Spin induced ferroelectric-like structural transition in a metal*

LiOsO$_{3}$ represents a previously only known example of ``ferroelectric metal,'' a concept presented by Anderson and Blount in 1965, with the properties being promoted by electron lattice coupling involving Li$^{+}$ ions displacement in the crystal structure [Y. Shi et al., Nat. Mater. 12, 1024(2013)]. We report that in Pb$_{2}$CoOsO$_{6}$, a new ordered double-perovskite with a centrosymmetric monoclinic space group of $P$2$_{1}$/n, a ferroelectric-like structural transition occurs at $\sim$ 38 K in the metallic state, i.e. a continuous second order transition to a noncentrosymmetric structure (space group: P1) associated by appearance of a nominal unique polar axis along the $c$-axis. The phase transition is coincident with a magnetic transition at the same temperature which corresponds to a long-range antiferromagnetic order. The magnetic structure analysis and theoretical calculations prove that the antiferromagnetic ordering is the driven force for the structural transition in Pb$_{2}$CoOsO$_{6}$ and it represents the first double-perovskite ``ferroelectric metal'' involving a magnetic ordering.