Synthesis, phase evolution and properties of phase-pure nanocrystalline BiFeO3 prepared by a starch-based combustion method

The preparation of phase-pure nano-sized BiFeO3 by a combustion-like method using starch as complexing agent is described herein. Phase evolution and development of the crystallite size during the synthesis were monitored depending on the heat treatment and the composition of the (BiFe)-gels. Phase-pure BiFeO3 was obtained at a low heating rate and calcination temperatures between 500 and 600 degrees C. Above 600 degrees C the BiFeO3 gradually decomposed to Bi25FeO40 and Bi2Fe4O9. The investigations showed that the appearance of secondary phases depends on the heating rate, calcination temperature, and the fuel to oxidizer ratio in the (BiFe)-gel. The use of HNO3 instead of acetic acid in the preparation of the (BiFe)gel promotes the formation of secondary phases. To study the phase stability the phase-pure BiFeO3 powder (1c) obtained after calcining at 550 degrees C (d(cryst) = 37 nm) was sintered to ceramic bodies up to 800 degrees C. During sintering the BiFeO3 phase decomposed to Bi25FeO40 and Bi2Fe4O9 gradually. The activation energy for the decomposition process during sintering was calculated to 337 +/- 19 kJ/mol using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. Magnetic measurements on phase-pure BiFeO3 powders show maximal magnetization of about 0.7 emu/g at 90 kOe and coercivities between 5 and 7 kOe at 300 K. Investigations at 10 K reveal a loop shift (exchange-bias) up to 2.9 kOe in the negative direction. The optical band gaps of the phase-pure BiFeO3 powders were determined as 2.28(4) eV. (C) 2013 Elsevier B.V. All rights reserved.