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

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.

Proton Resonance and Critical Relaxation in NH4MnF3and NH4ZnF3

“…The activation temperature EJkB = 650 K that we have found for NH4ZnF3 above 115K is somewhat higher than the 585 f 15K that Steenbergen et al [26] have found from neutron scattering. Burriel [27, quoted in 171 has attempted to calculate the electrostatic barriers in NH4ZnF3 and NH4MnF3 but his results of order 2000K are clearly too large.…”