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. 2012:2:426.
doi: 10.1038/srep00426. Epub 2012 May 28.

Observation of superconductivity at 30~46 K in A(x)Fe₂Se₂(A = Li, Na, Ba, Sr, Ca, Yb, and Eu)

T P Ying  1 , X L Chen, G Wang, S F Jin, T T Zhou, X F Lai, H Zhang, W Y Wang
Affiliations

Observation of superconductivity at 30~46 K in A(x)Fe₂Se₂(A = Li, Na, Ba, Sr, Ca, Yb, and Eu)

T P Ying et al. Sci Rep. 2012.

Abstract

New iron selenide superconductors by intercalating smaller-sized alkali metals (Li, Na) and alkaline earths using high-temperature routes have been pursued ever since the discovery of superconductivity at about 30 K in KFe₂Se₂, but all have failed so far. Here we demonstrate that a series of superconductors with enhanced T(c) = 30∼46 K can be obtained by intercalating metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the ammonothermal method at room temperature. Analysis on their powder X-ray diffraction patterns reveals that all the main phases can be indexed based on body-centered tetragonal lattices with a∼3.755-3.831 Å while c∼15.99-20.54 Å. Resistivities show the corresponding sharp transitions at 45 K and 39 K for NaFe₂Se₂ and Ba₀.₈Fe₂Se₂, respectively, confirming their bulk superconductivity. These findings provide a new starting point for studying the properties of these superconductors and an effective synthetic route for the exploration of new superconductors as well.

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Figures

Figure 1
Figure 1. Powder X-ray diffraction patterns for samples measured at 297 K, Cu Kα radiation.
(a) β-FeSe; (b) K0.8Fe1.7Se2 (by high-temperature route); (c) Nominal LiFe2Se2; (d) Nominal NaFe2Se2; (e) Nominal KFe2Se2 (background corrected), peaks marked by ‘formula image’ are due to unknown phase; (f) Nominal Ca0.5Fe2Se2; (g) Nominal Sr0.8Fe2Se2; (h) Nominal Ba0.8Fe2Se2; (i) Nominal EuFe2Se2; (j) Nominal Yb0.8Fe2Se2. Peaks marked by ‘*’ are due to residual β-FeSe.
Figure 2
Figure 2. Powder X-ray diffraction pattern and Rietveld refinement profile for nominal NaFe2Se2 at 297 K.
Vertical bars (|) indicate the positions of the Bragg peaks. The bottom trace depicts the difference between the experimental and calculated intensity values. The inset shows the crystal structure of NaFe2Se2.
Figure 3
Figure 3. Magnetization and electrical resistance of nominal NaFe2Se2.
The left inset shows the magnetic hysteresises of nominal NaFe2Se2 measured at 10 K and 60 K in the range −25 kOe < H < 25 kOe, respectively. The right inset shows the temperature dependence of the electrical resistance of cold-pressed powder NaFe2Se2.
Figure 4
Figure 4. Magnetization, electrical resistance, and heat capacity of nominal Ba0.8Fe2Se2.
(a) The magnetization of nominal Ba0.8Fe2Se2 as a function of temperature. The left inset shows the magnetic hysteresises of nominal Ba0.8Fe2Se2 measured at 10 K and 60 K in the range −25 kOe < H < 25 kOe, respectively. The right inset shows the temperature dependence of the electrical resistance of cold-pressed powder Ba0.8Fe2Se2. (b) Low temperature heat capacity of cold-pressed powder Ba0.8Fe2Se2. The red dotted line is the curve fitting of phonon contribution to the heat capacity.
Figure 5
Figure 5. Magnetizations of nominal AxFe2Se2 (A = Li, K, Ca, Sr, Eu, and Yb).
(a) The magnetization of nominal LiFe2Se2 as a function of temperature. The inset shows the magnetic hysteresises of nominal LiFe2Se2 measured at 10 K and 40 K. (b) The magnetization of nominal KFe2Se2 as a function of temperature. The inset shows the magnetic hysteresises of nominal KFe2Se2 measured at 10 K and 35 K. (c) The magnetization of nominal Ca0.5Fe2Se2 as a function of temperature. The inset shows the magnetic hysteresises of nominal Ca0.5Fe2Se2 measured at 10 K and 50 K. (d) The magnetization of nominal Sr0.8Fe2Se2 as a function of temperature. The inset shows the magnetic hysteresises of nominal Sr0.8Fe2Se2 measured at 10 K and 50 K. (e) The magnetization of nominal EuFe2Se2 as a function of temperature. The inset shows the magnetic hysteresises of nominal EuFe2Se2 measured at 10 K and 50 K. (f) The magnetization of nominal Yb0.8Fe2Se2 as a function of temperature. The inset shows the magnetic hysteresises of nominal Yb0.8Fe2Se2 measured at 10 K and 60 K.
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References

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