Chemical Intercalations in Layered Transition Metal Chalcogenides

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Chemical intercalations in layered transition metal chalcogenides: syntheses, structures and related properties Zhongnan Guo, Fan Sun, and Wenxia Yuan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00146 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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Crystal Growth & Design

Zhongnan Guo, Fan Sun and Wenxia Yuan Department of Chemistry, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083, China. ABSTRACT: Transition metal chalcogenides (TMChs) have recently attracted a great deal of interests in the chemical and physical research fields. These compounds have a common crystal structure: they usually consist of two-dimensional or quasi-two dimensional layers stacked along the direction perpendicular to the layers. The combination between layers is generally by van der Waals interaction or weak chemical bonding, making the layered chalcogenides potential hosts for intercalation. Alkali metals, alkaline earths, rare earths and organic groups or compounds can be intercalated into the structure as spacing layers, resulting in a variety of new compounds and exhibiting interesting physical & chemical properties. In this review, we introduce and summarize the latest advances in chemical intercalation and the role of these spacing layers in TMChs, and their relation to relevant properties. Especially, we focus on the developments of chemical intercalation in Fe chalcogenide superconductors to understand the effect from intercalation on formation, structure and property, in the hope to provide some new insights for novel material design. Finally, perspectives on the challenge and opportunity for future exploration on this topic are also discussed.

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on these layered TMCh “hosts”. After intercalation, the In recent years, transition metal chalcogenides (TMChs) chalcogenide layer usually acts as a conducting layer that have attracted great attention in materials research due to determines compound properties. The intercalating guest, their prominent properties, including high-temperature which could be alkali metal, alkaline earth, rare earth and superconductivity,1-3 superior thermoelectric property,4,5 and organic group or compound, is considered as a spacing layer transparent conductivity.6-8 The chalcogenide family is that increases the interlayer distance and meanwhile acts as non-toxic compared to the isostructural arsenide, making a “carrier reservoir” to provide charge carrier into the TMCh safer and more interesting for the experimental adjacent conducting layer. The conducting layer and handling. From the view of crystal structure, these TMChs spacing layer (also called as functional layer and carrier usually show the layered feature, with the two-dimensional reservoir layer, respectively) alternately stack, resulting in or quasi-two dimensional layers stacking along the direction strong anisotropy in these compounds. Very different from perpendicular to the plane. Since the interaction between the layered transition metal dichalcogenide (TMD), which is 9 layers is generally through van der Waals forces or weak commonly composed of TMCh6 polyhedrons (Figure. 1a), chemical bonding, chemical intercalation can be operated TMCh (where the molar ratio of transition metal TM and

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chalcogen Ch is close to 1) consists of edge-sharing TMCh4 tetrahedrons with metal atom in the centre, forming the anti-fluorite structure in the conducting layer (Figure. 1b). On the other hand, the spacing layers include a variety of structures such as single atom, fluorite, perovskite and so on, forming so-called 122-,10-12 1111-13,14 and 2322-type structures15 named according to their stoichiometry (Figure. 1c).

Figure 1. The crystal structure of (a) TMD, (b) TMCh and (c) three typical intercalated TMChs. (Green spheres: TM; Yellow spheres: Ch; Red spheres: oxygen; The other spheres: other metals).

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to remarkable enhancement on the various performances of these novel functional materials (Figure. 2). Knowledge of the chemical intercalation syntheses as well as the resulting structures and properties in layered TMChs can provide an important scientific basis for designing and adjusting new functional materials.

Figure 2. Functional materials in the layered TMCh family which can be introduced or regulated by the appropriate chemical intercalation.

