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

TL;DR

This study investigates the structural and magnetic phase transitions in SrFe2As2 and EuFe2As2 using x-ray diffraction and Mössbauer spectroscopy, revealing their second-order nature and detailed transition characteristics.

Contribution

It provides new insights into the phase transition mechanisms and critical exponents of the ternary iron arsenides SrFe2As2 and EuFe2As2, highlighting differences in transition continuity.

Findings

Both compounds undergo displacive structural transitions to orthorhombic symmetry.

Full hyperfine field splitting observed below transition in SrFe2As2.

Transition of BaFe2As2 is more continuous but still second order.

Abstract

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.