Optical and dielectric properties of ultra-fine Mn doped BiFeO3 nanoparticles

Mn doped and pure BiFeO3 (BFO) with formula BiFe1-xMnxO3 (0 ≤ x ≤ 0.08) were prepared by hydrothermal process. The effects of doping on the microstructure and optical properties have been investigated using X-ray diffraction and both Raman and reflectance diffuse spectroscopies. XRD study revealed that pure and 2 and 4% doped samples present only the rhombohedral symmetry (R3c) typical structure of BFO while 6 and 8% doped samples contain also a cubic (I23) structure that can be attributed to Bi25MnO40 phase. Results showed a variation of structure parameters indicating that Mn can successfully substitute Fe until 4%. Raman spectroscopy revealed a structure distortion caused by the Mn insertion in oxygen octahedra. Temperature depending Raman study confirmed the stability of samples in the range of temperature measurement (from 300 to 800 k) and marked the presence of the Neel magnetic transition around the expected TN value. Compared with pure BiFeO3, Mn doped samples present remarkable changes in band structure as shown in plotted Kubelka-Munk function which fit well with the obtained structure results. Direct optical bandgap determined from Tauc plot showed an important decrease in value, which is interesting for photoinduction applications. Dielectric analysis reveals an increase of the dielectric constant and the AC conductivity after Mn doping which can be related to the replacement of Fe3+ by Mn2+ that could enlarge the density of vacancies in the material.