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

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 <i>T</i>_c = 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 <i>T</i>_c. 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