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The local structure of superconducting single crystals of K0.8Fe1.6+xSe2 withTc = 32.6 K was studied by x-ray absorption spectroscopy. Near-edge spectrareveal that the average valence of Fe is 2+. The room temperature structureabout the Fe, K and Se sites was examined by iron, selenium and potassiumK-edge measurements. The structure about the Se and Fe sites shows a highdegree of order in the nearest neighbor Fe-Se bonds. On the other hand, thecombined Se and K local structure measurements reveal a very high level ofstructural disorder in the K layers. Temperature dependent measurements at theFe sites show that the Fe-Se atomic correlation follows that of the Fe-Ascorrelation in the superconductor LaFeAsO0.89F0.11 - having the same effectiveEinstein temperature (stiffness). In K0.8Fe1.6+xSe2, the nearest neighbor Fe-Febonds has a lower Einstein temperature and higher structural disorder than inLaFeAsO0.89F0.11. The moderate Fe site and high K site structural disorder isconsistent with the high normal state resistivity seen in this class ofmaterials. For higher shells, an enhancement of the second nearest neighborFe-Fe interaction is found just below Tc and suggests that correlations betweenFe magnetic ion pairs beyond the first neighbor are important in models ofmagnetic order and superconductivity in these materials.

Local Structure of the Superconductor K0.8Fe1.6+xSe2: Evidence of Large Structural Disorder