Influence of humidity on complex impedance and dielectric properties of iron manganite (FeMnO₃)

Bulk samples of iron manganite with a cubic $Ia\overline{3}$ crystal structure were obtained by solid state synthesis (combination of milling and calcination) of starting hematite (Fe₂O₃) and manganese carbonate (MnCO₃) powders followed by sintering green samples at 1000 °C for 4 h. Structural characterization performed by XRD and SEM analysis confirmed the formation of iron manganite in the form of a porous network of grains and pores with average particle size of 530 and 430 nm, respectively. The influence of humidity on complex impedance and dielectric properties was monitored in the relative humidity range 30–90% in a climatic chamber in the frequency range 42 Hz–1 MHz at working temperatures of 25 and 50 °C. At the lowest frequency of 42 Hz the impedance reduced with relative humidity from 57.9 (RH 30%) to 3.21 MΩ (RH 90%) at 25 °C, and from 23.3 (RH 30%) to 1.3 MΩ (RH 90%) at 50 °C that is approximately 20 times. Analysis of complex impedance using an equivalent circuit showed the dominant influence of grain boundaries. The influence of grains (bulk) was taken into account for higher relative humidity values (70% at 25 °C and 60% at 50 °C) that is related to the mechanism of water interaction with the sample surface. The dielectric constant and loss tangent decreased with increase in frequency in accordance with the Maxwell–Wagner type interfacial polarization mechanism and also increased with increase in relative humidity.