Properties of layered iron oxychalcogenides with checkerboard structure*

The layered iron oxychalcogenides La$_2$O$_3$Fe$_2$X$_2$ (X = S, Se) have a unique checkerboard-like Fe$_2$OX$_2$ sublattice. Their Mott insulating behavior makes them promising candidates for novel superconductors. In this talk, we present results on the isostructural A$_2$F$_2$Fe$_2$OS$_2$ (A = Sr, Ba) compounds. Both pure and doped samples are characterized by magnetization, resistivity, heat capacity and Raman spectroscopy measurements. In addition to the previously reported antiferromagnetic transition around T$_N~\approx$ 100 K, we observed another phase transition in the ordered state, as well as a possible structural phase transition near room temperature. A change in the excitation gap at the high temperature (structural) phase transition is indicated by the temperature dependent resistivity.