Structural and magnetic phase transitions in the ternary iron arsenides SrFe2As2 and EuFe2As2

The structural and magnetic phase transitions of the ternary iron arsenides SrFe(2)As(2) and EuFe(2)As(2) were studied by temperature-dependent x-ray powder diffraction and (57)Fe Mossbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr(2)Si(2)-type structure at room temperature and exhibit displacive structural transitions at 203 K (SrFe(2)As(2)) and 190 K (EuFe(2)As(2)), respectively, to orthorhombic lattice symmetry in agreement with the group-subgroup relationship between I4/mmm and Fmmm (beta-SrRh(2)As(2)-type). (57)Fe Mossbauer spectroscopy experiments with SrFe(2)As(2) 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 AFe(2)As(2) compounds, which shows that the transition of BaFe(2)As(2) is indeed more continuous than the transition of SrFe(2)As(2), but it remains second order even in the latter case.