Structural and magnetic phase transitions in the ternary iron arsenides SrFe2As2and 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 Mössbauer 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 I 4/ mmm and Fmmm (β - SrRh 2 As 2 -type). 57 Fe Mössbauer 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.