NaOH-Intercalated Iron Chalcogenides (Na_1-OH)Fe_1-X (X = Se, S): Ion-Exchange Synthesis and Physical Properties

The discovery of the (Li_1-FeOH)FeSe superconductor has aroused significant interest in metal hydroxide-intercalated iron chalcogenides. However, all efforts made to intercalate NaOH between FeSe and FeS layers have failed so far. Here we report two NaOH-intercalated iron chalcogenides (Na_1-OH)Fe_1-X (X = Se, S) that were synthesized by a low-temperature hydrothermal ion-exchange method. Their crystal structures were solved through single-crystal X-ray diffraction and refined against powder X-ray and neutron diffraction data. Different from the (Li_1-FeOH)FeX superconductors that crystallize in a tetragonal space group P4/nmm with Z = 2, (Na_1-OH)Fe_1-X belong to an orthorhombic space group Cmma with Z = 4. The structural solution also reveals that there are vacancies in both Na and Fe sites and there are not iron ions in the (Na_1-OH) layer. This is probably why both Fe(II) and Fe(III) species exist in the title compounds, as detected by X-ray photoelectron spectroscopy. Based on magnetization and electrical resistivity measurements, the two compounds were found to be paramagnetic semiconductors. The absence of superconductivity should be closely related to the iron vacancies in the Fe_1-X layer. Theoretical calculations suggest that inducing superconductivity in (Na_1-OH)Fe_1-Se is promising due to the similarity of the electronic structures between stoichiometric (NaOH)FeSe and the (Li_1-FeOH)FeSe superconductor.