The intercalation chemistry of layered iron chalcogenide superconductors
Graphical abstract
Introduction
We review the recent work on the intercalation chemistry of iron chalcogenide superconductors. Given that they exhibit zero resistance for electrical currents below a critical temperature (Tc), superconductors hold great promise in our future energy needs [1], [2], [3], [4]. Furthermore, superconductors have been proposed for devices that stabilize the electrical power grid by storing energy mechanically in flywheels or electromagnetically in toroidal magnets [5], [6]. Much as previously known superconducting materials, the iron-based compounds have a role to play in our future energy needs [7], [8].
The iron chalcogenide superconductors remain topical due to their versatile solid state chemistry that allows new materials to be discovered and their ability to be isolated as single layers. Furthermore, the highest Tc so far observed in any iron-based system has been in single layered FeSe where reports vary from 65 K to 100 K [9], [10], [11]. Just like the high-Tc cuprates, the iron-based systems appear to be unconventional in their superconducting mechanism [12], [13] and this raises more hope that solid state chemists will continue to make significant discoveries in this field.
Although similar in many respects to the iron-based pnictides, the chalcogenides do show considerable differences in their physical and chemical properties. While the arsenide phases are held together by ionic forces between the cationic layers, e.g. (LaO)+ and Ba2+, and the anionic (FeAs)− layers, the chalcogenides can be held by van der Waals interactions alone. These comparatively weaker interactions make the FeCh layers, where Ch=S2− and Se2−, ideal hosts for intercalation chemistry such as alkali metal insertion along with various other types of guest species such as ammonia. Since the pnictides do not express intercalation chemistry, we have left them out altogether in this review. The reader curious about the major differences between chalcogenides and pnictide superconductors can find several reviews written on these two major categories [12], [13], [14], [15], [16], [17].
We review the chemical techniques for inserting ionic and molecular species into FeCh hosts and their resulting physical properties. An example of the different layered structures we will be reviewing and their intercalation chemistry is presented in the schematic of Fig. 1. We will examine three major methods for intercalating guest species, or intercalates, which include insertion of 1.) electropositive metals in liquid ammonia, 2.) metals and organic amines, and 3.) extended hydroxides through hydrothermal conditions. Each method plus post-synthetic treatment can lead to guest-host compounds with different structures as shown in Fig. 1.
Before presenting the latest studies on the intercalation of iron-based materials, we briefly review the history of intercalation chemistry of superconductors such as C60 and metal dichalcogenides. It is also instructive to present the crystal chemistry and physical properties of the simple binary iron chalcogenides—FeS, FeSe, and FeTe, which act as the hosts. The main goals of such chemistry has been to both prepare new compounds and ultimately control the physical properties of the binary compounds in order to discover the underlying mechanism for superconductivity. We believe the reader will recognize, however, that this chemistry has broader implications than superconductivity and could represent a new area for the preparation of two-dimensional (2D) inorganic materials.
Section snippets
History of intercalation chemistry and superconductivity
This year, 2016, is a leap year since February has an extra day. Such an insertion of a day into the calendar is known as an intercalation, and chemistry has borrowed this term to analogously describe the expansion in a solid upon inserting a ‘foreign’ species. Intercalation chemistry in inorganic materials has a long history [18], and its applications range from electrochemical energy storage [19] to the sequestration of waste material in layered clays or sulfides [20], [21], [22]. It is
Chemistry and physics of the FeCh hosts
Before discussing the intercalation chemistry of the iron based superconductors, it is instructive to describe the structure, bonding, and electronic properties of the host materials. The FeCh (Ch=S, Se, Te) layers, like the transition metal dichalcogenides, are held by weak van der Waals forces that make them susceptible to intercalation. In the case of the iron telluride, however, the van der Waals gap is too small on account of the large anionic radius of Te2− (2.21 Å) [51], and this hinders
Metal intercalation with liquid ammonia
Before discussing the first attempts at intercalating FeSe and FeS, we briefly mention that compounds of the type AxFe2-ySe2 had been prepared before through solid state techniques (where =alkali metal or Tl1+ cations) [102], [103], [104], [105], [106], [107], [108]. These materials exhibit a significant increase in the Tc up to approximately 30 K. Since single crystals can readily be grown from congruent melts [109], [110], we do not consider their preparation as intercalation chemistry. We
Metal intercalation with organic amines
Just as in the transition metal dichalcogenides, the community soon recognized that other amines besides ammonia could be intercalated into the iron chalcogenides. The first such report was performed by Krzton-Maziopa et al. where they used anhydrous pyridine as the solvent for a variety of alkali metals including Li, Na, K, and Rb [138]. By reacting with pure β-FeSe at 40 °C, Krzton-Maziopa et al. managed to expand the c-axis and enhance the Tc up to 45 K, close to the observed values for the
Metal hydroxide intercalation under hydrothermal conditions
An interesting development in the intercalation chemistry of FeSe was the demonstration by Lu et al. that such reactions could occur in aqueous solutions as opposed to ammoniacal ones that were strictly anhydrous [146]. Under hydrothermal conditions, Lu et al. added a selenide source in the form of selenourea to a strongly basic solution of LiOH along with iron metal. The resulting compound was formulated as LiFeO2Fe2Se2, which was structurally related to the LnOFeAs superconductors and the
Conclusions and future directions
The iron chalcogenides, and in particular FeSe, have been proven to be versatile hosts for intercalation chemistry. Furthermore, intercalation chemistry affords superconducting samples that are phase pure and therefore reveal the true ground state properties. These low-temperature synthetic techniques are superior to that of simply melting the constituent elements (alkali metal, iron, and selenium), since the latter leads to phase separation and the inclusion of an antiferromagnetic, insulating
Acknowledgements
We thank the National Science Foundation CAREER, DMR-1455118, for financial support.
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