title: Monitoring and understanding the paraelectric-ferroelectric phase transition in the metal-organic framework [NH4 ][M(HCOO)3 ] by solid-state NMR spectroscopy.
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 (25) Mg, (67) Zn, (14) N, and (13) C 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. (25) Mg and (67) Zn 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 (25) Mg and (67) Zn 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 (14) N and (13) C 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.
authors.affiliation: Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 (Canada).
authors.forename: Jun
authors.initials: J
authors.lastname: Xu
authors.affiliation: Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 (Canada).
authors.forename: Bryan E G
authors.initials: BE
authors.lastname: Lucier
authors.affiliation: Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 (Canada).
authors.forename: Regina
authors.initials: R
authors.lastname: Sinelnikov
authors.affiliation: Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5 (Canada).
authors.forename: Victor V
authors.initials: VV
authors.lastname: Terskikh
authors.affiliation: Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 (Canada).
authors.forename: Viktor N
authors.initials: VN
authors.lastname: Staroverov
authors.affiliation: Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 (Canada). yhuang@uwo.ca.
authors.forename: Yining
authors.initials: Y
authors.lastname: Huang
keywords: NMR spectroscopy; density functional calculations; ferroelectrics; metal-organic frameworks; phase transitions