Some research groups have systematically reviewed the individual system (Fe-based superconductor, Cu-based thermoelectric material and transparent conductors) based Actually, it was believed that the specific properties of on the extensive studies in literatures.3,4,6 However, an these layered chalcogenides originate from the conducting observation from the viewpoint of chemical intercalation in layer, especially dependent on the species of TM. For TMChs with different TM species is still lacking, especially example, high-temperature superconductivity has been for the one that focus on the role of spacing layer on the 10,16,17 discovered in layered Fe chalcogenides; Cu-based structures and related properties. Enlightening the systems are usually semiconductors and among them, the correlation of intercalation effects between different TMChs superior thermoelectric property has been found in would help us to deeply understand the adjustment of compound: BiOCuSe, with Cu selenide as the conducting crystal structure, band gap and electromagnetic property in layer;18,19 Meanwhile the intercalated Co sulphides and Co the whole layered TMCh family. Here, we review the latest selenides all show the magnetic ordering behavior.12 Even so, advances of chemical intercalation in layered TMChs (we it has been also explained that the spacing layer from focus on the systems with 3d TM: Fe, Co, Ni, and Cu). The chemical intercalation still highly affects the properties of procedure of intercalating process and the resulted crystal corresponding compounds even when the conducting layer structures are described in detail. The effects of intercalated is fixed. The superconducting critical temperature Tc can be layers on related properties are specifically discussed. At last, enhanced from 8 K to 30-46 K in Fe-based superconductors the perspectives on challenge and opportunity for the future with various intercalated guests species;10,20-22 The band gap exploration of intercalation of layered TMChs are also given. can be widely adjusted in semiconducting copper chalcogenides by varying the intercalated guest,15,23,24 and  the thermoelectric performance can also be significantly In this section, we give a brief introduction on the improved by changing the chemical composition of the initiating structure of binary TMChs before intercalation, to spacing layer.18,25-27 These phenomena all suggested that the understand the effect of the intercalation on the crystal chemical intercalation is a very effective way of tuning the configuration. Different from the TMD containing the physical and chemical properties of layered TMChs, leading metals from ⅣB to ⅥB (e.g. V, Mo and Nb), layered TMChs


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are always composed of Fe, Co, Ni and Cu with the ratio of TM: Ch closed to 1:1. Usually, there are two common structures for the binary TMChs: Firstly, the binary TMChs can crystallize the tetragonal phase adopting anti-PbO-type structure (P4/nmm space group) with the edge-sharing TMCh4 tetrahedrons stacking layer by layer along the c direction (Figure. 3).28-30 Actually, this structure can be just considered as the ones that we mentioned above (Figure 1c) with the spacing layer absent. This binary tetragonal structure is specifically easy to form in the FeCh systems but showing quite different thermostability by varying the chalcogens. Tetragonal FeS and FeSe were reported to decompose at a temperature higher than 140 and 457 ˚C,28,29 respectively. On the contrary, tetragonal FeTe is quite stable and decomposes congruently above ~850 ˚C.30 It is worth noting that the unconventional superconductivity has been observed in these simple binary Fe chalcogenides,16,17,31 making them very hot parents for chemical intercalation in recent years. The other binary anti-PbO-type TMChs with the TM instead of Fe, such as tetragonal CoSe and CoS, are metastable phases that can only be synthesized by de-intercalating the spacing layer from the 122-type TMChs.32 Secondly, TMChs can also show the NiAs-type structure with the space group of P63/mmc which consists of face-sharing TMCh6 octahedrons (Figure. 3).33 Different from the layered tetragonal phase, these hexagonal TMChs show three-dimensional structure, although the anisotropy still exists. This hexagonal structure widely exists in the Fe, Co and Ni chalcogenide systems. It should be mentioned that if we have a look at the binary phase diagrams of Fe-Ch systems, it can be observed that the tetragonal FeCh and the hexagonal FeCh show an adjacent and competitive relationship.28,29 Different from the tetragonal phase that always exists in low temperature range, the hexagonal ones are commonly more thermostable. It has been reported that after reacting with the alkali metal, the hexagonal TMCh (e.g. CoSe and NiSe) will switch to the tetrahedral symmetry (KCo2Se2 and KNi2Se2) with 122-type structure (I4/mmm).12,34

