Reorientation of magnetic dipoles at the antiferroelectric-paraelectric phase transition of Bi1-xNdxFeO3 (0.15 <= x <= 0.25)

Ceramic Bi1-xNdxFeO3 solid solutions in the approximate compositional range 0.15 <= x <= 0.25 undergo first-order antiferroelectric (AFE) <-> paraelectric (PE) phase transitions between 300 degrees C and room temperature. The orthorhombic AFE (Pnam, root 2a(c) x 2 root 2a(c) x 4a(c), where a(c) approximate to 4 angstrom and is the lattice parameter of an ideal cubic perovskite) and PE (Pbnm, root 2a(c) x root 2a(c) x 2a(c)) structures differ not only in the type of octahedral tilt system but also in the pattern of A-cation (Bi/Nd) displacements. The AFE <-> PE transition is accompanied by a large (approximate to 2%) discontinuous volume change similar to that observed at the ferroelectric (FE) <-> PE transition in BiFeO3. For all compositions, the structural transition occurs within the G-type antiferromagnetic (AFM) state. Variable-temperature magnetic measurements reveal a strong coupling between the AFE <-> PE transition and the magnetic ordering. Rietveld refinements using neutron powder-diffraction data demonstrate that the structural transition is accompanied by reorientation of magnetic dipoles within the G-type AFM array resulting in a significant increase in magnetization.