Sb − 5 s lone pair dynamics and collinear magnetic ordering in Ba 2 FeSb Se 5

International audience; Neutron diffraction and X-ray pair distribution function (XPDF) experiments were performed in order to investigate the magnetic and local crystal structures of Ba2FeSbSe5 and to compare them to the average (i.e. long-range) structural model, previously obtained by single crystal X-ray diffraction. Changes in the local crystal structure (i.e. in the second coordination sphere) are observed upon cooling from 295 K to 95 K resulting in deviations from the average (i.e. longrange) crystal structure. This work demonstrates, that these observations cannot be explained by local or long-range magnetoelastic effects involving Fe-Fe correlations. Instead, we found, that the observed differences between local and average crystal structure can be explained by Sb-5s lone pair dynamics. We also find, that below the Néel temperature (TN = 58 K), the two distinct magnetic Fe 3+ sites order collinearly, such that a combination of antiparallel and parallel spin arrangements along the b-axis results. The nearest-neighbor arrangement (J1 = 6 Å) is fully antiferromagnetic, while next-nearest-neighbor interactions are ferromagnetic in nature.