Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons¹, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state². Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric () to non-centrosymmetric (R3c) transition in metallic LiOsO₃ that is structurally equivalent to the ferroelectric transition of LiNbO₃ (ref. 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 in ref. 2, and establishes a new class of materials whose properties may differ from those of normal metals.

The perovskite ${\text{NaOsO}}_3$ shows a Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low-temperature limit is absent, indicating that the band gap fully opens. In situ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role in the gap opening.

A dense pellet of the polycrystalline $\mathrm{Sr}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ $(0⩽\delta <0.2)$ was obtained by a high-pressure method, and electronic transport properties of the stoichiometric perovskite were measured. A large electronic specific-heat coefficient was found $\left[45.7\left(2\right)\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}\right]$, indicating a moderate degree of electrons correlation: the mass enhancement factor was 6.1, which is higher than that for the $4d$ ferromagnetic perovskite $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ (4.4). The thermoelectric properties of $\mathrm{Sr}\mathrm{Co}{\mathrm{O}}_3$ were found to be sensitive to band filling, in contrast to those of $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$.

Single crystals of $\mathrm{EuMnS}{\mathrm{b}}_2$ were successfully grown and their structural and electronic properties were investigated systematically. The material crystallizes in an orthorhombic-layered structure (space group: Pnma, No. 62) comprising a periodic sequence of -MnSb/Eu/Sb/Eu/- layers ($\sim 1\mathrm{nm}$ in thickness), and massless fermions are expected to emerge in the Sb layer, by analogy of the candidate Dirac materials $\mathrm{EuMnB}{\mathrm{i}}_2$ and $A\mathrm{Mn}P{n}_2$ ($A=\mathrm{Ca}$ or Sr or Ba, $Pn=\mathrm{Sb}$ or Bi). The magnetic and specific heat measurements of $\mathrm{EuMnS}{\mathrm{b}}_2$ suggest an antiferromagnetic ordering of Eu moments near 20 K. A characteristic hump appears in the temperature-dependent electrical resistivity curve at $\sim 25\mathrm{K}$. A spin-flop transition of Eu moments with an onset magnetic field of $\sim 15\mathrm{kOe}$ (at 2 K) was observed. Interestingly, $\mathrm{EuMnS}{\mathrm{b}}_2$ shows a negative magnetoresistance (up to $-95\%$) in contrast to the positive magnetoresistances observed for $\mathrm{EuMnB}{\mathrm{i}}_2$ and $A\mathrm{Mn}P{n}_2$ ($A=\mathrm{Ca}$ or Sr or Ba, $Pn=\mathrm{Sb}$ or Bi), providing a unique opportunity to study the correlation between electronic and magnetic properties in this class of materials.