Monitoring and Understanding the Paraelectric-Ferroelectric Phase Transition in the Metal-Organic Framework [NH4] [M(HCOO)3] by Solid-State NMR Spectroscopy

The paraelectric-ferroelectric phase transition in two isostructural metal-organic frameworks (MOFs) [NH4] [M(HCOO)(3)] (M= Mg, Zn) was investigated by in situ variable-temperature Mg-25, Zn-67, N-14, and C-13 solid-state NMR (SSNMR) spectroscopy. With decreasing temperature, a disorder-order transition of NH4+ cations causes a change in dielectric properties. It is thought that [NH4][Mg(HCOO)(3)] exhibits a higher transition temperature than [NH4][Zn(HCOO)(3)] due to stronger hydrogen-bonding interactions between NH4+ ions and framework oxygen atoms. Mg-25 and Zn-67 NMR parameters are very sensitive to temperature-induced changes in structure, dynamics, and dielectric behavior; stark spectral differences across the paraelectric-ferroelectric phase transition are intimately related to subtle changes in the local environment of the metal center. Although Mg-25 and Zn-67 are challenging nuclei for SSNMR experiments, the highly spherically symmetric metal-atom environments in [NH4][M(HCOO)(3)] give rise to relatively narrow spectra that can be acquired in 30-60 min at a low magnetic field of 9.4 T. Complementary N-14 and C-13 SSNMR experiments were performed to probe the role of NH4+-framework hydrogen bonding in the paraelectric-ferroelectric phase transition. This multinuclear SSNMR approach yields new physical insights into the [NH4][M(HCOO)(3)] system and shows great potential for molecular-level studies on electric phenomena in a wide variety of MOFs.