Besides the two structures mentioned above, some other space groups can also be found in binary TM-Ch systems, especially for the CuChs. For example, a typical structure which was obtained in minerals in Cu-Ch system adopts a hexagonal structure with P63/mmc space group,35 but its structure is quite different from the NiAs-type of NiCh and CoCh. Both Cu and Ch occupy two crystal sites, and the CuCh4 tetrahedron and CuCh3 triangle alternately stack to form the layer structure.35,36 However, anti-PbOand NiAs-type structures are still two of the most common structures in binary TMCh systems.  Several chemical intercalation methods have been carried out in TMCh systems, according to the literatures. It should be mentioned that since the intercalation chemistry of TMD was rather well studied in the past,37 synthetic routes on chemical intercalation in TMCh have mainly referred to the work on that of TMD. Meanwhile, the selection of methods is highly depended on the crystal structures and chemical stability of both TMChs and guest layers. In this section, we introduce the four representative methods for the chemical intercalation in layered TMCh. Solid state method Solid state method has been widely applied to synthesize the novel inorganic compounds, including the layered TMChs. The advantages of this high-temperature approach are high reacting rate and good crystallinity, which leads to the easy synthesis of single crystal.22,38,39 For the syntheses of TMCh, the solid state reaction has been usually carried out by mixing the raw materials (elementary substances or binary chalcogenide precursors) and then sealing them in quartz tubes before sintering at high temperature.10,12,34 Filling the tubes with nitrogen or argon gas can effectively prevent the oxidation of samples. In order to grow the single crystals, flux method could be used with the addition of flux (metal chlorides such as KCl-NaCl or KCl-AlCl3 mixture) to reduce the melting temperature.40-42 For the alkali metal or Tl intercalated systems, the alkali metal and Tl can also be used as the self-flux component during crystal growing (e.g. KCo2Se2 or TlNi2Se2).39,43 However, thermostability of the product is a pre-condition for this high-temperature synthetic method and metastable phases can hardly be prepared with the solid state method.17 Solvothermal synthesis

Figure 3. Two common crystal structures of binary TMChs with TM = Fe, Co, Ni, Cu and Ch = S, Se and Te. (Green sphere: TM; Yellow sphere: Ch; Orange line: unit cell).

Solvothermal synthesis is also an effective way to introduce the spacing layers in TMCh. Liquid ammonia, organic amine and even pyridine could be used as solvents.20,44-46 Especially alkali metals, which are very common guest layers, can be dissolved in liquid ammonia and be efficiently intercalated



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into the TMCh layers. Solvothermal synthesis reduces the reaction temperature during the intercalation and leads to the possibility to obtain metastable phases. For example, Hatakeda et al. have synthesized Li intercalated FeSe using C2H8N2 as the solvent at 45 ℃.44 Ying et al. intercalated several metals in between the [FeSe] layers with liquid ammonia even at room temperature.20 These syntheses always proceed in the autoclave with the high pressure inside during the reaction. Moreover, besides playing the role of the solvents, organic molecules can also be intercalated into the lattice, leading to the various precious structures,46,47 which will be discussed in the section “structure and related properties”. Hydrothermal synthesis Hydrothermal synthesis has also been used in synthetizing intercalated TMCh when the product is stable in aqueous solution. The operation of hydrothermal reactions is similar with the solvothermal synthesis, expect the deionized water is used as reaction medium to fill the autoclave instead of organic solvents. Some TMChs with specific guest layer can be synthesized by using TM in their elemental forms as the starting materials.48-50 Lu et al. have synthesized the layered Li1-xFexOHFeSe with hydrothermal reactions at 160 ℃ by using Fe powder and selenourea as the raw materials.49 Lai et al. have obtained the highly crystalline FeS and (NH3)Fe0.25Fe2S2 with the similar route from Fe powder mixed with the Na2S and (NH4)2S aqueous solution, respectively.17,48 In addition, hydrothermal synthesis can also be carried out in combination with ion exchange, where the precursor is needed whose spacing layer is less stable in water than that of the target product. Dong et al. designed to synthesize Li1-xFexOHFeSe by using KxFe2-ySe2 crystals as the precursor, where the high-quality single crystal sample has been successfully obtained.51 Electrochemical method Electrochemical synthesis is well known to be effective in intercalating numbers of metals into layered parents and has been commonly used in TMD system. However, the utilization of this method on TMCh is still limited. Shen et al. successfully intercalated K and Na into tetragonal FeSe by this electrochemical procedure, by processing of the metal K as anode and the binary FeSe as cathode.52 KNO3 solution together with ethylene carbonate and dimethyl carbonate was used as the electrolyte under a constant current of 10 µA. Alekseeva and co-workers also reported the Li intercalation using similar method in Ref.52 and the superconductivity (Tc = 44K) has been realized.53 For the samples from the electrochemical method, the crystallinity is generally worse than that from the other routes.

