The preparation and phase diagrams of (7Li1−xFexOD)FeSe and (Li1−xFexOH)FeSe superconductors

Abstract

We report the phase diagram for the superconducting system (⁷Li_1−xFe_xOD)FeSe and contrast it with that of (Li_1−xFe_xOH)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, 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 D. The NPD measurements definitively rule out an antiferromagnetic ordering in the non-superconducting (Li_1−xFe_xOD)FeSe samples below 120 K, which has been reported in non-superconducting (Li_1−xFe_xOH)FeSe. A likely explanation for the observed antiferromagnetic transition in (Li_1−xFe_xOH)FeSe samples is the formation of impurities during their preparation such as Fe₃O₄ and Li_xFeO₂, 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 T_c.