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

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2012-01, Vol.85 (2)
• Main Authors: Tyson T. A., Gu G., Yu, T., Han, S.J., Croft, M., Dimitrov, I.K., Li, Q.
• Format: Article
• Language: English
• Subjects: ABSORPTION SPECTROSCOPY; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; FLEXIBILITY; IRON; MONOCRYSTALS; POTASSIUM; SELENIUM; SPECTRA; SUPERCONDUCTORS; VALENCE
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.85.024504

The local structure of superconducting single crystals of K{sub 0.8}Fe{sub 1.6+x}Se{sub 2} 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{sub 0.89}F{sub 0.11}, having the same effective Einstein temperature (stiffness). In K{sub 0.8}Fe{sub 1.6+x}Se{sub 2}, the nearest-neighbor Fe-Fe bonds have a lower Einstein temperature and higher structural disorder than in LaFeAsO{sub 0.89}F{sub 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.