Electron paramagnetic resonance and electric characterization of a [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 a [CH₃NH₂NH₂][Zn(HCOO)₃] dense perovskite metal–organic framework (MOF). The pyroelectric current of a single crystal sample reveals two structural phase transitions at T_c1 = 325 and T_c2 = 173 K that are related to the ordering of CH₃NH₂NH₂⁺ cations. The dielectric permittivity exhibits a small kink at T_c1 implying improper ferroelectric phase transition, while much stronger anomaly is observed at T_c2. The 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 powder MOF samples doped with small amounts of paramagnetic Mn²⁺ and Cu²⁺ probe ions. CW EPR spectra reveal the successful incorporation of these ions at the Zn²⁺ lattice sites, while ENDOR measurements indicate several proton species that are in excellent agreement with the X-ray diffraction data. The CW EPR linewidth and intensity of the Mn²⁺ spectra demonstrate anomalies at the phase transition points. The direct measurements of the phase memory time T_m of the Mn²⁺ centers indicate a second motional process of CH₃NH₂NH₂⁺ cations below T_c2. 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 T_c2 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 T_c2.