Displacive Phase Transitions and Magnetic Structures in Nd-Substituted BiFeO3

Neutron powder diffraction was used to determine changes in the nuclear and magnetic structure; of Bi1-xNdx,FeO3 polymorphs involved in the first-order displacive phase transitions from the high-temperature nonpolar phase to the low temperature polar (x <= 0.125) and antipolar (0.125 <= x <= 0.25) phases, respectively. The high-temperature phase (O-1), which crystallizes with a structure similar to the room-temperature form of NdFeO3, exhibits Pbnm symmetry and unit cell root 2a(c) x root 2a(c) x 2a(c) (where a(c) approximate to 4 angstrom is the lattice parameter of an ideal cubic perovskite), determined by a(-) a(-) c(+) octahedral tilting. The low-temperature polar structure (R) is similar to the beta-phase of BiFeO3 and features rhombohedral symmetry determined by a(-)a(-)a(-) octahedral rotations and cation displacements. The recently discovered antipolar phase (O-2) resembles the antiferroelectric Pbam (root 2a(c) x 2 root 2a(c) x 2a(c)) structure of PbZrO3 but with additional displacements that double the PbZrO3 unit cell along the c-axis to root 2a(c) x 2 root 2a(c) x 4a(c) and yield Pbnm symmetry. The O-1 <-> R and O-1 <-> O-2 transitions are both accompanied by a large discontinuous expansion of the lattice volume in the low-temperature structures with a contrasting contraction of the [FeO6] octahedral volume and an abrupt decrease in the magnitude of octahedral rota:ions. The O-1 <-> O-2 transition, which occurs in the magnetic state, is accompanied by an abrupt approximate to 90 degrees reorientation of the magnetic dipoles. This coupling between the nuclear and magnetic structures is manifested in a significant magnetization anomaly. Below 50 K, reverse rotation of magnetic dipoles back to the original orientations in the high-temperature O-1 structure is observed.