Evolution of precipitate morphology during heat treatment and its implications for the superconductivity in KxFe1.6+ySe

Y. Liu

¹

,

Q. Xing

²

,

K. W. Dennis

³

et al.

Abstract

We study the relationship between precipitate morphology and superconductivity in K x Fe 1.6+y Se 2 single crystals grown by self-flux method. Scanning electron microscopy (SEM) measurements revealed that superconducting phase forms a network in the samples quenched above iron vacancy order-disorder transition temperature T s , whereas it aggregates into micrometer-sized rectangular bars and aligns as disconnected chains in the furnace-cooled samples. Accompanying this change in morphology the superconducting shielding fraction is strongly reduced. By post-annealing above T s followed by quenching in room temperature water, the network recovers with superconducting shielding fraction approaching 80% for the furnace-cooled samples. A reversible change from network to bar chains was realized by a secondary heat treatment in annealed samples showing large shielding fraction, i.e., heating above T s followed by slow cooling across T s . The large shielding fraction observed in K x Fe 1.6+y Se 2 single crystals actually results from a uniform and contiguous distribution of superconducting phase. Through the measurements of temperature dependent x-ray diffraction, it is found that superconducting phase precipitates while iron vacancy ordered * Corresponding author: yliu@ameslab.gov 2 phase forms together by cooling across T s in K x Fe 1.6+y Se 2 single crystals. It is a solid solution above T s , where iron atoms randomly occupy the both Fe1 and Fe2 sites in iron vacancy disordering status; and phase separation is driven by the iron vacancy order-disorder transition upon cooling. However, neither additional iron in the starting mixtures nor as-quenching at high temperatures can extend the miscibility gap to KFe 2 Se 2 side.

Giant increase of critical current density and vortex pinning in Mn doped KxFe2−ySe2 single crystals

“…18,19 The as-grown undoped crystal was annealed and quenched into an ice water bath to improve the superconductivity. 14,15,18 Energy dispersive x-ray (EDX) spectroscopy has been carried out for composition analysis. 18 The mean compositions of the four samples were determined to be K 0.76 Fe 1.89 Se 2 (S1), K 0.79 Fe 1.88 Se 2 (S1-quenched), K 0.80 Fe 1.84 Mn 0.012 Se 2 (S2), K 0.80 Fe 1.86 Mn 0.020 Se 2 (S3), and K 0.73 Fe 1.87 Mn 0.024 Se 2 (S4) from nine points of EDX spectrum.…”

“…Recent neutron diffraction studies demonstrate that the insulating phase has a chemical formula K 2 Fe 4 Se 5 forming a ffiffi ffi 5 p  ffiffi ffi 5 p iron vacancy ordered block antiferromagnetic (AF) structure at low temperature while the superconducting phase arises from alkali deficient K z Fe 2 Se 2 (ThCr 2 Si 2 structure), which is embedded in the matrix of the insulating block AF phase. 16 Different schemes, such as high temperature melt (within one or two steps), 14,15,[17][18][19][20] Bridgman method, 21 and optical floating-zone technique, 22 have been employed to grow single crystalline A x Fe 2Ày Se 2 . Lei and Petrovic show that pristine K x Fe 2Ày Se 2 single crystals can only carry small J c values.…”

“…To this end, a question is naturally raised whether the A x Fe 2Ày Se 2 SCs could be a candidate material for high field magnet. However, the bulk A x Fe 2Ày Se 2 single crystals are always infested by the intrinsic phase separation, [13][14][15][16] which is a big obstacle to practical applications. Recent neutron diffraction studies demonstrate that the insulating phase has a chemical formula K 2 Fe 4 Se 5 forming a ffiffi ffi 5 p  ffiffi ffi 5 p iron vacancy ordered block antiferromagnetic (AF) structure at low temperature while the superconducting phase arises from alkali deficient K z Fe 2 Se 2 (ThCr 2 Si 2 structure), which is embedded in the matrix of the insulating block AF phase.…”

(Li1−xFex)OHFeSe Superconductors: Crystal Growth, Structure, and Electromagnetic Properties

“…A x Fe 2−y Se 2 (A = K, Rb, and Cs) Precursor A x Fe 2−y Se 2 (A = K, Rb, and Cs) single crystals used as precursors can be obtained using self-flux, Bridgman, and optical floating-zone (OFZ) crystal growth methods [54][55][56][57][58][59][60][61][62][63]. Figure 1a,b show the schematic illustrations of the self-flux and Bridgman growth methods, respectively.…”

Anisotropic transport property anomaly in K0.8Fe1.65Se2 crystal

“…The phase separated pattern often adopts notable stripe configurations along 110 ½ and 1 10 ½ directions in iron-selenide plane. [11][12][13][14] This in-plane fourfold symmetric stripe-like structure will lead to a fourfold symmetric superconductivity as well as the fourfold symmetric pinning effect which have been reported by Xie et al 15 On the other hand, the anisotropic transport measurements between out-of-plane and ab-plane in A x Fe 2Ày Se 2 reveal that the upper critical field is high with a moderate anisotropy C about 3-4. [16][17][18] Therefore, the studies of the anisotropic superconducting properties and flux dynamics are of crucial importance for evaluating the potential application.…”