Study of Thermal Behavior and Single Crystal Growth of A 0.8 Fe 1.81 Se 2 ( A =K, Rb, and Cs)
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O R I G I N A L PA P E R
Study of Thermal Behavior and Single Crystal Growth of A0.8 Fe1.81Se2 (A = K, Rb, and Cs) F. Peng · W.P. Liu · C.T. Lin
Received: 6 November 2012 / Accepted: 30 November 2012 / Published online: 20 December 2012 © Springer Science+Business Media New York 2012
Abstract We have investigated the melting behavior of alkaline iron selenide compounds of composition A0.80 Fe1.81 Se2 (A = K, Rb, and Cs) by thermogravimetry/differential thermal analysis (TG/DTA). The analysis indicated that all of the compounds melt incongruently and complete melting occurs at 902, 927, and 900 ◦ C for A = K, Rb, and Cs, respectively, as confirmed by high temperature optical microscopy (HTOM). Study of DTA shows the structure of the phase separation present in the compounds and both endothermic and exothermic peaks on heating/cooling measurements are clearly observed. The optimum crystallization temperature range is indicated by the significant exothermic peeks on cooling DTA. Large and high quality single crystals can be obtained at a slow cooling/growth rate by the modified Bridgman, flux, or floating zone methods. XRD reflections in the (00l) plane show that all the single crystals represent an intergrowth of two sets of the c-axis characterized by slightly different lattice constants. Magnetization measurements show that the superconducting transition temperature occurs at Tc = 31.6 K with shielding fraction volume of ∼100 % for K0.80 Fe1.81 Se2 . Keywords A0.8 Fe1.81 Se2 (A = K, Rb, and Cs) · TG/DTA · Bridgman growth · Floating zone growth
1 Introduction The discovery of superconductivity at temperatures above 30 K in Ax Fe2−y Se2 (A = K, Rb, Cs, (Tl, K), and (Tl, Rb)) F. Peng · W.P. Liu · C.T. Lin () Max-Planck-Institut für Festköperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany e-mail: [email protected]
compounds has attracted considerable attention [1–7]. The crystal structure of Ax Fe2−y Se2 descends from the wellknown ThCr2 Si2 structure with vacancies at the Fe sites. Microstructure analysis by means of transmission electron microscopy (TEM) on K0.8 Fe2−y Se2 single crystals demonstrates a clear phase separation in the superconducting samples. Two phases in the form of parallel lamellae, namely iron-vacancy-ordered and -disordered phases, stack along the c axis of the crystal [8]. It was soon found that magnetism and superconductivity coexist in Ax Fe2−y Se2 (A = K, Rb, and Cs) [9, 10]. To study the compound structure, magnetic, as well as physical anisotropic behavior, the form of single crystals is desperately required. However, these complex microstructures make it difficult to grow bulk superconducting Ax Fe2−y Se2 single crystals. Success in obtaining superconducting samples strongly depends on the ratios of the starting chemical components, particularly on the initial Fe content, as well as heating, cooling, and growth rates. So far, different techniques such as flux growth and the Bridgman method have been used to grow Ax Fe2−y Se2 (A = K, Rb, and Cs) single crystals [11–18]. Nearly all
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