High-temperature phases of multiferroic BiFe_0.7Mn_0.3O₃

We report the results of a high-resolution powder neutron-diffraction study of multiferroic BiFe0.7Mn0.3O3. 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 (3) over barc. The gamma phase of BiFe0.7Mn0.3O3 is therefore different than that of the parent compound BiFeO3, which retains an orthorhombic structure up to its decomposition temperature. The lattice parameters of the gamma phase of BiFe0.7Mn0.3O3 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.