$\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$: a defect resilient expanded-lattice high-temperature superconductor

Abstract

Two-dimensional iron-chalcogenide intercalates display a remarkable correlation of the interlayer spacing with the enhancement of the superconducting critical temperature ($T_c$). In this work, synchrotron x-ray absorption ($XAS$, at Fe and Se K edges) and emission ($XES$) spectroscopies, allow to discuss how the important rise of $T_c$ (44 K) in the molecule intercalated $\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$ relates to the electronic and local structure changes felt by the inorganic host upon doping ($x$). $XES$ shows that widely-separated layers of edge-sharing $\rm FeSe_4$ tetrahedra, carry low-spin moieties with a local Fe magnetic moment slightly reduced compared to the parent $\beta$-$\rm Fe_{2-z}Se_2$. Pre-edge $XAS$ advises on the progressively reduced mixing of metal $3d-4p$ states upon lithiation. Doping-mediated local lattice modifications, probed by conventional $T_c$-optimization measures (cf. anion height and $FeSe_4$ tetrahedra regularity), become less relevant when layers are spaced far away. On the basis of extended x-ray absorption fine structure, such distortions are compensated by a softer Fe-network that relates to Fe-site vacancies, alleviating electron-lattice correlations and superconductivity. Density functional theory ($DFT$) guided modification of isolated $\rm Fe_{2-z}Se_2$ ($z$, vacant sites) planes, resembling the host layers, identify that Fe-site deficiency occurs at low energy cost, giving rise to stretched Fe-sheets, in accord with experiments. The robust high-$T_c$ in $\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$, arises from the interplay of electron donating spacers and the iron-selenide layers tolerance to defect chemistry, a tool to favorably tune its Fermi surface properties.