Bismuth ferrite
Bismuth ferrite is an inorganic chemical compound with perovskite structure and one of the most promising multiferroic materials. The room-temperature phase of BiFeO3 is classed as rhombohedral belonging to the space group R3c. It is synthesized in bulk and thin film form and both its antiferromagnetic Néel temperature and ferroelectric Curie temperature are well above room temperature. Ferroelectric polarization occurs along the pseudocubic direction with a magnitude of 90–95 μC/cm2.
Sample Preparation
Bismuth ferrite is not a naturally occurring mineral and several synthesis routes to obtain the compound have been developed.Solid state synthesis
In the solid state reaction method bismuth oxide and iron oxide in a 1:1 mole ratio are mixed with a mortar or by ball milling and then fired at elevated temperatures. Preparation of pure stoichiometric BiFeO3 is challenging due to the volatility of bismuth during firing which leads to the formation of stable secondary Bi25FeO39 and Bi2Fe4O9 phase. Typically a firing temperature of 800 to 880 Celsius is used for 5 to 60 minutes with rapid subsequent cooling. Excess Bi2O3 has also been used a measure to compensate for bismuth volatility and to avoid formation of the Bi2Fe4O9 phase.Single crystal growth
Bismuth ferrite melts incongruently, but it can be grown from a bismuth oxide rich flux. High quality single crystals have been important for studying the ferroelectric, antiferromagnetic and magnetoelectric properties of bismuth ferrite.Chemical routes
Wet chemical synthesis routes based on sol-gel chemistry, modified Pechini routes, hydrothermal synthesis and precipitation have been used to prepare phase pure BiFeO3. The advantage of the chemical routes is the compositional homogeneity of the precursors and the reduced loss of bismuth due to the much lower temperatures needed. In sol-gel routes, an amorphous precursor is calcined at 300-600 Celsius to remove organic residuals and to promote crystallization of the bismuth ferrite perovskite phase, while the disadvantage is that the resulting powder must be sintered at high temperature to make a dense polycrystal.Solution combustion reaction is a low-cost method used to synthesize porous BiFeO3. In this method, a reducing agent and an oxidizing agent are used to generate the reduction-oxidation reaction. The appearance of the flame, and consequently the temperature of the mixture, depends on the oxidizing/reducing agents ratio used. Annealing up to 600 °C is sometimes needed to decompose the bismuth oxo-nitrates generated as intermediates. Since the content of Fe cations in this semiconductor material, Mӧssbauer spectroscopy is a proper technique to detect the presence of a paramagnetic component in the phase.