The preparation and phase diagrams of (${}^7$Li${}_{1-x}$Fe${}_x$OD)FeSe and (Li${}_{1-x}$Fe${}_x$OH)FeSe superconductors

Abstract

We report the phase diagram for the superconducting system (${}^7$Li${}_{1-x}$Fe${}_x$OD)FeSe and contrast it with that of (Li${}_{1-x}$Fe${}_x$OH)FeSe both in single crystal and powder forms. Samples were prepared via hydrothermal methods and characterized with laboratory and synchrotron X-ray diffraction, high-resolution neutron powder diffraction (NPD), and high intensity NPD. We find a correlation between the tetragonality of the unit cell parameters and the critical temperature, $T_c$, which is indicative of the effects of charge doping on the lattice and formation of iron vacancies in the FeSe layer. We observe no appreciable isotope effect on the maximum $T_c$ in substituting H by by D. The NPD measurements definitively rule out an antiferromagnetic ordering in the non-superconducting (Li${}_{1-x}$Fe${}_x$OD)FeSe samples below 120 K, which has been reported in non-superconducting (Li${}_{1-x}$Fe${}_x$OH)FeSe.${}^1$ A likely explanation for the observed antiferromagnetic transition in (Li${}_{1-x}$Fe${}_x$OH)FeSe samples is the formation of impurities during their preparation such as Fe${}_3$O${}_4$ and LixFeO2, which express a charge ordering transition known as the Verwey transition near 120 K. The concentration of these oxide impurities is found to be dependent on the concentration of the lithium hydroxide reagent and the use of H${}_2$O vs. D${}_2$O as the solvent during synthesis. We also describe the reaction conditions that lead to some of our superconducting samples to exhibit ferromagnetism below $T_c$.

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• Strong anisotropy in nearly ideal-tetrahedral superconducting FeS single crystals
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• Reemergence of high-Tc superconductivity in the (Li1-xFex)OHFe1-ySe under high pressure
• Superconductivity and magnetism in Iron Sulfides Intercalated by Metal Hydroxides
• Frustrated magnetism in tetragonal CoSe, analogue to superconducting FeSe
• Spin resonance in the superconducting state of Li${}_{1-x}$Fe${}_x$ODFe${}_{1-y}$Se observed by neutron spectroscopy
• Electronic phase separation in iron selenide (Li, Fe)OHFeSe superconductor system
• Doping Mn into (Li1-xFex)OHFe1-ySe superconducting crystals via ion-exchange and ion-release/introduction syntheses
• Long range magnetic order in hydroxide layer doped (Li${}_{1-x-y}$Fe${}_x$Mn${}_y$OD)FeSe
• Spin correlations in the parent phase of Li${}_{1-x}$Fe${}_x$ODFeSe