A new avenue to relaxor-like ferroelectric behaviour found by probing the structure and dynamics of [NH3NH2]Mg(HCO2)3

Abstract

The field of relaxor ferroelectrics has long been dominated by ceramic oxide materials exhibiting large polarisations with temperature and frequency dependence. Intriguingly, the dense metal–organic framework (MOF) [NH₃NH₂]Mg(HCO₂)₃ was reported as one of the first coordination frameworks to exhibit relaxor-like properties. This work clarifies the origin of these relaxor-like properties through re-examining its unusual phase transition using neutron single crystal diffraction, along with solid-state NMR, quasielastic neutron scattering and dielectric spectroscopy studies. This reveals that the phase transition is caused by the partial reorientation of NH₃NH₂ within the pores of the framework, from lying in the planes of the channel at lower temperatures to along the channel direction above the transition temperature. The transition occurs via a dynamic process such that the NH₃NH₂ cations can slowly interconvert between parallel and perpendicular orientations, with an estimated activation energy of 50 kJ mol⁻¹. Furthermore these studies are consistent with proton hopping between the hydrazinium cations oriented along the channel direction via a proton site intermediate. This study suggests the ferroelectric properties of [NH₃NH₂]Mg(HCO₂)₃ are likely driven by a hydrogen bonding mechanism. The relaxor behaviour is proposed to be the result of polar regions, which likely fluctuate due to increased cation dynamics at high temperature. The combination of cation reorientation and proton hopping fully describes this material's relaxor-like behaviour, suggesting a route to the future design of non-oxide-based relaxor ferroelectrics.