of NH₄⁺ in the NH₄MF₃ perovskites: I. A quasielastic neutron scattering study

Quasielastic neutron scattering experiments have been carried out to study the thermally activated rotational process of the NH₄⁺ ion in the perovskite family NH₄MF₃ (M=Mg, Zn, Mn and Cd). The activation energies of this process in the high- and low-temperature structural phases have been obtained, and range from 44.8 meV for NH₄MgF₃ to 128 meV for NH₄CdF₃ in the high-temperature phase, and from 75 meV for NH₄ZnF₃ to 246 meV for NH₄CdF₃ in the low-temperature phase. Geometric models for the orientations of the NH₄⁺ ion occupied during the rotational process are proposed in both phases. In the high-temperature phase, the dominant rotations are around twofold axes. A preferential axis, determined by the orientations of the ion in the crystal, shows the greatest jump rate, in the low-temperature phase the rotational disorder persists through two energetically non-equivalent orientations.