Structural, optical and complex impedance spectroscopy study of   multiferroic Bi2Fe4O9 ceramic
  Multiferroic bismuth ferrite Bi_2Fe_4O_9 (BFO) ceramic was synthesized by conventional solid state reaction route. X-ray diffraction and Rietveld refinement show formation of single phase ceramic with orthorhombic crystal structure (space group Pbam). The morphological study depicted a well-defined grain of size $\simeq$2{\mu}m. The optical studies were carried out by using UV-Vis spectrophotometer which shows a band gap of 1.53 eV and a green emission spectrum at 537 is observed in the Photoluminescence study. The frequency dependent dielectric study at various temperature revealed that the dielectric constant decreases with increase in frequency. A noticeable peak shift towards higher frequency with increasing temperature is observed in the frequency dependent dielectric loss plot. The impedance spectroscopy shows a substantial shift in imaginary impedance (Z") peaks toward the high frequency side described that the conduction in material favoring the long range motion of mobile charge carriers. The presence of non-Debye type multiple relaxations has been confirmed by complex modulus analysis. The frequency dependent ac conductivity at different temperatures indicates that the conduction process is thermally activated. The variation of dc conductivity exhibited a negative temperature coefficient of resistance behavior. The activation energy calculated from impedance, modulus and conductivity data confirmed that the oxygen vacancies play a vital role in the conduction mechanism. 
S. R. Mohapatra, B. Sahu, T.Badapanda, M. S. Pattanaik, S. D. Kaushik,   A. K. Singh
cond-mat.mtrl-sci