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

• Published in: Chemistry : a European journal Vol. 21; no. 41; pp. 14348 - 14361
• Main Authors: Xu , Jun, Lucier , Bryan E. G., Sinelnikov, Regina, Terskikh, Victor V., Staroverov, Viktor N., Huang, Yining
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 05.10.2015; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Cations; Chemistry; density functional calculations; Dielectric strength; Electric potential; ferroelectrics; Metal-organic frameworks; Metalorganic compounds; NMR spectroscopy; Nuclear magnetic resonance; Phase transformations; Phase transitions; Spectra; Spectroscopy; Spectrum analysis; density functional calculations; ferroelectrics; phase transitions; NMR spectroscopy; metal-organic frameworks
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201501954

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 25Mg, 67Zn, 14N, and 13C 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. 25Mg and 67Zn 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 25Mg and 67Zn 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 14N and 13C 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. Through the SSNMR lens: Miniscule molecular‐level modifications can lead to major differences in macroscale properties. The paraelectric–ferroelectric phase transition in [NH4][M(HCOO)3] (M=Mg, Zn) metal–organic frameworks was investigated by in situ variable temperature 25Mg, 67Zn, 14N, and 13C solid‐state NMR (SSNMR) spectroscopy. The NMR parameters can be obtained rapidly and reflect temperature‐induced changes in the material; stark spectral differences across the phase transition and within each phase are related to subtle structural evolution (see figure).