Ferroelectricity is a property of certain materials, usually insulators, that have a spontaneous electric polarization that can be reversed by the application of an external electric field. It is widely believed that ferroelectricity cannot occur in metals because polarisation would be screened by the conduction electrons. Typically, materials demonstrate ferroelectricity only below a certain phase transition temperature, called the Curie temperature, Tc, where inversion symmetry is broken. In 1965, Philip Anderson, the Nobel prize laureate, together with his co-worker Blount proposed a concept of a so-called “ferroelectric” metal, which could exhibit similar structural transition behaviour as that of ferroelectrics materials, such as LiNbO₃ and LiTaO₃. Unfortunately, up to now, no clear example of such a material has been identified.

Recently, through the collaboration with other international research teams, including the members from NIMS of Japan, and University of Oxford, SHI Youguo, an associate professor in WANG Nanlin ’s group in Institute of Physics, Chinese Academy of Sciences (IOP), reported the discovery of a new 5d material called LiOsO₃ which remains a metal down to the lowest temperatures and yet undergoes a structural phase transition (Fig.1) that is identical to the ferroelectric transition in the well-known displacive ferroelectrics LiNbO₃ and LiTaO₃. Using a variety of techniques including neutron diffraction which allowed measuring structural parameters with a great precision, they found that the phase transition in LiOsO₃ is characterised by a large shift in the position of the Li ions thereby breaking inversion symmetry (fig.2 and 3). The same structural effect has been known for many years to cause ferroelectricity in insulating LiNbO₃ and LiTaO₃.

The discovery represents the first clear-cut example of a “ferroelectric” metal proposed by Anderson and Blount. It is also scientifically interesting because the mechanisms for structural phase transitions are usually quite distinct in metals and insulators, so it is surprising to find a metal (LiOsO₃) that undergoes the same structural transition as occurs in the insulating analogues. The discovery of a “ferroelectric” metal establishes a new class of materials which could have interesting properties, such as the possibility of non-centrosymmetric superconductivity stabilised by the “ferroelectric” structural instability.

This work was published in Nature Materials (doi:10.1038/nmat3754) and highlighted by Nature Materials in its News and Views (doi:10.1038/nmat3774). The research was partially funded by the National Basic Research Program of China (973 program, No. 2011CB921701 and 2011CBA00110), and the Chinese Academy of Sciences.