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 Superconductivity in Fe-based family Superconductivity induced by intercalation is not a new topic. It has been reported that the superconductivity of TMDs54-56 and C6057 could be enhanced with pyridine and ammonia intercalated respectively. More recently in year of 2003, layered NaxCoO2·yH2O was discovered, in which the [CoO2] acting as the conducting layer and the intercalated H2O plays a key role on its superconductivity.58,59 However, the above systems show low-temperature superconductivity with Tc limiting below 39 K. In contrast, it has been confirmed that the Fe-based family belongs to high-temperature superconductors, for which Tc could be higher than the McMillan limitation (39 K) and no doubt that the intrinsic superconducting mechanism of these iron materials differs from the electric-phonon coupling described by Bardeen-Cooper-Schrieffer (BSC) theory.60-62 After the discovery of Tc = 8.5 K in pristine FeSe in year 2008,16 scientists have soon recognized that the chemical intercalation could be an effective way to enhance the superconductivity in this system. Guo and co-workers firstly reported the K intercalation in binary FeSes by solid state reaction, forming a layered iron selenide with the nominal composition K0.8Fe2Se2.10 This compound showed Tc at 31 K and the structure was reported to crystallize into the ThCr2Si2-type structure (122-type) with a body-centered tetragonal cell and I4/mmm space group (Figure. 4a).10 Following this work, several isostructural systems with intercalated Rb, Cs and Tl have been reported.21,22,63,64 The [FeSe] interlayer distance is enlarged from 5.52 Å in binary FeSe to 6.99-7.64 Å due to the intercalated metals as the spacing layers (the cation with larger radius leads to a larger distance). The lattice parameter a is also expanded from 3.77 to 3.88-3.96 Å in these ternary chalcogenides, suggesting the weaker Fe-Fe interaction in the ab plane. It should be mentioned that besides the enlarged lattice, the structural configuration of [FeSe] layer in FeSe-based 122-type compounds is also different from that of FeSe host: after the metal intercalation, the space group switched from P4/nmm (primitive tetragonal lattice) to I4/mmm (body-centred tetragonal lattice) with the n-glide along the ab plane disappeared. Considering the potential high Tc, the intercalated FeSe rapidly gained high attention from the research community.65-70 And the single crystals have been obtained by self-flux method (Figure. 4b).38,64,71,72 By using angle-resolved photoemission spectroscopy (ARPES) on high-quality crystal samples, it has been demonstrated that the large electron Fermi surfaces are observed around the zone corners while there is no hole Fermi surface near the zone centre (Figure. 4c), quite different from the superconducting Fe arsenide discovered before.73,74


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another √8×√10 superstructure in ab plane.84 Very recently, by separating the superconducting phase from crystal sample with the microsampling technique, Tanaka et al. showed the accurate composition of AxFe2Se2 in K-Fe-Se system to be K0.43Fe2.02Se2, with the typical 122-type structure and stoichiometric [FeSe] layer.85 Anyhow, the appropriate synthetic method to obtain the pure superconducting phase in this intercalated FeSe-based 122-type system was still urgently needed.

Figure 4. (a) Powder X-ray diffraction (PXRD) and Rietveld refinement of K0.8Fe2Se2. The inset shows the structure of K0.8Fe2Se2 with 122-type. Reproduced from ref. 10. Copyright 2010 American Physical Society. (b) Optical image of the K0.8Fe2-ySe2 single crystal from self-flux method. Reproduced from ref. 38. Copyright 2012 American Physical Society. (c) The Fermi surfaces of K0.8Fe2Se2 from ARPES. Reproduced from ref. 73. Copyright 2011 Macmilan Publishers Limited. Concerning the unusual properties in AxFe2Se2 (A = alkali metals or Tl), subsequent studies have demonstrated that a micron-scale phase separation actually occurred in these materials.75-78 It has been indicated that the main phase of AxFe2Se2 is actually an insulating phase (Figure. 5a).79 This insulating phase was later found to be the stoichiometry compound A0.8Fe1.6Se2 (so-called 245 phase), showing √5×√5 superstructure of Fe vacancy with the reduced symmetry (I4/m) and exhibiting the antiferromagnetic ordering with Neel temperature TN around 540 K (Figure. 5b, c).80,81 It seems that the 245 phase with 20% Fe vacancy is more stable than the superconducting phase, considering the valence balance (metal A: +1, Fe: +2 and Se: -2). The superconducting phase only existed as the minority phase in the crystal sample with 10-20% percent and this feature hindered the measurement on its real composition.38,76,82,83 Some works indicated that the chemical formula of superconducting phase was AxFe2Se2, with the value x varied from 0.3 to 0.8.82,83 Conversely, Ding et al. suggested the superconducting phase in this system to be K2Fe7Se8 with

