Since the discovery of superconductivity at 26 K in an iron-arsenide layered compound LaFeAsO1-xFx [1], the area of iron-based superconductors has been growing. Other parent compounds have been subsequently identified and the critical temperature Tc has increased to 55 K [2]. Ternary iron arsenide, BaFe2As2 with ThCr2Si2-type structure, is one of the non-superconducting parent compounds, where proper doping, for example, partial substitution of potassium for barium induces superconductivity with the highest Tc of 38K [3]. Iron-based superconductors exhibit high critical temperature, low anisotropy and large upper critical field and thus are attractive candidates for practical applications, once facile methods of wire and tape fabrication are developed. Traditional synthesis routes for the preparation of Ba1-xKxFe2As2 powders involves solid state reaction between the constituent elements. The reaction is hampered by slow kinetic, thus multiple regrinding and sintering processes are required in order to obtain single phase materials [3]. Moreover, parasitic impurities, such as unreacted FeAs tend to wet grain boundaries and act as current blocking secondary phase. Here we propose a facile synthesis of Ba1-xKxFe2As2 utilizing the reaction of metal hydrides such as BaH2 and KH with either FeAs or elemental Fe and As. The use of hydrides provides uniform mixing and thus promotes complete reaction. The synthetic conditions, including the method of reagent mixing, the initial stoichiometric ratios and type of the precursors, the reaction temperature and time will be discussed in the detail. The thermodynamic characterization of parent BaFe2As2 and KFe2As2 compounds as well as Ba1-xKxFe2As2superconductor by means of oxide melt solution calorimetry in sodium molybdate at 700�C [4] will be presented. This method provides the essential information about the thermodynamic parameters, such as thermodynamic stability and enthalpy of mixing, thus facilitating our understanding of the complex superconducting system. The possible origin of the phase separation, in particular Ba/K concentration gradients, revealed by high temperature drop solution calorimetry will be discussed.References :[1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono. JACS, 130, 3296, 2008 .[2] Z.A. Ren, W. Lu, J. Yang, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Chinese Phys. Lett. 25, 2215, 2008 .[3] M. Rotter, M. Tegel, D. Johrendt. Phys. Rev Lett. 101, 107006, 2008 .[4] A. Navrotsky. Phys. Chem. Minerals, 24, 222, 1997  .

Facile synthesis and thermochemistry of the Ba1-xKxFe2As?2 superconductors

248th National Meeting (2014)

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