“…After the discovery of cuprate high temperature superconductors (HTSC), Mössbauer spectroscopy was widely used for studies of these materials and reports of observation of some anomalies in the spectral parameters in the vicinity of the superconducting critical temperature (T c ) [3][4][5][6][7] were published. Due to the absence of commonly available Mössbauer nuclides in the cuprates, most studies were accomplished either by partial substitution of copper atoms by 57 Fe and/or 119 Sn, or by using resonant isotopes of the rare earth metals, like 151 Eu, which increases the degree of difficulty of the measurements and reduces the clarity of the results [3,5,8,9]. Recently, the discovery of iron-based superconductors, that naturally contain the common Mössbauer nuclide, 57 Fe, has triggered intense Mössbauer studies of these superconductors [10][11][12][13][14][15].…”