Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn2+ doped [NH4][Zn(HCOO)3] hybrid formate framework

• Published in: Physical chemistry chemical physics : PCCP 2020-01, Vol.22 (16), p.8513-8521
• Main Authors: Navickas, Marius, Giriūnas, Laisvydas, Kalendra, Vidmantas, Biktagirov, Timur, Gerstmann, Uwe, Wolf Gero Schmidt, Mączka, Mirosław, Pöppl, Andreas, Banys, Jūras, Mantas Šimėnas
• Format: Article
• Language: English
• Subjects: Continuous radiation; Density functional theory; Electron paramagnetic resonance; Ferroelectric materials; Ferroelectricity; Manganese ions; Order parameters; Phase transitions; Relaxation time; Spectra; Splitting; Temperature; Temperature dependence; Transition temperature
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d0cp01612h

We present an X- and Q-band continuous wave (CW) and pulse electron paramagnetic resonance (EPR) study of a manganese doped [NH4][Zn(HCOO)3] hybrid framework, which exhibits a ferroelectric structural phase transition at 190 K. The CW EPR spectra obtained at different temperatures exhibit clear changes at the phase transition temperature. This suggests a successful substitution of the Zn2+ ions by the paramagnetic Mn2+ centers, which is further confirmed by the pulse EPR and 1H ENDOR experiments. Spectral simulations of the CW EPR spectra are used to obtain the temperature dependence of the Mn2+ zero-field splitting, which indicates a gradual deformation of the MnO6 octahedra indicating a continuous character of the transition. The determined data allow us to extract the critical exponent of the order parameter (β = 0.12), which suggests a quasi two-dimensional ordering in [NH4][Zn(HCOO)3]. The experimental EPR results are supported by the density functional theory calculations of the zero-field splitting parameters. Relaxation time measurements of the Mn2+ centers indicate that the longitudinal relaxation is mainly driven by the optical phonons, which correspond to the vibrations of the metal–oxygen octahedra. The temperature behavior of the transverse relaxation indicates a dynamic process in the ordered ferroelectric phase.