The phenomenon of ferroelectricity, in which a compound exhibits a spontaneous electric polarization, is generally thought to be incompatible with metallicity. This seems intuitive, since unbound carriers would quickly act to screen any polarization in a metal. In a strange twist, researchers Youguo Shi of the National Institute for Materials Science in Ibaraki, Japan, and co-workers from China, Japan, and the United Kingdom, now report a ferroelectric-like transition in metallic LiOsO3 at 140 K. Their results open the door to an entirely new class of materials and shed light on fundamental electron behavior in ferroelectrics.
Writing in the November issue of Nature Materials (DOI: 10.1038/NMAT3754; p. 1024), the researchers describe structure refinement studies of rhombohedral LiOsO3. Using neutron powder diffraction, they find that this compound forms a LiNbO3-type crystal structure; at room temperature, it can be described by a centrosymmetric R
$\overline 3 $
cspace group, which is not compatible with ferroelectricity. However, upon cooling below 140 K, the structure undergoes a phase transition to a noncentrosymmetric R3c space group, which is compatible with ferroelectricity. The researchers confirmed this symmetry change using convergent-beam electron diffraction.
Interestingly, they find that the material remains a metal through the phase change, and they find no associated change in magnetic behavior across the phase transition. This suggests that the transition to ferroelectric behavior in this compound is driven by a displacive, order-disorder-type transition, rather than collective electron motion. The group said that their study may guide other researchers as they seek to understand high-temperature ferroelectric transitions in related LiNbO3 and LiTaO3 compounds.
Temperature dependence of the LaOsO3 unit cell parameters as it undergoes a structural transition at 140 K. Refinement of this structure demonstrates a transition to a noncentrosymmetric space group. Reproduced with permission from Nat. Mater. 12 (2013), DOI: 10.1038/NMAT3754; p. 1024. © 2013 Macmillan Publishers Ltd.
