Determination of temperature-dependent atomic displacements in the ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{(\mathrm{Fe}}_2{\mathrm{As}}_2{)}_5$ superconductor with a metallic spacer layer

We have used the combination of Fe $K$-edge and Ir $L_3$-edge x-ray absorption measurements as a function of temperature to investigate local atomic displacements in the newly discovered ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ superconducting system. We find relatively relaxed Fe-Fe atomic pair correlations with large displacements in the ${\mathrm{FeAs}}_4$ tetrahedra, revealed by Fe $K$-edge extended x-ray absorption fine structure (EXAFS) analysis. Similarly, the temperature dependence of Ir $L_3$-edge EXAFS shows nanoscale disorder in the IrAs layer that should have a significant effect on the active FeAs-layer characteristics. Furthermore, x-ray absorption near edge structure data are presented to discuss the evolution of the unoccupied electronic states revealing the marginal role of spin-orbit coupling, while the interlayer interactions and disorder should be important for describing the physics of the ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ system.

Figure 1

(a) Magnitude of the Fourier transform of Fe $K$-edge EXAFS ($k^2$-weighted) at 35 K (open circles) together with the two-shell-model fit (solid line) including Fe-As and Fe-Fe pairs. (b) Magnitude of the Fourier transform Ir $L_3$-edge EXAFS ($k^2$-weighted) at 45 K (open circles) together with the single-shell-model fit (solid line). Insets show the filtered EXAFS oscillations relative to the region of interest ($R=1$–3 Å for the Fe $K$ edge and 1.2–3.2 Å for the Ir $L_3$ edge) together with the model fits (solid line).

Figure 2

Temperature dependence of Fe-Fe (a), Fe-As (b), and Ir-As (c) distances. The former two are obtained from the Fe $K$-edge EXAFS analysis while the latter one is from the Ir $L_3$-edge EXAFS analysis. The dashed lines are guides to the eyes. A structural cartoon showing two sites of Ir is shown as an inset.

Figure 3

Temperature dependence of the mean square relative displacements relative to Fe-Fe (a), Fe-As (b) and Ir-As (c) distances. Dashed lines represent Einstein model least-square fits. The Einstein temperatures are also reported.

Figure 4

Room temperature Ir $L_3$-edge absorption spectrum of ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ compared with ${\mathrm{IrCl}}_3$ and ${\mathrm{IrO}}_2$ references. Inset shows a zoom in the edge region of first and second (black line) derivatives. Vertical dashed lines are guides to the absorption edge positions for $\mathrm{Ir}{}^{3+}$ and $\mathrm{Ir}{}^{4+}$.

Figure 5

(a) Ir $L_3$-edge and $L_2$-edge normalized absorption spectra of ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ at 125 K. The $L_2$-edge spectrum is normalized at half of the $L_3$-edge step size. The dashed line is an arctangent background function. In the bottom the background-subtracted white line peaks are shown. The inset shows the temperature dependence of the ratio between $L_3/L_2$ white line intensities determined by Lorentzian fits to the background-subtracted white lines.

Figure 6

Fe $K$-edge XANES spectrum of ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ at 25 K compared with the one measured on the SmFeAsO system at 60 K. The upper inset shows a zoom over the near-edge features (D and E) at several temperatures. FT of the Fe $K$-edge EXAFS on ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_4{\mathrm{As}}_8{\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ is compared with that measured on the SmFeAsO system in the lower inset.