Lattice dynamics of BaFe 2 Se 3 .

This paper presents a study of the lattice dynamics in BaFe 2 Se 3 . We combined first-principle calculations, infrared measurements and a thorough symmetry analysis. Our study confirms that Pnma cannot be the space group of BaFe 2 Se 3 , even at room temperature. The phonons assignment requires Pm to be the BaFe 2 Se 3 space group, not only in the magnetic phase, but also in the paramagnetic phase at room temperature. This is due to a strong coupling between a short-range spin-order along the ladders, and the lattice degrees of freedom associated with the Fe-Fe bond length. This coupling induces a change in the bond-length pattern from an alternated trapezoidal one (as in Pnma ) to an alternated small/large rectangular one. Out of the two patterns, only the latter is fully compatible with the observed block-type magnetic structure. Finally, we propose a complete symmetry analysis of the BaFe 2 Se 3 phase diagram in the 0-600 K range.