Structural phase transition and superlattice misfit strain of $R\text{FeAsO}$ $\left(R=\text{La},\text{Pr},\text{Nd},\text{Sm}\right)$

The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO (La-1111) and SmFeAsO (Sm-1111) single crystals measured by high-resolution x-ray diffraction is found to be sharp while the $R\text{FeAsO}$ $\left(R=\text{La},\text{Nd},\text{Pr},\text{Sm}\right)$ polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single-crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature-dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the $A{\text{Fe}}_2{\text{As}}_2$ $\left(A=\text{Ca},\text{Sr},\text{Ba}\right)$ “122” systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 (55 K) versus 122 (35 K) systems and implies the surface reconstruction in 1111 single crystals.

Figure 1

(Color online) Upper panels show a (b) lattice constant of the LaFeAsO (left) and SmFeAsO (right) single-crystal samples as a function of temperature during cooling (empty blue circles) and warming (filled red circles) cycle. Middle panels show the same for the corresponding polycrystalline powder samples. Upper insets in these panels show the zoomed region over the SPT indicating the presence of a hysteresis whereas the lower insets in these panels show the evolution of the 220 spot/peak during cooling. Lower panel presents the order parameters for the single crystals and polycrystalline powders during cooling. The order parameter of the ${\text{BaFe}}_2{\text{As}}_2$ system taken from Ref. 22 is shown in the inset for comparison.

Figure 2

(Color online) Upper panel: intensity variation of the 220 peak as a function of temperature for the single crystal and polycrystalline powder samples of LaFeAsO (marked as La 1111) and SmFeAsO (marked as Sm 1111) systems together with the intensity variation in a similar peak observed in the ${\text{BaFe}}_2{\text{As}}_2$ system (marked as Ba 122 polycrystalline powder). Lower panel: $\Delta \theta /\theta$ of the 220 peak in the tetragonal phase and 400 peak in the orthorhombic phase as a function of $T/T_s$ for the Sm-1111 and La-1111 single crystals (left panel) and polycrystalline powders (right panel). Size of dots shows the dimension of error bar.

Figure 3

(Color online) Normalized FWHM of the tetragonal 220 peak and corresponding orthorhombic 400 (or 040) peak as a function of temperature for the $R\text{FeAsO}$ $\left(R=\text{La},\text{Pr},\text{Sm},\text{Nd}\right)$ systems. $T<T_s$ and $T>T_s$ are shown in the left and right panels, respectively. Upper panel compares the behavior of the polycrystalline powder samples. Middle panels compare the normalized FWHM of the single-crystal and polycrystalline powder systems. Fit to the data are included as lines. The exponents obtained from the fits are compared in the lower panels as a function of the rare-earth ionic size. Size of dots shows the dimension of error bar.