M\"ossbauer studies of powdered single crystals of FeTe0.5Se0.5
  The M\"ossbauer measurements performed on powdered single crystals of FeTe0.5Se0.5 (Tc ~ 14.7 K) reveal minor content of two impurity phases, identified as Fe3O4 and Fe7Se8, among the major tetragonal phase. From the shape of impurity subspectra it follows that Fe7Se8 behaves in superparamagnetic manner, most likely because of randomly distributed Fe vacancies in the lattice structure of Fe Te Se. The magnetite content in the powdered absorber exposed to ambient air conditions remains unchanged during period of 16 months. Ageing effects were observed for the samples stored under argon atmosphere and small increase of the isomer shift of the doublet was detected. Presented temperature dependence of the hyperfine parameters can be explained as due to possible orthorhombic distortion or to temperature behaviour of impurity phases Fe3O4 and Fe7Se8. Strong tendency to formation of crystalline texture of powdered sample is observed. 
K. Szyma\'nski, W. Olszewski, L. Dobrzy\'nski, D. Satu{\l}a, D. J.   Gawryluk, M. Berkowski, R. Pu\'zniak, A. Wi\'sniewski
cond-mat.supr-con