Local structure of the superconductor K0.8Fe1.6+xSe2

Abstract

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

Local Structure of Sodium‐ and Iron‐deintercalated NaFeAs

“…Analogues of this deficient ThCr 2 Si 2 -type system are afforded by the A 1-x Fe 2-y Se 2 series (A = K, Rb, Cs; x ≈ 0.2; y ≈ 0.4) and EXAFS measurements of some of these materials reveal comparable behavior to that presented herein. [30,31] This deintercalate synthesized by oxidation with iodine solution has a much higher oxidation state for iron than other iron-based superconductors, where typically the iron valence ranges between extremes of +1.5 and +2.5 and is optimal at +1.88 for LaFeAsO 0.88 F 0.12 for electron-doped materials [1] and +2.2 for the hole-doped case of K 0.4 Ba 0.6 Fe 2 As 2 . [8] While NaFe 1.67 As 2 would appear to lie outside the range of superconductivity, our sample derived from stoichiometric www.zaac.wiley-vch.de NaFeAs, like that of Friederichs et al, [26] does exhibit superconductivity but with a superconducting volume fraction derived from the ZFC susceptibility measurement of only 7 % (Friederichs et al reported a volume fraction of about 20 %).…”

“…The loss of order is similar to that reported to occur in the related system K 0.8 Fe 1.6 Se 2 compared with β-FeSe from which it is formally derived via deintercalation of iron and a compensating intercalation of potassium. [30,31] In this latter system the comparison of FeSe and K 0.8 Fe 1.6 Se 2 at the Fe K-edge is strikingly similar to that shown in Figure 5, showing that the introduction of vacancies in the anti-PbO type iron selenide layers leads to a loss of well resolved scattering paths outside the first-coordination shell and this is evident in the EXAFS despite the long-range ordering of vacancies and significantly greater long range structural order in the K 0.8 Fe 1.6 Se 2 system as probed by diffraction methods [17] relative to the semi amorphous nature of the Na 1-x Fe 2-y As 2 system (Figure 1 and reference [26] ). Modeling of the EXAFS for the parent material NaFeAs shows strong features from direct and multiple scattering paths out to 6 Å away from the absorbing site.…”

Local structure response of phase separation and iron-vacancy order inKxFe2−ySe2superconductor

“…Local structure of AA -Fe2_ ySe2 has been studied by ex tended x-ray absorption fine-structure (EXAFS) measure ments [16][17][18], providing important information on the atomic displacements and nanoscale disorder. Among others, local structure of Ky Fe2_JSe2, studied by polarized EXAFS mea surements, reveals large local disorder with the nanoscale structure being similar to that of a glass [17].…”

EXAFS study of thermoelectric BiCuOSe: Effects of Cu vacancies

“…To understand the electrical transport properties and in particular the effects of Cu vacancies on these properties we employ extended X-ray absorption fine structure (EXAFS) spectroscopy, an element-specific atomic-scale probe, at the Cu-K edge to investigate the local surroundings of Cu atoms in both stoichiometric (x¼ 0) and Cu-deficient (x¼0.05) BiCu 1À x OSe samples from room temperature down to 25 K. Previously ZrCuSiAstype superconductive and related iron oxypnictides have been researched by EXAFS, particularly near the superconductivity transition temperature [23][24][25][26], but to our knowledge the present work is the first EXAFS study on the thermoelectric oxychalcogenide compounds. By temperature-dependent measurements, fruitful information like local geometry and lattice dynamics can be deduced.…”