We investigate the interplay between lattice symmetry, phonons, and magnetism in the quasi-one-dimensional ladder compound BaFe2S3 by combining polarized synchrotron infrared spectroscopy, hybrid-functional density functional theory calculations, and inelastic neutron scattering. Lattice-dynamics analysis reveals that the crystal symmetry is lower than previously proposed and is consistent with a P1 space group at low temperature. Several infrared-active phonon modes exhibit pronounced anomalies at both the structural transition temperature TS≈125--130~K and the N\'eel temperature TN≈95~K. First-principles calculations show that the modes affected at TS predominantly involve displacements that modulate magnetic exchange pathways. Neutron scattering demonstrates that below TN the magnetic order is three-dimensional, long-ranged, and static. Between TN and TS, the system displays three-dimensional short-range dynamic magnetic correlations, which disappear above TS. The structural transition thus coincides with the onset of magnetic fluctuations rather than with static magnetic order. Our results indicate that short-range, dynamical magnetic correlations are sufficient to drive a static structural instability, providing a magnetically driven mechanism reminiscent of the iron-pnictide 122 family, yet realized here in a quasi-one-dimensional Mott system. These findings highlight the central role of magnetoelastic coupling in iron-based superconductors beyond the itinerant regime.
@article{arxiv.2602.16899,
title = {Ground State of BaFe2S3 from Lattice and Spin Dynamics},
author = {Y. Oubaid and S. Deng and NS. Dhami and M. Verseils and D. Bounoua and A. Forget and D. Colson and P. Foury-Leylekian and M. B. Lepetit and V. Balédent},
journal= {arXiv preprint arXiv:2602.16899},
year = {2026}
}