Schematic representation of the - and -phases of BiFeO3 showing the change in structural symmetry from rhombohedral to orthorhombic at the ferroelectric-paraelectric phase transition.
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Multiferroics are technologically-important materials which simultaneously exhibit electric order (alignment of electric dipoles in an electric field) and magnetic order (alignment of magnetic spins in a magnetic field).
This makes them potentially useful in devices such as random access memories, sensors and actuators. BiFeO3 is the most widely studied multiferroic as both the magnetic and electric ordering occur at room temperature. The room temperature structure of BiFeO3 (∝-phase) has been well established as rhombohedral. However the transition away from electric ordering at high temperatures (ferroelectric, ∝, to paraelectric, β, transition around 820 – 830oC) is coupled with a change in structural symmetry. The exact nature of the β-phase has been a subject of much dispute with many different symmetries reported. We used high resolution neutron diffraction to investigate the β-phase with the aim of resolving these discrepancies. We were able to clearly demonstrate that this phase is orthorhombic, ruling out some of the previously suggested models, and subsequently present the first full crystallographic model.
DC Arnold, FD Morrison, P Lightfoot (University of St Andrews), KS Knight (ISIS)
Research date: September 2009
Contact: Prof Phil Lightfoot, firstname.lastname@example.org
Further reading: DC Arnold et al, Phys Rev Lett 102 (2009) 027602
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