Monitoring and Understanding the Paraelectric–Ferroelectric Phase Transition in the Metal–Organic Framework [NH4][M(HCOO)3] by Solid‐State NMR Spectroscopy |
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| Authors: | Dr Jun Xu Dr Bryan E G Lucier Regina Sinelnikov Dr Victor V Terskikh Prof Viktor N Staroverov Prof Yining Huang |
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| Affiliation: | 1. Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 (Canada);2. Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5 (Canada) |
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| Abstract: | 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. |
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| Keywords: | density functional calculations ferroelectrics metal– organic frameworks NMR spectroscopy phase transitions |
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