Chalcogenides and Pnictides
We have measured Mössbauer spectra of several chalcogenide oxides AEFe2Ch2O (AE = Ba, Sr, Ch = S, Se). As an example spectra of BaFe2S2O  are shown in Fig. 1. BaFe2S2O is a spin-ladder-type compound which is an analogue of the pure sulfide BaFe2S3 where recently pressure-induced superconductivity was reported. BaFe2S2O is a Mott-type insulator with a localized electronic system (e3t23 configuration for tetrahedral symmetry). This is reflected in the Mössbauer spectra which show isomer shifts typical for Fe2+ and large quadrupole interaction due to the structural distortion of the FeS3O units constituting the ladders in the crystal structure. The spectrum in the magnetically ordered phase is a signature of combined magnetic and quadrupole hyperfine interactions, typical for magnetically ordered Fe2+ compounds. In contrast to the six-line patterns usually observed for many magnetically ordered Fe-based systems, here the combined hyperfine interactions lead to the appearance of an additional weak line which is intrinsic to the sample. The spectra were analyzed using the full hyperfine Hamiltonian.