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

Abstract

The discovery of the (Li1–x Fe x OH)­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 (Na1–x OH)­Fe1–y 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 (Li1–x Fe x OH)­FeX superconductors that crystallize in a tetragonal space group P4/nmm with Z = 2, (Na1–x OH)­Fe1–y 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 (Na1–x 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 Fe1–y X layer. Theoretical calculations suggest that inducing superconductivity in (Na1–x OH)­Fe1–y Se is promising due to the similarity of the electronic structures between stoichiometric (NaOH)­FeSe and the (Li1–x Fe x OH)­FeSe superconductor.

Facile synthesis and selected characteristics of two-dimensional material composed of iron sulfide and magnesium-based hydroxide layers (tochilinite)

“…Synthetic valleriite and tochilinite have been prepared and characterized too; [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] however, the yield of the substances was typically lower than 50%, and those were substantially contaminated by the reaction by-products. Another group of layered iron chalcogenide-hydroxide materials with the composition Fe(Se,S)Á(Li,Fe)OH and some their derivatives [52][53][54][55][56] synthesized in the recent years are attracting much attention because of the superconducting properties but little efforts have been made to study other characteristics and potential applications.…”

Temperature dependent local inhomogeneity and magnetic moments of (Li1−xFex)OHFeSe superconductors

“…Thus, the results for the local structure provide a direct evidence of nanoscale inhomogeneity in (Li 1− x Fe x )OHFeSe. It should be mentioned that the presence of a smaller, compressed structure had been indicated earlier (although never definitively proven) in some diffraction studies on the same material 40,44 and in a sister compound (Na 1− x OH)Fe 1− y Se, 52 originating likely from the atomic displacements in the FeSe 4 tetrahedron following Li substitution in the active layer.…”

Advances in Chalcogenide Crystal Growth: Flux and Solution Syntheses, and Approaches for Postsynthetic Modifications

“…An intriguing SCSC hydrothermal ion exchange was observed in a sample of the layered K 0.8 Fe 1.6 Q 2 ( Q = S and Se) structures (Figure ). In these cases, K + ions were exchanged not just with alkali cations but with alkali hydroxide and resulted in (Na 1– x OH)­Fe 1– y Q and (Li 1– x Fe x OH)­Fe Q single crystals (Figure ). − The latter, (Li 1– x Fe x OH)­FeSe, is a superconductor with T c ≈ 40 K, while pristine K 0.8 Fe 2 Se 2 and FeSe have T c of 30 and 8 K, respectively; therefore, intercalation in the layered FeSe achieved a positive effect on the superconducting behavior .…”

“… In these cases, K + ions were exchanged not just with alkali cations but with alkali hydroxide and resulted in (Na 1– x OH)­Fe 1– y Q and (Li 1– x Fe x OH)­Fe Q single crystals (Figure ). − The latter, (Li 1– x Fe x OH)­FeSe, is a superconductor with T c ≈ 40 K, while pristine K 0.8 Fe 2 Se 2 and FeSe have T c of 30 and 8 K, respectively; therefore, intercalation in the layered FeSe achieved a positive effect on the superconducting behavior . Another fascinating example of a SCSC postsynthetic transformation is the reductive K + -deintercalation of K 0.8 Fe 1.6 S 2 , which resulted in high-quality single crystals of mackinawite FeS .…”