The preparation and phase diagrams of (7Li1−xFexOD)FeSe and (Li1−xFexOH)FeSe superconductors†
We report the phase diagram for the superconducting system (7Li1−xFexOD)FeSe and contrast it with that of (Li1−xFexOH)FeSe both in single crystal and powder forms. Samples were prepared via hydrothermal methods and characterized by 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, Tc, 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 Tc in substituting H by D. The NPD measurements definitively rule out an antiferromagnetic ordering in the non-superconducting (Li1−xFexOD)FeSe samples below 120 K, which has been reported in non-superconducting (Li1−xFexOH)FeSe. A likely explanation for the observed antiferromagnetic transition in (Li1−xFexOH)FeSe samples is the formation of impurities during their preparation such as Fe3O4 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 H2O vs. D2O as the solvent during synthesis. We also describe the reaction conditions that lead to some of our superconducting samples to exhibit ferromagnetism below Tc.
- This article is part of the themed collections: 2016 Journal of Materials Chemistry C Hot Papers and Emerging Investigators 2016: Novel design strategies for new functional materials