Sb-5s Lone Pair Dynamics And Collinear Magnetic Ordering In Ba2fesbse5

Neutron diffraction and x-ray pair distribution function experiments were performed to investigate the magnetic and local crystal structures of Ba2FeSbSe5 and to compare them with 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 to 95 K, resulting in deviations from the average (i.e., long range) crystal structure. In this paper, we demonstrate 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 Ned temperature (T-N = 58 K), the two distinct magnetic Fe3+ sites order collinearly, such that a combination of antiparallel and parallel spin arrangements along the b axis results. The nearest-neighbor arrangement (J(1) = 6 angstrom) is fully antiferromagnetic, while next-nearest-neighbor interactions are ferromagnetic in nature.