Figure 5. (a) Phase separation in KxFe2-ySe2 crystal. The left inset shows the diffraction of the superstructure in main phase. The right inset shows the superconducting gap in minority phase. Reproduced from ref. 79. Copyright 2011 American Physical Society. (b) Crystal Structure of the 245 phase and (c) the view along c direction, showing √5×√5 superstructure. Reproduced from ref. 81. Copyright 2012 American Physical Society. Ammonothermal method, which can be considered as the solvothermal synthesis with liquid ammonia as solvent, was timely carried out to solve this problem. Since alkali metals can be dissolved in the ammonia solution, a series of superconductors can be obtained by intercalating metals in binary FeSe precursor using this ammonothermal method. Hence, this method has effectively avoided the formation of thermodynamically stable 245 phase in the final product, resulting in the pure superconducting phases. Scheidt et al. reported the Li/NH3 intercalated Fe: LixFe2Se2(NH3)y by



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ammonothermal method with Tc = 44 K.86 Ying et al. demonstrated that numbers of metals: Li, Na, Ba, Sr, Ca, Yb and Eu could be intercalated into the pristine FeSe via similar method. The formation of insulating 245 phase was successfully avoided from PXRD and Tc was enhanced to 30-46 K (Figure 6a and b).20 The crystal structure remains to be the body-centered tetragonal cell (I4/mmm) with lattice parameter of a = 3.755-3.831 Å and c = 15.99-20.54 Å , which is larger than that of the samples prepared from the solid state reaction. It is reasonable to speculate that ammonia molecules were also intercalated in FeSe host together with the metals, which further enlarged the interlayer distance. Meanwhile, these two works all suggested that the superconducting phase is stoichiometric in [FeSe] layers without any Fe vacancy.20,86

Figure 6. (a) X-ray diffraction patterns for FeSe intercalated by different metals, where the 245 phase is absent. (b) Magnetization of NaFe2Se2, showing a superconductivity with Tc = 46 K (the maximum value in Fe chalcogenides so far). The left inset shows the magnetic hysteresis. The right inset shows the electrical resistance from the cold-pressed sample. Reproduced from ref. 20. Copyright 2012 Nature Publishing group. The detailed crystal structure of ammonia intercalated Fe selenide was subsequently reported based on the neutron

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diffraction. Burrard-Lucas et al. have confirmed that the ammonia (NH3) together with amide molecules was co-intercalated with Li+ cations into the structure during this ammonothermal synthesis. It was suggested that the compound also adopted I4/mmm space group with Tc = 43 K.46 Interestingly, Li+ cations were determined to be situated between the Se2- anions in Ref.46, instead of the body center of the lattice, which is quite different from the structure given by Ying et al. (Figure. 7a).20,46 Correspondingly, the ND3 molecule was suggested to locate at the center when deuteroammonia was used for the sample synthesis.46 Burrard-Lucas et al. also indicated the existence of hydrogen bonds in the structure between H and Se (shown as the dash line in Figure 7a), suggesting that the ammonia may also play a role to stabilize the structure. Furthermore, Sedlmaier et al. reported the existence of an intermediate phase with Tc = 39 K during the intercalation, which contains a novel double layer of ammonia molecules and remains the primitive tetragonal lattice similar to FeSe host (P4/nmm) (Figure. 7b).47 Yusenko et al. found even more intermediate phases appeared during the intercalation of Ba2+ cations before the formation of final product Ba0.28(NH3)1.92Fe2Se2 (I4/mmm) (Figure. 7c).87 It has to be concluded that the products gained from ammonothermal intercalation are typically very complex. Several primitive tetragonal lattices exist in metastable phases, and all of them rapidly turn into the stable body-centre tetragonal lattice during the reaction. Anyway, ammonothermal method provides the possibility to carefully study the composition of superconducting phase in this family. Ying et al. reported that there are at least two superconducting phases in the K-intercalated FeSe systems: K0.3(NH3)0.47Fe2Se2 corresponds to the Tc = 44 K phase and K0.6(NH3)0.37Fe2Se2 to Tc = 30 K phase, with the parameter c = 15.56(1) and 14.84(1) Å , respectively.88 More importantly, their work reveals that the superconducting phases in this system only appear at the certain doping levels, resulting in a platform-like appearance (Figure. 8a). An unidentified phase with Tc = 36 K has also been observed in their work, which is likely to be the intermediate phase with primitive tetragonal lattice reported by Sedlmaier et al.47 It can be concluded that the optimal amount of doping in AxFe2Se2 is equal to or even smaller than 0.15 electrons per Fe, since the Tc was driven down with x in Kx(NH3)yFe2Se2 larger than 0.15. However, the sample with less K content than 0.15 showed the residual of binary FeSe. Moreover, Ying et al. suggested that the amount of intercalated K has a higher influence on Tc than that of ammonia between the layers, since Tc was close to the raw value after the NH3 was extracted out of the structure.88 Guo et al., intercalated Na with ammonia in the FeSe1-xSx solid solution and show that the intercalation of NH3 still affected the superconductivity, where the NH3-free phase showed lower Tc than the samples


