Structural and magnetic phase transitions in the ternary iron arsenides   SrFe2As2 and EuFe2As2
  The structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray powder diffraction and 57-Fe Moessbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr2Si2-type structure at room temperature and exhibit displacive structural transitions at 203 K (SrFe2As2) or 190 K (EuFe2As2) to orthorhombic lattice symmetry in agreement with the group-subgroup relationship between I4/mmm and Fmmm. 57-Fe Moessbauer spectroscopy experiments with SrFe2 As2 show full hyperfine field splitting below the phase transition temperature (8.91(1) T at 4.2 K). Order parameters were extracted from detailed measurements of the lattice parameters and fitted to a simple power law. We find a relation between the critical exponents and the transition temperatures for AFe2As2 compounds, which shows that the transition of BaFe2As2 is indeed more continuous than the transition of SrFe2As2 but it remains second order even in the latter case. 
Marcus Tegel, Marianne Rotter, Veronika Weiss, Falko M. Schappacher,   Rainer Poettgen, Dirk Johrendt