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Author(s): Karki, B; Alfailakawi, A; Frandsen, BA; Everett, MS; Neuefeind, JC; Xu, B; Wang, H; Fang, M; Freelon, B | Abstract: We describe the local structural properties of the iron oxychalcogenides, La2O2Fe2OM2(M=S,Se), by using pair distribution function analysis applied to total scattering data. Our results from neutron powder diffraction show that M = S and Se possess similar nuclear structures at low and room temperatures. The local crystal structures were studied by investigating deviations in atomic positions and the extent of the formation of orthorhombicity. Analysis of the total scattering data suggests that buckling of the Fe2O plane occurs below 100 K. The buckling may occur concomitantly with a change in octahedral height. Furthermore, within a typical range of 1-2 nm, we observed a short-range orthorhombiclike structure suggestive of nematic fluctuations in both of these materials.

We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides ${\mathrm{La}}_{2}{\mathrm{O}}_{2}{\mathrm{Fe}}_{2}\mathrm{O}M{}_{2}(M=\text{S},\text{Se})$. Elastic neutron scattering reveals that $M=\text{S}$ and Se have similar nuclear structures at room and low temperatures. Our neutron diffraction data reveals that both materials obtain antiferromagnetic ordering at a N\'eel temperature ${T}_{N}90.1\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}0.16$ K and $107.2\ifmmode\pm\else\textpm\fi{}0.06$ K for $M=\text{Se}$ and S, respectively. The magnetic arrangements for both $M=\text{S}$, Se compounds are obtained through Rietveld refinement. We find the order parameter exponent $\ensuremath{\beta}$ to be 0.$129\ifmmode\pm\else\textpm\fi{}0.006$ for $M=\text{Se}$ and $0.133\ifmmode\pm\else\textpm\fi{}0.007$ for $M=\text{S}$. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen replacement, the nature of the magnetic interactions is similar in these materials.

Using density functional dynamical mean-field theory we investigate the site-selective electronic structure of ${\mathrm{Ca}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{3}{\mathrm{S}}_{2}$. We confirm that the parent compound with two distinct iron sites is a multiorbital Mott insulator similar to ${\mathrm{La}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{2}{\mathrm{S}}_{2}$. Upon electron/hole doping, carrier localization is found to persist in the two active iron channels because the chemical potential lies in a gap structure with anisotropic and almost vanishing states near the Fermi energy. This emergent behavior stems from large electronic reconstruction caused by dynamical spectral weight transfer involving states with distinct $d$-shell occupancies and orbital character at low energies. We detail the implications of our microscopic analysis and discuss the underlying physics which will emerge in future experiments on ${\mathrm{Ca}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{3}{\mathrm{S}}_{2}$.