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Figure 7. (a) The structure of Li0.6(ND2)0.2(ND3)0.8Fe2Se2 at 298 K. Reprinted from ref. 46. Copyright 2013 Macmillan Publishers Limited. (b) Intercalation of lithium and ammonia into FeSe, where the intermediate phase (in the middle) shows a primitive tetragonal lattice; Reproduced from ref. 47. Copyright 2014 American Chemical Society. (c) The various structures during Ba-NH3 intercalation in binary FeSe. Reproduced from ref. 87. Copyright 2015 Royal Society of Chemistry. containing ammonia molecules.89 Similar phase diagram was also observed in the intercalated FeSe1-xTex system.90,91 In order to understand the peculiar phase relationship in intercalated FeSe system, Liu et al. carefully investigated the KxFe2-ySe2 system by first-principles calculation.92 In their work, two factors were suggested to govern the phase formation: the amount of negative charge from alkali metal K into FeSe, and the Coulomb attraction between K+ and [FeSe]δ-. As a function of the intercalated K content x in KxFe2-ySe2 system, phase evolution with x was summarized as follows: only at 0.25 ≤ x ≤ 0.6, Fe is favoured to fully occupy, forming the superconducting phase; at x > 0.6, FeSe layers tend to create Fe vacancy. While at x < 0.25, the structure collapses due to the dynamic instability. A schematic phase diagram is constructed in Figure. 8b. Based on this work, we can suggest that Tc might be higher than 46 K with less charge inserted in [FeSe] layer (the area with x < 0.25 in KxFe2-ySe2), if the structural instability could be solved.

Other amines and organic solvents were also used for chemical intercalation of pristine FeSe. Krzton-Maziopa et al. reported the intercalation of FeSe with Li in anhydrous pyridine, resulting in a new compound Lix(C5H5N)yFe2-zSe2.45 It is interesting to find that the space group switches from I4/mmm to P4/mmm in this Li-pyridine intercalated FeSe after annealing (215˚C, 50 hours), which has not been observed in ammonia intercalated samples. Noji’s group showed that ethanediamine (C2H8N2) could also act as a solvent for intercalation.44,93-95 It was reported that Tc was raised up to 45 K with Li and C2H8N2 intercalation, basically the same with that of the ammonia intercalated samples.20,44 As expected, the C2H8N2 intercalated FeSe was suggested to crystallize into the typical 122-type structure (Figure. 9a).95 However, although the interlayer distance of two adjacent [FeSe] was effectively enlarged to 8.02-10.37 Å , the obtained Tc is still not beyond the limit (46 K) reported before. Hosono et al. further increased the [FeSe] distance to 16.23



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Figure 9. (a) Schematic structure of the ethylenediamine intercalated FeSe with space group I4/mmm. Reproduced from ref. 95. Copyright 2015 American Physical Society. (b) The relationship between Tc and interlayer distance in intercalated FeSe system with different diamines (DA) but the same Na/Sr content. Reproduced from ref. 97. Copyright 2015 American Chemical Society.

