Investigation of the optical and dielectric properties of NaCu_0.2Fe_0.8-xMn_xO₂ (x=0.4; 0.5; 0.6; 0.7) layered oxide materials

NaCu0.2Fe0.8-xMnxO2 compounds were prepared by the solid state method. The structural analyses confirmed that the samples crystallize in hexagonal system with R-3 m space group. Fourier transform infrared spectroscopy shows vibrational modes of the compounds NaCu0.2Fe0.8-xMnxO2 and proves the presence of the octahedral environment MO6 (M = Cu, Fe, Mn) which is in good agreement with the structural study. The investigation by UV-visible spectroscopy revealed a strong absorption of the prepared samples in the UV-visible range and allowed extracting the band gap (2.8 eV < E-g < 3 eV) which shows a small increase with manganese content and exhibits the samples semiconductor behavior. Additionally, the study of the real part of the dielectric permittivity using impedance spectroscopy demonstrated a high dielectric constant at low frequency and suggested the existence of space charge polarization and dipolar polarization. However, the study of the imaginary part of the dielectric permittivity proved that the dipolar polarization may be masked due to the high ionic DC conductivity. Finally, modulus formalism indicated two relaxations attributed to the grain boundary and grain relaxations.