Local structural study of novel mott-insulating cousins of the iron pnictides*

Iron oxy-chalcogenides, La₂O₂Fe₂OM₂ (M = S, Se), is the layered materials formed from stacking layered units of La₂O₂ and Fe₂OM₂ (M = S, Se). We describe the structural properties of the La₂O₂Fe₂OM₂ (M = S, Se), by using pair distribution function (PDF) analysis and Rietveld refinement methods applied to neutron diffraction data. Our results show that M = S and Se possess similar nuclear structure at low and room temperatures. Local crystal structure was studied by investigating deviations in atomic positions and orthorhombicity. By tracking the orthorhombicity parameter, we observe the local scale distortions between the tetragonal and orthorhombic structure in a typical range of 1-2 nm. These spatially limited distortions represent fluctuating nematic order which suggests the ubiquity of nematic fluctuations in iron-based superconductors and related materials. In addition, we found the discontinuity in c-lattice which may arise due to the buckling of Fe₂O plane. We anticipate that this buckling might be due to the change in octahedral height or tilting of the octahedral structure. Overall, these results suggest that the structural distortion may play a role in absence of superconductivity in these materials.