Intercalating Anions between Terminated Anion Layers: Unusual Ionic S–Se Bonds and Hole-Doping Induced Superconductivity in S_0.24(NH₃)_0.26Fe₂Se₂

Abstract

The pairing of ions of opposite charge is a central principle of chemistry. Even though the ability to intercalate anions is desirable for many applications, it remains a key challenge for numerous host materials with their outmost layers beingn anions. In this work, we introduce a hydrothermal ion-exchange synthesis to intercalate oxidative S and Se anions between the Se layers of FeSe, which leads to single crystals of novel compounds (Se/ S) x (NH 3 ) y Fe 2 Se 2 . In particular, the unusual anion−anion bonding between the intercalated S (or Se) and Se layers exhibits strong ionic characteristics. The charge transfer through the Se layer to S (or Se) intercalants is further confirmed by the elevated oxidation state of Fe ions and the dominant hole carriers in the intercalated compounds. By intercalating S, for the first time superconductivity emerged in hole-doped iron chalcogenides. The generality of this chemical approach was further demonstrated with layered FeS and NiSe. Our findings thus open an avenue to exploring diverse aspects of anionic intercalation in similar materials.

Synthesis, modular composition, and electrochemical properties of lamellar iron sulfides

“…These lamellar iron sulfide materials belong to a family of layered iron chalcogenides containing various interlayer atoms, including the aforementioned compounds investigated for superconductivity and magnetism. 16,17,27,60,18,[20][21][22][23][24][25][26] Many of these materials contain amine or hydroxide ligands bound to the intercalated cations; we speculate that this may help stabilize layered structures vs. bulk iron chalcogenides. Presumably the relatively strong binding of Li + to DMF or to a negatively charged iron sulfide layer in 1-Li could play a similar role and may explain why this material is not obtained from similar precursors under other conditions.…”

“…[6][7][8][9][10][11][12][13][14][15] Layered iron chalcogenides, such as lamellar iron selenides, have been specifically targeted for applications in superconductivity. [16][17][18][19][20][21][22][23][24][25][26][27] While layered iron sulfides are known in some minerals like tochilinite, there has been comparatively less work on synthetic approaches to generate layered iron sulfides as compared with their selenide congeners. This is despite the fact that iron sulfides have favorable properties, including the low cost and abundance of iron and sulfur, that make them promising candidates for batteries, supercapacitors, and superconductors.…”

Synthesis, Modular Composition, and Electrochemical Properties of Lamellar Iron SulfidesSynthesis, Modular Composition, and Electrochemical Properties of Lamellar Iron Sulfides

“…These lamellar iron sulfide materials belong to a family of layered iron chalcogenides containing various interlayer atoms, including the aforementioned compounds investigated for superconductivity and magnetism. 16,17,27,60,18,[20][21][22][23][24][25][26] Many of these materials contain amine or hydroxide ligands bound to the intercalated cations; we speculate that this may help stabilize layered structures vs. bulk iron chalcogenides. Presumably the relatively strong binding of Li + to DMF or to a negatively charged iron sulfide layer in 1-Li could play a similar role and may explain why this material is not obtained from similar precursors under other conditions.…”

Unconventional Superconductivity Induced by Suppressing an Iron-Selenium-Based Mott Insulator CsFe4−xSe4

“…The iron-selenium (FeSe) based superconductors, which are different from the iron-arsenic based superconductors, have attracted a lot of attention recently because of their intriguing properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The FeSe has the simplest structure, and the critical temperature (T c ) of the bulk samples [1] can be enhanced from about 8 K to 37 K by pressure [2][3][4].…”

“…The Meissner shielding signal was observed up to 65 K for FeSe monolayer thin film grown on a Nbdoped SrTiO 3 substrate [5][6][7], which could be the highest T c of all iron-based superconductors discovered so far (in terms of the Meissner effect). By intercalating alkali metals between FeSe layers, superconductivity was observed in a series of compounds, such as A x Fe 2−y Se 2 (A ¼ Na, K, Rb, Cs, Tl=Rb, and Tl=K) [8][9][10][11][12][13][14][15] and x Fe x OHFeSe [16], and T c can be enhanced to 32 K, or even to 46 K. Also, the recently found hole-type FeSe based superconductor S 0.24 ðNH 3 Þ 0.26 Fe 2 Se 2 enriches the physics in the family of FeSe based superconductors [17]. An obvious common feature is that they all have FeSe layers as conducting sheets from which the superconductivity originates.…”