Room temperature polar structure and multiferroicity in BaFe2Se3

The understanding of the superconducting phase under pressure in the famous iron-based spin ladder BaFe2Se3 commands a perfect understanding of the structural properties. We present an accurate single crystal x-ray analysis which demonstrates that the structure admitted until now at room temperature is totally ruled out. We show that the system has a polar structure already at 300 K and is thus ferroelectric. Its structure is modified below the Neel transition and the polarization is intensified. These features unambiguously prove the multiferroic character of BaFe2Se3 as proposed theoretically. This system evidences a competition between multiferroicity and superconductivity.