Structure of vacancy-ordered single-crystalline superconducting potassium iron selenide

Abstract

With single crystal X-ray diffraction studies, we compare the structures of three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2, K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a (√5 x √5 x 1) super cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250K of a = b = 8.729(2) Å and c = 14.120(3) Å, a = b = 8.7186(12) Å and c = 14.0853(19) Å and at 295 K, a = b = 8.8617(16) Å and c = 15.304(3) Å for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty, whilst the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2) % to 85.3(3) %. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se -F -Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors distinguishes themselves from the structure of the non-superconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site as well as lower alkali metal occupancy that ensures an exact Fe 2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity.

Large local disorder in superconducting K0.8Fe1.6Se2studied by extended x-ray absorption fine structure

“…The Fe-Se distances determined by EXAFS data in different polarizations on two edges are similar within experimental uncertainties (upper panel). However, both Fe-Se and Fe-Fe distances (lower panel) appear slightly shorter than the average bondlengths determined by diffraction studies [17,18,19,20,33]. This may be due to largely disordered structure, as seen in the glasses [34,35].…”

Fe57Mössbauer study of magnetic ordering in superconductingK0.80<

“…At the temperatures of interest here, K 0.80 Fe 1.76 Se 2.00 adopts a vacancy-ordered I4/m modification of the parent ThCr 2 Si 2 −type I4/mmm structure with iron essentially filling a 16i site and leaving ordered vacancies on the (almost) empty 4d site 26,31 . Occupations of ∼8% for the Fe-4d site have been reported 31 .…”

“…Occupations of ∼8% for the Fe-4d site have been reported 31 . If we assume full occupation of the Fe-16i site in our sample, this leaves 9% of the iron in the 4d site.…”

Structure, magnetic order and excitations in the 245 family of Fe-based superconductors

“…One superconducting sample from a systematic synthesis study of another group [21] is found to be K 0.737(6) Fe 1.631(3) Se 2 in the singlecrystal x-ray diffraction study [20], while another superconducting sample to be K 0.83(2) Fe 1.64(1) Se 2 in a powder neutron diffraction refinement study [10]. Thus, the iron valance in the superconducting samples is very close to but not exactly at 2+, similar to that in the previously discovered iron chalcogenide superconductors Fe 1+δ (Se,Te) [8,9], but very different from the conclusion of the ARPES studies which claim intact FeSe layers in A 0.8 Fe 2 Se 2 [17,22,23].…”

“…1. The ordered structure is refined in the tetragonal I4/m unit cell, and the structure parameters are tabulated in [10,20]. The prominent structural feature of the superconducting sam- ples is the almost empty 4d Fe1 site with an occupancy n(4d) at a few percent and the full occupancy of the 16i Fe2 site, see TABLE II, below room temperature.…”