Probing the Structure and Dynamics of the [NH4]M(HCO2)3 Ferroelectric Phases: Dielectric Relaxation through Orientational Disorder†

Neutron diffraction studies of the low-temperature relaxor ferroelectric phases of [NH₄]M(HCO₂)₃, where M = Mn²⁺ and Zn²⁺, show that a third of the NH₄⁺ cations remain subtly structurally disordered to low temperature. All NH₄⁺ cations within the channels are well separated from each other, with significant hydrogen bonds only with the anionic M(HCO₂)₃ framework. Complementary studies of the dynamics using ²H solid state NMR and quasielastic neutron scattering indicate significant rotational motion in both paraelectric and ferroelectric phases, which evolves gradually with increasing temperature with no abrupt change at the phase transition. Nudged elastic band calculations suggest that the activation barrier for flipping between “up” and “down” orientations of the NH₄⁺ cations is low in the ferroelectric phase, with the NH₄⁺ cations primarily interacting with the framework rather than neighbouring NH₄⁺ cations. It is likely this motion that is responsible for scrambling the NH₄⁺ cation orientation locally in the ferroelectric phase. We propose that this disorder, with the same basic motion active above and below the phase transition, induces the significant dielectric relaxation in these materials. This suggests that orientational disorder may be an effective substitution for compositional disorder commonly associated with relaxor ferroelectrics in molecular materials.