Soft chemical control of superconductivity in lithium iron selenide hydroxides Li1-xFex(OH)Fe1-ySe

Abstract

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li1-xFex(OH)Fe1-ySe (x ~ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesised samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasised by the demonstration that reductive post-synthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li1-xFex(OH) reservoir layer to fill vacancies in the selenide layer

7 pages supplementary material available from Journal, Inorganic Chemistry 2015

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• [(Li0.8Fe0.2)OH]FeS and the ferromagnetic superconductors [(Li0.8Fe0.2)OH]Fe(S1-xSex) (0<x<1)
• Spin resonance in the superconducting state of Li${}_{1-x}$Fe${}_x$ODFe${}_{1-y}$Se observed by neutron spectroscopy
• New Alkali-Metal- and 2-Phenethylamine-Intercalated Superconductors $A_x$(C${}_8$H${}_{11}$N)${}_y$Fe${}_{1-z}$Se ($A$ = Li, Na) with the Largest Interlayer Spacings and $T_{\mathrm{c}}$ $\sim$ 40 K
• Basic electronic properties of iron selenide under variation of structural parameters
• Non-trivial role of interlayer cation states in iron-based superconductors
• Superconductivity and Intercalation Statein the Lithium-Hexamethylenediamine-Intercalated Superconductor Li${}_x$(C${}_6$H${}_{16}$N${}_2$)${}_y$Fe${}_{2-z}$Se${}_2$: Dependence on the Intercalation Temperature and Lithium Content
• Two-dimensional Cs-vacancy superstructure in iron-based superconductor $C{s}_{0.8}F{e}_{1.6}S{e}_2$
• Possible pairing symmetry in the FeSe-based superconductors determined by quasiparticle interference
• Coexistence of localized and itinerant magnetism in intercalated iron-selenide (Li,Fe)OHFeSe
• Importance of Fermi surface and magnetic interactions for the superconducting dome in electron doped FeSe intercalates