Mössbauer spectroscopy study of FeSe$_{0.91}$S$_{0.09}$ superconductor single crystals

Mössbauer spectra of single crystals of sulfur-doped iron selenide FeSe$_{0.91}$S$_{0.09}$ are studied in a wide temperature range, including the vicinity of structural and superconducting transitions. It is found that iron atoms exhibit a nonmagnetic state even in the range of helium temperatures, which can be attributed to the low-spin state of Fe$^{2+}$ ($3d^6$, $S = 0$) ions. It is shown that this state remains nearly unchanged at temperatures close to the superconducting transition temperature. This means that the low-spin state of iron ions most probably results from some structural features and is not directly related to superconductivity. The temperature dependence of the parameters characterizing the hyperfine interaction exhibits several anomalies in the ranges of structural and electronic transitions. The Debye temperature $\Theta_{\mathrm{M}}=478$ K determined for the iron sublattice turns out to be much higher than the value $\Theta_{\mathrm{M}}=285$ K for the undoped FeSe$_{1-\delta}$ compound.