Local structure of the superconductor K_0.8Fe_1.6+xSe₂: Evidence of large structural disorder

The local structure of superconducting single crystals of K_0.8Fe_1.6+xSe₂ with Tc = 32.6 K was studied by x-ray absorption spectroscopy. Near-edge spectra reveal that the average valence of Fe is 2+. The room temperature structure about the Fe, K, and Se sites was examined by iron, selenium, and potassium K-edge measurements. The structure about the Se and Fe sites shows a high degree of order in the nearest-neighbor Fe-Se bonds. On the other hand, the combined Se and K local structure measurements reveal a very high level of structural disorder in the K layers. Temperature-dependent measurements at the Fe sites show that the Fe-Se atomic correlation follows that of the Fe-As correlation in the superconductor LaFeAsO_0.89F_0.11, having the same effective Einstein temperature (stiffness). In K_0.8Fe_1.6+xSe₂, the nearest-neighbor Fe-Fe bonds have a lower Einstein temperature and higher structural disorder than in LaFeAsO_0.89F_0.11. The moderate Fe site and high K site structural disorder is consistent with the high normal state resistivity seen in this class of materials. For higher shells, an enhancement of the second-nearest-neighbor Fe-Fe correlation is found just below Tc, possibly due to changes in magnetic or local structural ordering.