We report a temperature-dependent investigation of the multiferroic perovskite bismuth ferrite ${\text{BiFeO}}_3$ (BFO) by using x-ray powder diffraction together with differential scanning calorimetry measurements. Our results provide evidence that the paraelectric phase above $T_c=820°\text{C}$ is not cubic but distorted and can be well refined in a monoclinic $P{2}_1/m$ 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-$P{2}_1/m$ transition. The high-temperature $P{2}_1/m$ 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 ${\text{BiO}}_{12}$ 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.

• Received 12 December 2007

DOI:https://doi.org/10.1103/PhysRevB.78.134108