Direct imaging of the structural domains in the iron pnictides $A{\text{Fe}}_2{\text{As}}_2$ $\left(A=\text{Ca},\text{Sr},\text{Ba}\right)$

The parent compounds of recently discovered iron-arsenide superconductors, $A{\text{Fe}}_2{\text{As}}_2$ with alkaline earth $A=\text{Ca},\text{Sr},\text{Ba}$, 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 $10–50\mu \text{m}$. 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.

Figure 1

(Color online) A pattern of structural domains is found in parent compounds of iron-pnictide superconductors, $A{\text{Fe}}_2{\text{As}}_2$, with $A=\text{Ca}$ (top panel), Sr (middle panel), and Ba (bottom panel) at 5 K, well below $T_{SM}$, the temperature of the simultaneous structural/magnetic transition. The characteristic spacing between the lines is about $10\mu \text{m}$, and the contrast in optical images follows the magnitude of orthorhombic distortion in the compound. Inset in top panel shows terraces on $A=\text{Ca}$ crystal at room temperature. In all images $c$ axis is perpendicular to the surface.

Figure 2

(Color online) High resolution optical image of pure ${\text{BaFe}}_2{\text{As}}_2$ above (left) and below (right) the temperature of the coupled structural/magnetic transition, $T_{SM}=135\text{K}$ (top row). A pattern of domain walls is formed below $T_{SM}$ due to formation of twin boundaries. Second row of panels shows schematics of the displacements of atoms in the tetragonal lattice (above $T_{SM}$, left) during structural transition, leading to orthorhombic distortion and formation of domain walls. In the orthorhombic phase (right) the unit cell of the lattice doubles in size in the $a\text{−}b$ plane, new unitary vectors $a_O$ and $b_O$ become of different length $\left(a_O>b_O\right)$, and rotate by approximately $45°$. The third row shows sketches of the expected transformation of the x-ray diffraction pattern during domain formation. The bottom row shows actual x-ray data and zooms of (200) and (220) reflections (insets).

Figure 3

(Color online) Below the transition, the domain boundaries form regular patterns. Three typical areas can be found in both optical images and x-ray microdiffractograms around the split tetragonal (220) reflection. The two rows compare typical patterns as found in optical and x-ray images. Of note, since x rays penetrate the whole sample thickness, the lack of crossed patterns in left and right panels shows clearly that domains are extending through the whole thickness of the samples along (001) axis. The area $A$ represents lamellae of the domains O1 and O2; the area $C$ represents lamellae of domains O3 and O4. The area $B$ shows crossed lamellae of the domain walls O1-O2 and O3-O4. It is characterized by increased structural deformation.