Microstructure and tetragonal-to-orthorhombic phase transition of AFe2As2 (A=Sr,Ca) as seen via transmission electron microscopy

The structural properties of the AFe(2)As(2) (A=Sr,Ca) compounds have been extensively analyzed by transmission electron microscopy (TEM) from room temperature down to 20 K. The experimental results demonstrate that the SrFe2As2 compound, consistent with the previous x-ray data, undergoes a visible tetragonal-to-orthorhombic (T-O) phase transition at low temperature. Moreover, twinning lamellae arising from T-O transition clearly appear in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudoperiodic structural modulation with a periodicity of around 40 nm at room temperature. This modulation is likely related to the local structural distortions within the Ca layers. In situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase.