Cation vacancy order in the K0.8+xFe1.6−ySe2 system: Five-fold cell expansion accommodates 20% tetrahedral vacancies

Abstract

Ordering of the tetrahedral site vacancies in two crystals of refined compositions K 0.93(1) Fe 1.52(1) Se 2 and K 0.862(3) Fe 1.563(4) Se 2 produces a fivefold expansion of the parent ThCr 2 Si 2 unit cell in the ab plane which can accommodate 20% vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, producing a potential doping mechanism which can operate at both average and local structure levels. 65 K 0.93(1) Fe 1.52(1) Se 2 (crystal 1) and K 0.862(3) Fe 1.563(4) Se 2 (crystal 2) corresponding to formal iron charge states of 2.02(2) and 2.008(7), respectively (Table 1). The different K and Fe vacancy levels in the two crystals thus give rise to the same iron charge

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].…”

“…Similar results were obtained with WINXAS package [31] in which the backscattering amplitudes and phase shifts were calculated using FEFF [32]. The E 0 , N i and S 2 0 were fixed after a number of trials on different EXAFS scans with structural input from diffraction studies [17,18,19,20,33]. Only the radial distances R i and the corresponding σ 2 i , were allowed to vary in the least squares model fits for the temperature dependent study.…”

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

“…Except under the small dome of the metal- lic crossover and the superconducting phase underneath, most region of the phase diagram stays in insulating phase [26]. Basca et al are the first to perform single crystal xray refinement study of K 0.862(3) Fe 1.563(4) Se 2 at 90 K and K 0.93(1) Fe 1.52(2) Se 2 at 100 K [25] in this insulating phase, and their compositions are marked by the crosses in the basal plane of FIG. 5.…”

High-Pressure Single-Crystal Neutron Scattering Study of Magnetic and Fe Vacancy Orders in (Tl,Rb) 2 Fe 4 Se 5 Superconductor

“…The superconductors crystallize with a highly ordered √ 5 × √ 5 superstructure, in which the Fe1 site of the I4/m structure is only a few percent occupied and the Fe2 site fully occupied [4,6]. The nonsuperconducting samples at low-T also crystallize in the I4/m structure, but both Fe sites are fractionally occupied [7,8], since the numbers of the Fe vacancies in the samples and the vacant sites in the √ 5 × √ 5 pattern are mismatched. The partially ordered √ 5 × √ 5 vacancy order becomes one of three competing phases for temperature below the room temperature up to ∼ 500 K, namely, these samples are phase-separated and in the miscibility gap at ambient condition [8,9].…”