Displacive Phase Transitions and Magnetic Structures in Nd-Substituted BiFeO₃

Abstract

Neutron powder diffraction was used to determine changes in the nuclear and magnetic structures of Bi1−xNdxFeO3 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 (O1), which crystallizes with a structure similar to the room-temperature form of NdFeO3, exhibits Pbnm symmetry and unit cell √2ac × √2ac × 2ac (where ac ≈ 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 BiFeO3 and features rhombohedral symmetry determined by a−a−a− octahedral rotations and cation displacements. The recently discovered antipolar phase (O2) resembles the antiferroelectric Pbam (√2ac × 2√2ac × 2ac) structure of PbZrO3 but with additional displacements that double the PbZrO3 unit cell along the c-axis to √2ac × 2√2ac × 4ac and yield Pbnm symmetry. The O1 ↔ R and O1 ↔ O2 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 rotations. The O1 ↔ O2 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 O1 structure is observed.