Phase stability and structural temperature dependence in powdered multiferroic BiFeO3

We report a temperature-dependent investigation of the multiferroic perovskite bismuth ferrite BiFeO3 (BFO) by using x-ray powder diffraction together with differential scanning calorimetry measurements. Our results provide evidence that the paraelectric phase above T-c=820 degrees C is not cubic but distorted and can be well refined in a monoclinic P2(1)/m space group. An equivalent structure can be reconstructed based on the C2/m monoclinic space group and by assuming two types of bismuth sites. The marked change of the cell volume at T-c provides evidence for the first-order nature of the R3c-to-P2(1)/m transition. The high-temperature P2(1)/m phase is centrosymmetric and characterized by (i) strong oxygen octahedra tilting along the b axis; (ii) the occurrence of antiferroelectric displacements of the Fe cations; and (iii) an interesting lamellar structure characterized by two different types of BiO12 cages. The temperature-induced lamellar structure suggests a significant electronic rearrangement in terms of chemical bonding, which in turn might condition anisotropic electronic properties. The occurrence of a lamellar structure provides also an understanding of why BFO decomposes suddenly at higher temperatures. Finally, an anomaly in the evolution of the cell parameters at T-N underlines the spin-lattice coupling in proximity of the magnetic transition.