5 years ago

Single Crystal Electron Paramagnetic Resonance of Dimethylammonium and Ammonium Hybrid Formate Frameworks: Influence of External Electric Field

Single Crystal Electron Paramagnetic Resonance of Dimethylammonium and Ammonium Hybrid Formate Frameworks: Influence of External Electric Field
Ju̅ras Banys, Sergejus Balčiu̅nas, Andreas Pöppl, Mirosław Ma̧czka, Gunnar Jeschke, Monika Trzebiatowska, Anastasia Kultaeva, Daniel Klose, Mantas Šimėnas
We present a continuous wave electron paramagnetic resonance (EPR) study of a Mn2+ doped [(CH3)2NH2][Zn(HCOO)3] hybrid dense metal–organic framework (MOF) that exhibits an order–disorder structural phase transition at Tc = 163 K. The W-band EPR measurements of a powder sample are performed to verify the previously reported spin Hamiltonian parameters of the Mn2+ centers in the low-temperature phase. The temperature dependent single crystal X-band EPR experiments reveal that Mn2+ probe ions are susceptible to the phase transition, as the spectrum changes drastically at Tc. The angular dependent EPR spectra of Mn2+ centers are obtained by rotating the single crystal sample about three distinct directions. The simulation of the determined angular dependences reveals six MnO6 octahedra in the ordered phase that originate from a severe crystal twinning of the [(CH3)2NH2][Zn(HCOO)3] MOF. The possible ferroelectric origin of the crystalline twins is investigated by single crystal EPR measurements with an applied external electric field. No significant effect of the electric field on the spectra is observed. The EPR results are supported by the measurements of the electric field dependence of the macroscopic electric polarization. Analogous EPR measurements are performed on a single crystal sample of ferroelectric Mn2+ doped [NH4][Zn(HCOO)3] MOF. Contrary to the dimethylammonium framework, the EPR signal and electric polarization of the ammonium compound demonstrate clear ferroelectric behavior.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b06257

DOI: 10.1021/acs.jpcc.7b06257

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