Iron-based superconductors are extremely sensitive to impurity phases and defects in the crystal structure, which could significantly change the physical and chemical properties both in normal and superconducting states. This vulnerability is due to the fact that both microscopic and macroscopic inhomogeneities could be magnetically active, therefore influencing bulk and surface superconductivity. In the basic families of iron superconductors, the 11-type materials stand apart, owing to the absence of intermediate charge reservoir layers. The FeSe12x compounds consist only of a stack of electronically active layers weakly coupled by the van der Waals interaction. These layers are based on the edge-shared FeSe4 tetrahedra. FeSe12x demonstrates superconductivity with Tc ¢ 8 K, but its single crystals can be used for either production of intercalated materials AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb and Eu) with Tc = 30–46 K 5 or superconducting monolayers with Tc = 65 K. 6–8 In this paper we describe the synthesis of FeSe12x single crystals from the halide eutectic flux under steady temperature gradient conditions and provide proofs of their high quality by the measurements of transport and thermodynamic properties. The superconducting tetragonal phase of FeSe12x exists only below 730 K (457 uC) in a rather narrow composition range. If these conditions are not fulfilled, the samples are contaminated by Fe, Fe7Se8, and Fe3O4 impurities. The attempts to grow tetragonal FeSe12x single crystals from alkali–halide flux have been undertaken in ref. 10–13. The synthesis of iron selenides from KCl flux with the melting temperature 776 uC was described in ref. 10. The ampoule with Fe, Se and flux was heated up to 840 uC and sustained at this temperature for 30 hours (h) to homogenize the solution. It was then cooled down to 820 uC for 1 h to provide the necessary supersaturation for nucleation. Further cooling was done with the rate of 0.3–0.5 uC h from 820 to 770 uC. After that, the ampoule was cooled rapidly down to 400 uC and held for 24 h to stabilize Fe and Se. The single crystals with the hexagonal shape were produced in this synthesis. Ref. 11 presented the synthesis based on NaCl/KCl flux with the eutectic temperature 657 uC. The ampoule with preliminary obtained FeSe0.89 powder and flux was heated to 900 uC. Three steps of cooling were employed: firstly with 3 uC h rate down to 740 uC, secondly with 1 uC h rate down to 600 uC, and, finally, the furnace was cooled rapidly down to room temperature. Structural measurements revealed that both tetragonal (a) and hexagonal (b) phases coexist in the sample. A more promising method with LiCl–CsCl mixture which melts at 326 uC was used in ref. 12. The ampoule with elemental Fe and Se mixed with the flux was heated to 715 uC, and kept for 1 h at this temperature before shifting to a preheated furnace at 457 uC. After slow cooling down to 300 uC it was quenched in water. The main disadvantage of this method is that it likely results in either a or b phase depending on the various growth conditions. Application of vapor transport method to the synthesis of tetragonal FeSe12x in ref. 13 also had the same problem as above. Therefore, to grow tetragonal FeSe12x single crystals of high quality, it is necessary to use a eutectic flux which melts at low temperatures (preferably below 250 uC), Institute of Experimental Mineralogy, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, Russia P.N. Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C Low Temperature Physics and Superconductivity Department, M.V. Lomonosov Moscow State University, 119991 Moscow, Russia. E-mail: vasil@mig.phys.msu.ru; Fax: +07 495 9329217; Tel: +07 495 9329217 CrystEngComm
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