Rotational motions of NH+4 groups in NH4ZnF3 studied by quasielastic neutron scattering

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Abstract
The rotational behavior of the ammonium groups in the high‐temperature cubic phase of the perovskite NH4ZnF3 has been investigated, using incoherent quasielastic neutron scattering. The measurements were carried out for various wave vector transfers between 0.68 and 5.52 Å−1 and for temperatures between the transition temperature of 115.1 and 300 K. The data are compared with some reorientational jump models. It is assumed that in the cubic phase the NH4 group is a slightly distorted tetrahedron, apparently due to strong hydrogen–fluorine interactions. The degree of distortion could, however, not be determined. On the time scale of the experiment (?3 ps) the reorientational behavior of the NH4 group can be described by 90° jumps about its axis of smallest moment of inertia. Residence times between 0.4 and 7 ps were found. Their temperature dependence follows an Arrhenius behavior, yielding Ea/kB=580±15 K, which agrees with calculations for the quantum mechanical energy level scheme.