Mössbauer spectroscopy determination of iron foreign phases in the superconducting systems; RAsFeO _1-x, RAsFeO _1-xF _x, and Sr _1-xK _xFe ₂As ₂

The recently discovered superconducting-spin density wave materials, containing Fe and As, have raised huge interest. However, most materials prepared to date, suffer from a varying degree of content of foreign Fe-As phases, Fe ₂As, FeAs ₂, and FeAs, which can lead to wrong conclusions concerning the properties of these materials. We show here that Mössbauer spectroscopy is able to determine quite easily the relative content of the foreign phases. This procedure is demonstrated by a study of seven samples of superconducting or spin density wave materials, prepared in three different laboratories.