Space Group Symmetry of BaFe$_2$Se$_3$: ab initio-Experiment Phonon Study

TL;DR

This study combines first-principles calculations and experimental measurements to determine the correct space group of BaFe$_2$Se$_3$, revealing a strong coupling between spin order and lattice structure that affects its symmetry and phonon properties.

Contribution

It provides a definitive symmetry assignment for BaFe$_2$Se$_3$, showing that $Pm$ is the correct space group, not $Pnma$, due to spin-lattice coupling effects.

Findings

$Pm$ is the correct space group for BaFe$_2$Se$_3$.

Spin-order coupling influences lattice symmetry and phonon modes.

The bond-length pattern changes with magnetic order, affecting symmetry analysis.

Abstract

This paper presents a study of the structure 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.