Intercalant dependence of superconductivity in Ax(NH3)yFe2-δSe2 single crystals

Abstract

Metal intercalation using a solvent has produced significant advances in the development of Fe-chalcogenide superconducting materials. Recently, the superconducting transition temperature (TC) of metal-intercalated FeSe has been raised to 46 K using ammonia as the solvent for the alkali-metal atom. However, multiple superconducting phases have been found, which may arise from different concentrations of intercalants, further complicating the situation. Here, we report the synthesis of single-crystals of metal-intercalated FeSe superconductors using liquid ammonia, and their physical properties. Particularly, utilization of single-crystals allows us to investigate the resistivity in these ammoniated metal-intercalated FeSe superconductors, Ax(NH3)yFe2-δSe2, for the first time. Firstly, we compared with their TC s and the interlayer distance between the FeSe layers (dI) as a function of ionic radius (r) of the exchangeable intercalant. We found that both TC and dI show weak dependence on the r, if the r is smaller than the effective size of another intercalant, i.e., NH3 molecule. Besides the enhancement of the dI by the insertion of NH3 molecule, one expects that the charge-transfer due to the intercalation of cation would leads to the shift of the Fermi energy. The intercalants (Ba and K) have similar rs, but different valences (Ba2+ and K+), which may throw light on the significance of charge transfer. When the metal concentrations were investigated on the cleaved surfaces of these single crystals, clear differences were found between the two compounds, with the K-concentration about double the Ba-concentration, which may imply the identical charge-transfer. This was also supported for the investigation of the Li-concentration. These results suggest not only the local environment within the conductive FeSe layers but also the significance of the number of electronic charges supplied to the FeSe layers by the intercalated metal and/or ammonia molecules.

Intercalated Iron Chalcogenides: Phase Separation Phenomena and Superconducting Properties

“…The products are extremely air-sensitive and moisturesensitive due to the presence of alkali metals in the structure and a highly developed specific surface area, therefore any contact with atmosphere or humidity must be avoided, otherwise, they immediately decompose. Through proper adjustment of the molar ratio of reagents and other conditions of the intercalation process, one can obtain materials of high superconducting volume fraction with T c = 46 K. In case of ammonia-based intercalates, the highest purity of materials (the lowest content of magnetic impurities) was obtained at molar ratios of alkali metal and iron selenide close to 1:2 (Scheidt et al, 2012;Burrard-Lucas et al, 2013;Zheng et al, 2013;Lee et al, 2018). Ammonia-based and amine-based intercalates do not seem to be very pure systems as in the final products, depending on the concentration of alkali metal in the solvent, various types of solvates with metal ions are formed, which may concomitantly enter to the structure.…”