Figure 8. (a) Dependence of Tc in Kx(NH3)yFe2Se2 on different amounts of K by ammonothermal method. Higher-Tc regime exists for K0.3(NH3)yFe2Se2, lower-Tc for samples with stoichiometry K0.6(NH3)yFe2Se2, and middle regime for an unknown phase. Reproduced from ref. 88. Copyright 2013 American Chemical Society. (b) Schematic phase diagram of KxFe2−ySe2 from first-principles calculation. Reproduced from ref. 92. Copyright 2016 WILEY. Å by intercalating hexamethylenediamine (C6H16N2).96 But Tc was lowered to 38 K with this extreme expansion on layer distance. All these facts suggested that the interlayer distance is not the critical factor for determining the superconductivity in FeSe-based materials. The same conclusion was also given by Hayashi et al., who intercalated FeSe with various diamines (DA), and found that Tc was basically unchanged irrespective of rather different interlayer distance from 8.7 to 11.4 Å.97 Instead, the charge doping is suggested to be conclusive, since Tc was basically the same in Na or Sr intercalated systems respectively, with the fixed Na/Sr content but different interlayer distance (Figure. 9b). Meanwhile, Srx(DA)y-FeSe shows lower Tc compared with Nax(DA)y-FeSe, suggesting the overdoping in Sr intercalated samples.97 Very recently, Jin and co-workers successfully synthesized Na-ethylenediamine co-intercalated FeSe.98 Based on the carefully neutron and X-ray diffraction, they indicated that there are several different lattices even including the orthorhombic cell with the various occupation sites of

C2H8N2 molecules in the products. These complex structures remind us the scenario in ammonia intercalated samples. More importantly, Jin et al., reported FeSe intercalated only by C2H8N2 molecules, where the samples are paramagnetic without any charge doping from the spacing layer but the superconductivity can be retrieved by inducing Na.98 Since this phase showed no magnetic ordering in low temperature range, it suggests that the essential parent of FeSe-based superconductor could be paramagnetic, which is quite different from the parent with spin density wave (SDW) in the FeAs-based high temperature superconductors.61 Although the chemical intercalation of alkali metals can be efficiently achieved by using solvothermal method with liquid ammonia or some other organic solvents, there is still a serious drawback on this series of samples: the stability in air. Since the alkali metals are very easy to react with the water in air, it causes great difficulties for storage and characterization. An important development was then reported by carrying out the hydrothermal synthesis. Lu et al. have synthesized another FeSe-based superconductor with intercalating the lithium hydroxide in between the [FeSe] layers.49 Under hydrothermal conditions, selenourea and Fe powder were used as the raw materials. The composition of this superconductor was determined to be (Li0.8Fe0.2)OHFeSe, with the ZrCuSiAs-type structure (1111-type), very rare in intercalated FeCh system (Figure 10a).50,99 After the hydroxide intercalated, the [FeSe] interlayer distance was increased to be around 9.3 Å . Interestingly, the spacing layer contains around 20% percent of substituted Fe on Li site, whose chemical valence was suggested to be +2.50 These Fe2+ cations play a role as an electron doper for the [FeSe] layer, very similar with the alkali metal cations A+ in 122-type structure.


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been confirmed again that charge doping from the spacing layer is indispensable for inducing the superconductivity in intercalated FeSe materials.

Figure 11. (a) Superconductivity is successfully turned on from the non-superconducting (Li1-xFexOH)Fe1-ySe by lithiation. (b) Schematic diagram of filling the Fe vacancies in [FeSe] layer using Fe from spacing layer. Reproduced from ref. 100. Copyright 2015 American Chemical Society.

