  Temperature dependent crystal structures of the quasi-one-dimensional ladder
material BaFe2Se3 are examined. Combining the optical second harmonic
generation (SHG) experiments and neutron diffraction measurements, we elucidate
the crystal structure with Pmn2_1 space group in the low-temperature phase
below Ts2 = 400 K, further above Neel temperature. This low-temperature phase
loses the spatial inversion symmetry, where a resultant macroscopic
polarization emerges along the rung direction. The transition is characterized
by block-type lattice distortions with the magneto-striction mechanism. Change
in the electrical resistivity and the magnetic susceptibility across the
polar-nonpolar transition also suggests a modification of the electronic states
reflecting the structural instability. Consistency and discrepancy with the
existing theory are discussed.
