Investigations on Bi25FeO40 powders synthesized by hydrothermal and combustion-like processes

The syntheses of phase-pure and stoichiometric iron sillenite (Bi25FeO40) powders by a hydrothermal (at ambient pressure) and a combustion-like process are described. Phase-pure samples were obtained in the hydrothermal reaction at 100 degrees C (1), whereas the combustion-like process leads to pure Bi25FeO40 after calcination at 750 degrees C for 2 h (2a). The activation energy of the crystallite growth process of hydrothermally synthesized Bi25FeO40 was calculated as 48(9) kJ mol(-1). The peritectic point was determined as 797(1) degrees C. The optical band gaps of the samples are between 2.70(7) eV and 2.81(6) eV. Temperature and field-depending magnetization measurements (5-300 K) show a paramagnetic behaviour with a Curie constant of 55.66 x 10(-6) m(3) K mol(-1) for sample 1 and C=57.82 x 10(-6) m(3) K mol(-1) for sample 2a resulting in magnetic moments of mu(mag)=5.95(8) mu(B) mol(-1) and mu(mag)=6.07(4) mu(B) mol(-1). The influence of amorphous iron-oxide as a result of non-stoichiometric Bi/Fe ratios in hydrothermal syntheses on the magnetic behaviour was additionally investigated. (C) 2014 Elsevier Inc. All rights reserved.