We report the results of a high-resolution powder neutron-diffraction study of multiferroic BiFe${}_{0.7}$Mn${}_{0.3}$O${}_3$. We have confirmed the previous assignment of the $\alpha$ phase and $\beta$ phase as having $R3c$ and $Pbnm$ symmetry, respectively. The $\gamma$ phase, however, has been shown unequivocally not to be cubic, as previously reported, but rather to retain octahedral tilting distortions leading to a lower symmetry, most likely rhombohedral with space group $R\overline{3}c$. The $\gamma$ phase of BiFe${}_{0.7}$Mn${}_{0.3}$O${}_3$ is therefore different than that of the parent compound BiFeO${}_3$, which retains an orthorhombic structure up to its decomposition temperature. The lattice parameters of the $\gamma$ phase of BiFe${}_{0.7}$Mn${}_{0.3}$O${}_3$ are pseudocubic throughout its entire stability range, despite the fact that there are still significant distortions from the cubic aristotype structure even at the onset of decomposition.

• Received 16 April 2013

DOI:https://doi.org/10.1103/PhysRevB.87.224109