Polar state induced by block-type lattice distortions in BaFe2Se3 with quasi-one-dimensional ladder structure

A stripe-type magnetic structure and related orbital ordering are universally observed in iron-based superconductors. As an exception, the quasi-one-dimensional ladder material BaFe2Se3, which shows pressure-induced superconductivity, exhibits a block-type antiferromagnetic ordering. The research of the electronic and structural properties in BaFe2Se3 at ambient conditions is important to reveal the origin of multiorbital superconductivity. Here, we report the temperature-dependent crystal structure of BaFe2Se3, combining optical second harmonic generation experiments and neutron diffraction measurements. We elucidate the crystal structure with the Pmn21 space group in the low-temperature phase below Ts2=400 K, further above the Néel temperature. This low-temperature phase loses spatial inversion symmetry, where a resultant macroscopic polarization emerges along the rung direction. The transition is characterized by block-type lattice distortions possibly accompanied by polar orbital ordering.