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

We report detailed structural studies of the low-temperature ferroelectric phases of [NH4]M(HCO2)3, where M = Mn2+ and Zn2+, finding that a third of the NH4+ cations remain subtly rotationally disordered to low temperature in both compounds. 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 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 the neighbouring molecular 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 associated with these materials’ relaxor-like dielectric properties.