Large local disorder in superconducting K_0.8Fe_1.6Se₂studied by extended x-ray absorption fine structure

Abstract

We have measured the local structure of superconducting K 0.8 Fe 1.6 Se 2 chalcogenide (T c = 31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe-Se and Fe-Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for the Fe-Se bondlength (k ∼ 5.8 eV Å−2 ) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to be flexible (k ∼ 2.1 eV Å−2 ) in comparison to the binary FeSe (k ∼ 3.5 eV Å−2 ), an indication of partly relaxed Fe-Fe networks in K 0.8 Fe 1.6 Se 2 . The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials.

Temperature Dependence of $\sqrt{2} \times \sqrt{2}$ Phase in Superconducting K0.8Fe1.6Se2 Single Crystal

“…The present observation of multiple phase transitions with the co-existence of a minority phases seems to suggest the importance of inhomogeneities in the A x Fe 2-y Se 2 systems, similar to that observed in the FeSe 1-x Te x [28]. The K x Fe 2-y Se 2 system with a large local disorder [34] and interesting temperature dependent vacancy ordering and coexisting phases [15,16] calls for parallels with the oxygen ordering effects on the superconductivity of cuprates [35,36].…”

“…At 520 K, the 1 2 2 × × phase merge with the parent tetragonal phase. The superlattice peaks corresponding to the magnetic ordering disappears at 580 K. All of these results strongly support the existence of a multiscale phase separation scenario in the A x Fe 2-y Se 2 systems driven by lattice misfit strain [37,38] indicating the importance of local lattice inhomogeneities [28,34], particularly the presence of competing ground states in determining the functional properties of these systems.…”

Comparison of the local structures of Ca 0.82 La 0.18 FeAs 2 and Ba 0.64 K 0.36 Fe 2 As 2 pnictide superconductors using atomic pair distribution function analysis

“…Another feature to be noticed is that the Fe-Fe bond contributions appearing at 2.7 Å is particularly damped. The damping is more prominent in Ca 0.82 La 0.18 FeAs 2 , similar to that of the phase separated K 0.8 Fe 1.6 Se 2 type chalcogenides [38,39] where a large local disorder [40] is found to exist. Fig.…”

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.…”