Lattice dynamics of BaFe₂Se₃

Abstract

This paper presents a study of the lattice dynamics in BaFe2Se3. We combined first-principle calculations, infrared measurements and a thorough symmetry analysis. Our study confirms that Pnma cannot be the space group of BaFe2Se3, even at room temperature. The phonons assignment requires Pm to be the BaFe2Se3 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 BaFe2Se3 phase diagram in the 0-600\,K range.

Chiral magnetic structure of spin-ladder multiferroic BaFe2Se3

“…The unusual character of the block arrangement has been recently explained by the ladder distortion evidenced in [3]. This distortion is characterized by a tetramerization of the Fe ladder which creates an alternation of larger and smaller squarelike Fe plaquettes [13] and thus a block character. The block-A magnetic order evidenced by our work is of great interest regarding the multiferroic properties.…”

“…In the magnetic phase, we have shown recently by x-ray diffraction that the space group of the nuclear phase is Pm (unit cell parameters: [3]. The symmetry breaking from Pnma was also observed by transmission electron microscopy [12] and lattice dynamic study [13]. However, it is important to note that it is very weak.…”