Direct imaging of the structural domains in the iron pnictides AFe₂As₂ (A=Ca,Sr,Ba)

The parent compounds of recently discovered iron-arsenide superconductors, AFe₂As₂ with alkaline earth A=Ca,Sr,Ba, undergo simultaneous structural and magnetic phase transitions at a temperature T_SM. Using a combination of polarized light microscopy and spatially resolved high-energy synchrotron x-ray diffraction we show that the orthorhombic distortion leads to the formation of 45°-type structural domains in all parent compounds. Domains penetrate through the sample thickness in the c direction and are not affected by crystal imperfections such as growth terraces. The domains form regular stripe patterns in the plane with a characteristic dimension of 10–50 μm. The direction of the stripes is fixed with respect to the tetragonal (100) and (010) directions but can change by 90° on thermal cycling through the transition. This domain pattern may have profound implications for intrinsic disorder and anisotropy of iron arsenides.