Local structure response of phase separation and iron-vacancy order in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ superconductor

We have studied the local structure of a ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ superconductor across the phase separation and iron-vacancy order-disorder temperatures (respectively at $\sim 520$ K and $\sim 580$ K). The combination of Fe K edge and Se K edge extended x-ray absorption fine-structure (EXAFS) measurements reveal the glassy local structure of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$, with anomalous behavior of local atomic displacements. We find that the Fe-Se distance remains temperature independent while the Fe-Fe distance suffers a substantial effect of the phase separation. Furthermore, the Fe-Se network shows a reduced disorder in the phase separation regime at lower temperature. The x-ray absorption near-edge structure features follow the local structure anomaly observed by EXAFS and reveal substantially reduced Fe $3d$–Se $4p$ hybridization in the iron-vacancy disordered phase at higher temperature. The results provide further information on the role of nanoscale atomic displacements in the peculiar coexistence of different electronic phases in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ with a filamentary superconducting state embedded in the iron-vacancy ordered magnetic texture.

Figure 1

Fourier transform (FT) magnitudes of the Fe K edge EXAFS (upper) and Se K edge EXAFS (lower) measured on ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ at different temperatures. The FTs are performed using a Gaussian window. The k range is about 4–15 Å${}^{-1}$ for the Se K edge EXAFS and about 4–16 Å${}^{-1}$ for the Fe K edge (see insets). Model fits to the FTs are also shown as solid lines. The insets show EXAFS oscillations with the $k$-space model fits at several temperatures.

Figure 2

Near neighbor distances in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ as a function of temperature. The Fe-Se distance (lower) is obtained from the analysis of both Se K edge (open circles) and Fe K edge (filled circles) EXAFS, while the Fe-Fe distance (upper) is determined from Fe K edge (filled circles) EXAFS analysis. The mean Fe-Fe distance measured by diffraction is also shown (crosses) for a prompt comparison.

Figure 3

Correlated DWFs of the Fe-Se (lower) and Fe-Fe (upper) distances, determined by temperature-dependent EXAFS. Solid lines represent correlated Einstein model fits (see text). Corresponding Einstein temperatures are also reported.

Figure 4

(a) Fe K edge XANES spectra of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ at several temperatures. The arctan background is also shown. The inset shows a zoom over the pre-edge peak P. (b) The pre-edge peak and arctan background. The background subtracted pre-edge peak is shown with Gaussian fit and two components P1 and P2. (c) Temperature dependence of the relative weight of P1 showing an anomalous change across the iron-vacancy order and phase separation temperatures.

Figure 5

(a) Se K edge XANES spectra of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ at several temperatures. The difference spectra with respect to a constant arctan function are also shown. (b) A zoom over the peak A is shown at several temperatures. (c) Temperature dependence of the peak intensity, showing a clear change in the Fe $3d$–Se $4p$ hybridization across the iron-vacancy order and phase separation temperatures.