A ferroelectric-like structural transition in a metallic 5$d$ oxide LiOsO$_{3}$

Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ``ferroelectric'' metal, in which a ferroelectric-like transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric ($R$-3$c)$ to non-centrosymmetric ($R$3$c)$ transition in metallic LiOsO$_{3}$ that is structurally equivalent to the ferroelectric transition of LiNbO$_{3}$. The transition involves a continuous shift in the mean position of Li$^{+}$ ions on cooling below 140 K. Its discovery realizes the scenario described by Anderson and Blount, and establishes a new class of materials whose properties may differ from those of normal metals. This research was supported in part by a Grant-in-Aid for Scientific Research (22246083, 25289233) from JSPS, Japan; the Funding Program for World-Leading Innovative R{\&}D on Science and Technology (FIRST Program) from JSPS, Japan; and the United Kingdom Engineering and Physical Sciences Research Council (EPSRC).