Effect of Al substitution on the magnetic, optical and electronic properties of KBiFe2O5 brownmillerite

We reveal the influence of non-magnetic Al3+ ion on magnetic, optical and electronic properties of KBiFe2O5. Al doped KBiFe2-xAlxO5 (x = 0, 0.2, 0.4) are synthesized by citrate combustion method. All the samples are found to be crystalized in monoclinic structure with P2/c space group. With Al doping, the grain size of the samples is reduced however, Fe ions maintain its +3 state. A mere difference in the low temperature zero field cooling and field cooling curve discards the multiple magnetic phases at low temperature, however, with Al, an improved magnetic moment is detected in the magnetic isotherms. A schematic model is adopted to explain the enhanced magnetization. Furthermore, the magnetic anomaly around the dielectric transition (760 -780 K) depicts an existence of magnetoelectric coupling in all the samples. The optical band gap is reduced from 1.73 eV to 1.69 eV with Al doping. Extensive dielectric and electric modulus analysis is carried out to explore the conduction as well as relaxation process in the parent and Al doped system. The activation energy estimated from the electric modulus is (0.412 ± 0.021) eV for x = 0 and (0.370 ± 0.025) eV for x = 0.2 respectively but surprisingly, Al doping with x = 0.4 shows a completely different relaxation mechanism. Moreover, the frequency dependent ac conductivity is also explained and agrees well with the electric modulus data.