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

The structural properties of the AFe₂As₂ (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.