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