Distinct local structure of superconducting ${\mathrm{Ca}}_{10}{M}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5(M=\mathrm{Pt},\text{Ir})$

We have studied the local structure of superconducting ${\mathrm{Ca}}_{10}{\mathrm{Pt}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ (Pt10418) and ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ (Ir10418) iron arsenides, showing different transition temperatures $(T_c=38$ and 16 K, respectively), by polarized Fe $K$-edge extended x-ray absorption fine-structure measurements. Despite the similar average crystal structures, the local structures of the ${\mathrm{FeAs}}_4$ tetrahedra in the two compounds are found to be very different. The ${\mathrm{FeAs}}_4$ in Pt10418 is close to a regular tetrahedron, while it deviates largely in Ir10418. The Fe-Fe correlations in the two compounds are characterized by similar bond-length characteristics; however, the static disorder in Pt10418 is significantly lower than that in Ir10418. The results suggest that the optimized local structure and reduced disorder are the reasons for higher $T_c$ and well-defined electronic states in Pt10418 unlike Ir10418 showing the coexistence of glassy and normal electrons at the Fermi surface, and hence provide direct evidence of the local-structure-driven optimization of the electronic structure and superconductivity in iron arsenides.

Figure 1

Fourier-transformed magnitude of in-plane polarized Fe $K$-edge EXAFS oscillations extracted from the absorption spectra measured on $\text{Pt}10418\left({\mathrm{Pt}}_4{\mathrm{As}}_8\right)$ and $\text{Ir}10418\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right)$ at $T=60$ K and not corrected by photoelectron phase shifts. The corresponding EXAFS oscillations are displayed as the inset.

Figure 2

Fourier transform magnitudes of the polarized Fe $K$-edge EXAFS on Pt10418 at several temperatures. The inset shows the corresponding EXAFS oscillations.

Figure 3

(a) Fourier transform (FT) of the $\chi \left(k\right)k^2$ signal in ${\mathrm{Ca}}_{10}{\mathrm{Pt}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ at the Fe $K$ edge (red bubbles) compared with the best fit (blue line) for $T=20$ K. The real-space contributions of the Fe-As and Fe-Fe distances are shown as colored peaks. The inset shows the back Fourier transform of the spectra in the main panel taken between 1.5–3 Å. The calculated $k$-space contributions of the Fe-As and Fe-Fe distances are shown with a vertical shift. (b) FTs of ${\mathrm{Ca}}_{10}{\mathrm{Pt}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ (black bubbles) as a function of temperature are shown with the model fits (red solid lines). (c) Filtered $\chi \left(k\right)k^2$ [range of the back Fourier transform is indicated by vertical dashed lines in (b)] signals (data points) with the corresponding model fits (line) are shown.

Figure 4

(a) Temperature dependence of the Fe-As and Fe-Fe distances (pictorially represented) obtained from two-shell model fits of the Fe $K$-edge EXAFS for Pt10418. (b),(c) The $T$ dependence of As-Fe-As bond angle $\left(\alpha \right)$ and pnictogen height from the Fe-Fe layer $\left(h_p\right)$. $\alpha$ and $h_p$ have been computed considering a perfect tetragonal coordination (see text). The errors bars are estimated by analysis of different scans and considering the correlation between different fit parameters. A picture of the local structure is also shown, defining the two parameters (As-Fe-As angle $\alpha$ and the anion height).

Figure 5

(a) Temperature dependence of the mean-square relative displacement ${\sigma }^2$ for the Fe-As and Fe-Fe distances in Pt10418, obtained from the polarized Fe $K$-edge EXAFS analysis. The dash-dotted line is the best-fit result of the Einstein model (see text). (b) Comparison between the ${\sigma }^2$ of Pt10418 [open symbols; same data as (a)] and Ir10418 (full symbols), measured in the low-$T$ region. The ${\sigma }^2$ obtained for Ir10418 are compatible with the Einstein model obtained for Pt10418, however with a vertical shift that mimics a higher value of ${\sigma }_0^2$ in the former.