  The local structure of superconducting single crystals of K0.8Fe1.6+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 LaFeAsO0.89F0.11 - having the same effective
Einstein temperature (stiffness). In K0.8Fe1.6+xSe2, the nearest neighbor Fe-Fe
bonds has a lower Einstein temperature and higher structural disorder than in
LaFeAsO0.89F0.11. 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 interaction is found just below Tc and suggests that correlations between
Fe magnetic ion pairs beyond the first neighbor are important in models of
magnetic order and superconductivity in these materials.
