Displacive Phase Transitions and Magnetic Structures in Nd-Substituted BiFeO 3

Neutron powder diffraction was used to determine changes in the nuclear and magnetic structures of Bi 1− x Nd x FeO 3 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 NdFeO 3, exhibits Pbnm symmetry and unit cell √2 a c × √2 a c × 2 a c (where a c ≈ 4 Å 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 β-phase of BiFeO 3 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 (√2 a c × 2√2 a c × 2 a c ) structure of PbZrO 3 but with additional displacements that double the PbZrO 3 unit cell along the c -axis to √2 a c × 2√2 a c × 4 a 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 [FeO 6 ] octahedral volume and an abrupt decrease in the magnitude of octahedral rotations. The O 1 ↔ O 2 transition, which occurs in the magnetic state, is accompanied by an abrupt ≈90° 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.