Thermal behaviour of the system Fe(NO3)3 · 9H2O–Bi5O(OH)9(NO3)4 · 9H2O–glycine/urea and of their generated oxides (BiFeO3)

Bismuth ferrite (BiFeO3) was obtained by a combustion reaction staring from two precursors systems, namely Fe(NO3)3 · 9H2O–Bi5O(OH)9(NO3)4 · 9H2O–glycine/urea with different metal nitrate/fuel molar ratios. The precursors’ thermal behavior is dependent on the fuel nature but practically independent to the fuel content. In glycine containing systems not all Bi2O3 is included into mixed oxides during the decomposition. Its presence was identified through the existence of two endothermic phase transitions (TDTA max at 745 and 818 °C) assigned to Bi2O3 α→δ transition, and its melting. The thermal investigations performed on oxides samples reveal for all oxides, independent on the precursor system, a similar behavior. For all the oxides was identified both the Curie temperature (which decreases with the annealing cycles) and the incongruent melting point (which is with ~10 °C higher for glycine generated oxides comparative with urea ones). The structural analysis shows in the case of the oxides prepared using urea as fuel, a faster evolution toward a single phase composition with the temperature, the formation of the BiFeO3 perovskite phase being completed in the temperature range of 500–550 °C. Only some traces of Bi36Fe2O57 were identified at the detection limit. TEM analysis performed on the BiFeO3 thermally treated at 500 °C for 3 h revealed the presence of small particles with an average size of ~33 nm and polycrystalline agglomerates with an average size of ~100 nm for glycine/urea derived oxides.