Superconductivity in LiFeO${}_2$Fe${}_2$Se${}_2$ with anti-PbO-type spacer layers

We report on a superconductor LiFeO${}_2$Fe${}_2$Se${}_2$ with $T_{\mathrm{c}}\sim 43$ K, synthesized via the hydrothermal method. The anti-PbO-type spacer layer of LiFeO${}_2$ has been successfully intercalated between the anti-PbO-type FeSe layer, which results in a high-$T_{\mathrm{c}}$ superconductivity in this iron selenide material. In this material, the LiFeO${}_2$ layer is electrically neutral and does not act as an electron donor, which is remarkably different from other heavily electron-doped intercalated FeSe-derived superconductors. Based on our results and previous reports on FeSe-based superconductors, we figure out a $\mathsf{V}$-shaped anion height from the Fe layer dependence of $T_{\mathrm{c}}$ in the FeSe-derived superconductors, which is in contrast to the inverse $\mathsf{V}$-shaped one for the FeAs-based superconductors. Our studies expand the categories of the iron-based superconductors and open a door in the quest for potential superconductors with various spacer layers, which is helpful for the study of the underlying physics in iron-based high-$T_{\mathrm{c}}$ superconductors.

Figure 1

(a) A schematic view of the structure of LiFeO${}_2$Fe${}_2$Se${}_2$. The layered structure consists of alternately stacking anti-PbO-type FeSe layers and LiFeO${}_2$ layers. (b) XRD patterns together with Rietveld refinement results. The observed diffraction intensities are represented by yellow-green open circles, and the fitting results by the red solid line. The blue curve at the bottom is the difference between observed and fitting intensities. Short black vertical bars below the XRD patterns indicate the positions of allowed Bragg reflections.

Figure 2

(a) The magnetic susceptibility of the as-synthesized LiFeO${}_2$Fe${}_2$Se${}_2$ sample from the hydrothermal method. (b) The magnetic susceptibility of the sample after annealing under a high pressure of 5 GPa at $T=150{}^{\circ }\mathrm{C}$ for 5 h. Solid symbols: Zero-field cooling (ZFC); open symbols: field cooling (FC). These data were collected under a magnetic field of 10 Oe. The inset in (a) is the zoom-in view around the superconducting transition of the as-synthesized sample. The bottom inset of (b) is the zoom-in view for ZFC data around the superconducting transition of the annealed sample, which indicates $T_{\mathrm{c}}\sim 43$ K. The top inset of (b) is the $M$-$H$ loop taken at 5 K for the annealed sample, where a linear $H$ dependence of $M$ with a negative slope can be recognized below 300 Oe in the low-field range (as the magenta arrow indicates).

Figure 3

(a) The magnetizations ($M$) taken under various external pressures with $H=10$ Oe in the ZFC mode. Before the measurement, the LiFeO${}_2$Fe${}_2$Se${}_2$ sample was annealed under a high pressure of 5 GPa at $T=150{}^{\circ }\mathrm{C}$ for 5 h. The inset shows the zoom-in view around the superconducting transitions. The arrow shows the beginning of the drop of $M$ at 1 GPa, which represents the way to define $T_{\mathrm{c}}$ as shown in (b). (b) Pressure dependence of the superconducting transition temperatures $T_{\mathrm{c}}$.

Figure 4

$T_{\mathrm{c}}$ vs chalcogen (Se) height for the FeSe-derived superconductors. The data for FeSe under high pressure are from Ref. [29], indicated by blue solid squares. Red solid circles are the data of the intercalated FeSe superconductors from Ref. [12] (K${}_x$Fe${}_{2-y}$Se${}_2$), Ref. [16] [Li${}_{0.6}$(NH${}_2$)${}_{0.2}$(NH${}_3$)${}_{0.8}$Fe${}_2$Se${}_2$], and the present work.