Bismuth ferrite (BiFeO3, also commonly referred to as BFO in Materials Science) 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 (G type ordering) Néel temperature and ferroelectric Curie temperature are well above room temperature (approximately 653 K and 1100K, respectively). Ferroelectric polarization occurs along the pseudocubic 111 direction with a magnitude of 90–95 μC/cm2.
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 (Bi2O3) and iron oxide (Fe2O3) in a 1:1 mole ratio are mixed with a mortar, or by ball milling and then fired at elevated temperatures. The volatility of bismuth and the relatively stable competing ternary phases Bi25FeO39 (sillenite) and Bi2Fe4O9 (mullite) makes the solid state synthesis of phase pure and stoichiometric bismuth ferrite challenging. 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 (e.g. a 4:1:1 mixture of Bi2O3, Fe2O3 and B2O3 at approximately 750-800 Celsius). High quality single crystals have been important for studying the ferroelectric, antiferromagnetic and magnetoelectric properties of bismuth ferrite.
Wet chemical synthesis routes based on sol-gel chemistry, modified Pechini routes or hydrothermal synthesis 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.
Thin and films
The electric and magnetic properties of high quality epitaxial thin films of bismuth ferrite reported in 2003 revived the scientific interest for bismuth ferrite. Epitaxial thin films have the great advantage that they can be integrated in electronic circuitry. Epitaxial strain induced by singly crystalline substrates with different lattice parameters than bismuth ferrite can be used to modify the crystal structure to monoclinic or tetragonal symmetry and change the ferroelectric, piezoelectric or magnetic properties. Pulsed laser deposition (PLD) is a very common route to epitaxial BiFeO3 films, and SrTiO3 substrates with SrRuO3 electrodes are typically used. Sputtering, metal organic chemical vapor deposition (MOCVD) and chemical solution deposition are other methods to prepare epitaxial bismuth ferrite thin films.
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