Intercalated Iron Chalcogenides: Phase Separation Phenomena and Superconducting Properties.

Anna Krzton-Maziopa

doi:10.3389/fchem.2021.640361

Abstract

Organic molecule-intercalated layered iron-based monochalcogenides are presently the subject of intense research studies due to the linkage of their fascinating magnetic and superconducting properties to the chemical nature of guests present in the structure. Iron chalcogenides have the ability to host various organic species (i.e., solvates of alkali metals and the selected Lewis bases or long-chain alkylammonium cations) between the weakly bound inorganic layers, which opens up the possibility for fine tuning the magnetic and electrical properties of the intercalated phases by controlling both the doping level and the type/shape and orientation of the organic molecules. In recent years, significant progress has been made in the field of intercalation chemistry, expanding the gallery of intercalated superconductors with new hybrid inorganic–organic phases characterized by transition temperatures to a superconducting state as high as 46 K. A typical synthetic approach involves the low-temperature intercalation of layered precursors in the presence of liquid amines, and other methods, such as electrochemical intercalation, intercalant or ion exchange, and direct solvothermal growths from anhydrous amine-based media, are also being developed. Large organic guests, while entering a layered structure on intercalation, push off the inorganic slabs and modify the geometry of their internal building blocks (edge-sharing iron chalcogenide tetrahedrons) through chemical pressure. The chemical nature and orientation of organic molecules between the inorganic layers play an important role in structural modification and may serve as a tool for the alteration of the superconducting properties. A variety of donor species well-matched with the selected alkali metals enables the adjustment of electron doping in a host structure offering a broad range of new materials with tunable electric and magnetic properties. In this review, the main aspects of intercalation chemistry are discussed, involving the influence of the chemical and electrochemical nature of intercalating species on the crystal structure and critical issues related to the superconducting properties of the hybrid inorganic–organic phases. Mutual relations between the host and organic guests lead to a specific ordering of molecular species between the host layers, and their effect on the electronic structure of the host will be also argued. A brief description of a critical assessment of the association of the most effective chemical and electrochemical methods, which lead to the preparation of nanosized/microsized powders and single crystals of molecularly intercalated phases, with the ease of preparation of phase pure materials, crystal sizes, and the morphology of final products is given together with a discussion of the stability of the intercalated materials connected with the volatility of organic solvents and a possible degradation of host materials.

Highlights

Discovery of superconductivity in a non-stoichiometric binary iron selenide compound FeSe1−x (Hsu et al, 2008) and shortly after in its ternary analogs: FeSe1−x−yChy (Ch = S, Te) with isovalent substitutions on the selenium site (Fang et al, 2008; Yeh et al, 2008; Mizuguchi et al, 2009a), widened the gallery of layered superconductors
Research on the effects of chemical modification, i.e., the substitution of an isovalent anion on critical temperature, intertwined with the studies of hydrostatic pressure effects resulted in a multitude of reports combining distortions in the crystal structure of parent compounds with the critical temperature of superconducting transition
It can be hypothetically assumed that more regular geometry should result in higher critical temperatures this is only a rough assumption, which ignores other important factors, such as the injection of charge carriers resulting from the presence of structural defects, vacancies, interstitial atoms, or inclusions that may serve as doping centers

Summary

INTRODUCTION

Reports on surprisingly high critical temperatures above 70 K observed in the monolayers of Fese1−x grown on diverse substrates (Wang et al, 2012; Sun et al, 2014; Kang and Fernandes, 2016; Zhao et al, 2016), support these assumptions, though the Fermi surface topology observed for the FeSe monolayers (only electron pockets) differs significantly from the bulk materials, and what is more interesting, both long-range magnetic order and nematicity are absent in monolayer films (Liu et al, 2012; He et al, 2013). The mechanical stress, if well-controlled, can serve as an additional tuning factor for the electronic properties of these materials (Zhou et al, 2019; Chaluvadi et al, 2021)

CRYSTAL STRUCTURE

Findings

CONCLUSIONS AND FUTURE DIRECTIONS