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Electron paramagnetic resonance and electric characterization of [CH3NH2NH2][Zn(HCOO)3] perovskite metal formate framework

Abstract

We present a combined continuous-wave (CW) and pulse electron paramagnetic resonance (EPR), pulse electron-nuclear double resonance (ENDOR), pyrocurrent as well as broadband dielectric study of [CH3NH2NH2][Zn(HCOO)3] dense perovskite metal-organic framework (MOF). The pyroelectric current of a single crystal sample reveals two structural phase transitions at Tc1 = 325 and Tc2 = 173 K that are related to the ordering of CH3NH2NH2+ cations. The dielectric permittivity exhibits a small kink at Tc1 implying improper ferroelectric phase transition, while much stronger anomaly is observed at Tc2. Dielectric spectra of the intermediate phase reveal a Cole-Cole relaxation process that is assigned to the hopping motion of the CH3NH2NH2+ cations. EPR and ENDOR experiments are performed on the powder MOF samples doped with small amount of paramagnetic Mn2+ and Cu2+ probe ions. CW EPR spectra reveal successful incorporation of these ions at the Zn2+ lattice sites, while ENDOR measurements indicate several proton species that are in an excellent agreement with the X-ray diffraction data. CW EPR linewidth and intensity of the Mn2+ spectra demonstrate anomalies at the phase transition points. The direct measurements of the phase memory time Tm of the Mn2+ centers indicate a second motional process of CH3NH2NH2+ cations below Tc2. The measurements of the longitudinal relaxation time T1 of the low-temperature phase reveal a coupling between the electron spins and a hard optical phonon mode which undergoes a damping due to the coupling with the relaxational mode as Tc2 is approached. The temperature dependent Mn2+ and Cu2+ spectra reflect the structural changes of the metal-oxygen octahedra. The fine structure splitting of Mn2+ ions is increasing as the temperature is decreased reflecting a distortion of the MnO6 octahedra. The Cu2+ hyperfine interaction demonstrates a first-order character close to the tricritical limit of the phase transition at Tc2.

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Publication details

The article was received on 17 Mar 2017, accepted on 05 Apr 2017 and first published on 06 Apr 2017


Article type: Paper
DOI: 10.1039/C7TC01140G
Citation: J. Mater. Chem. C, 2017, Accepted Manuscript
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    Electron paramagnetic resonance and electric characterization of [CH3NH2NH2][Zn(HCOO)3] perovskite metal formate framework

    M. Simenas, S. Balciunas, M. Trzebiatowska, M. Ptak, M. Maczka, G. Völkel, A. Poppl and J. Banys, J. Mater. Chem. C, 2017, Accepted Manuscript , DOI: 10.1039/C7TC01140G

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