Figure 10. (a) PXRD patterns and crystal structure of (Li0.8Fe0.2)OHFeSe. Reproduced from ref. 50. Copyright 2015 WILEY. (b) Magnetic susceptibility with the field of 10 Oe, showing Tc = 41 K. (c) Magnetic susceptibility with the field of 1 T, exhibiting an antiferromagnetic behaviour below 8.6 K. Reproduced from ref. 99. Copyright 2015 Macmillan Publishers Limited. Magnetic susceptibility measurement shows that Tc of (Li0.8Fe0.2)OHFeSe was around 41 K (Figure 10b),94 which is relatively high in intercalated FeSe family. More interestingly, the iron cation substitution on Li site in spacing layer induced the magnetic moment in this compound, which resulted in an unusual co-existence of superconductivity and magnetic ordering (Figure 10c).50,99 Sun et al. have prepared the (Li1-xFexOH)FeSe samples with a modified hydrothermal reaction where binary FeSe was used for synthesis instead of the iron powder and selenourea.100 The obtained sample showed the suppressed Tc but it restored to 41 K after “reductive lithiation” (treating the samples with the ammoniacal solution of Li metal) (Figure 11a). The authors indicated that the additional Li displaced the Fe ions from the intercalating layer to fill the Fe vacancies in [FeSe] layers (Figure 11b), and hence enhanced the superconductivity.100 Here, the spacing layer also acts as an “assistant” to adjust the defect of the conducting layer. Woodruff et al., then reported the synthesis and properties of non-superconducting LiOHFeSe, being considered as the parent of the superconducting (Li1-xFexOH)FeSe materials.101 No magnetic ordering was observed in this compound, in agreement with the suggested paramagnetic state of FeSe-based materials described in Ref. 98. Meanwhile, it has

With the relative high thermostability, (Li1-xFexOH)FeSe materials could be synthesized with higher temperature compared to the 122-type compounds with alkali metals. Small single crystals was firstly grown by hydrothermal route (Figure 12a),50 and Dong et al., also reported the successful synthesis of large single crystal via hydrothermal ion-exchange technique with K2Fe4Se5 as the precursor (Figure 12b).51 Based on the crystal samples, the difference of electromagnetic properties between the ab plane and c axis has been studied. It has been found that the ration of resistivity between two different directions ρc/ρab is large (around 2500 at 50 K). Unexpectedly, Dong reported another work on observing of an antiferromagnetic transition in (Li1-xFex)OHFeSe system with the sample having smaller parameter c, and hence the they suggested that the parent compound of this superconductors is antiferromagnet.102 This is quite contradictory to the scenario in Ref. 98 and 101. However, this antiferromagnetic “parent” was subsequently disputed by Zhou et al., since they demonstrated that this antiferromagnetic ordering comes from impurities such as the ferromagnetic Fe3O4.103

Figure 12. (a) The (Li0.8Fe0.2)OHFeSe single crystal sample with rectangular shape from hydrothermal synthesis. Reproduced from ref. 50. Copyright 2015 WILEY. (b) XRD pattern of (Li0.8Fe0.2)OHFeSe single crystal with the photograph of the sample right inset. Reproduced from ref.51. Copyright 2015 American Physical Society.



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Following the work on FeSe, the chemical intercalation was also carried out on another Fe chalcogenide: tetragonal FeS. Intercalation by solid state reaction, solvothermal and hydrothermal methods have been reported to synthesize the intercalated FeS with K, Fe-NH3 and LiOH as the spacing layers, respectively.48,104-107 However, superconductivity in parent FeS (Tc = 4.5K) has been totally degraded in these compounds after chemical intercalation, with the samples showing semiconducting behavior. Very recently, the superconducting signal has been finally obtained with Tc = 2.8 K in (Li1-xFex)OHFeSe sample from a two-step synthesis. Zhou and co-workers also retained the superconductivity in intercalated FeS with hydroxides as the spacing layer.108 They reported that the FeS with lithium hydroxide intercalated (I4/mmm)shows the varying Tc from 3 to 8 K, and the sodium hydroxide intercalated sample with a supposed body-centre unit cell shows Tc = 16 K. However, the intercalated FeS family still needs to be enriched, to investigate the effect of intercalation on its structure and physical properties. On the other hand, the intercalation in tetragonal FeTe has been reported in only one work up to now, where the Li/Na-ammonia was intercalated in binary FeTe and the antiferromagnetic ordering was maintained.90 The difficulty for synthesis in FeTe-based system is mainly attributed to the large interlayer distance and the interstitial Fe in the structure. The syntheses, structures and related properties of typical intercalated FeCh are summarized in Table 1. It can be concluded that the influence of spacing layer from the chemical intercalation in FeCh systems includes two aspects: enlarging the interlayer distance and providing the extra carriers for the [FeCh] conducting layers, although the superconductivity in these materials is mainly depended on the latter factor. In the sections below, we will discuss the intercalating effect in layered Ni, Co and Cu chalcogenides. The role of intercalated spacing layers would be fully understood, in spite of the variety of the compounds in these systems is not as rich as the Fe species. Structural evolution in Ni-based family Besides the Fe-based system, superconductivity has also been observed in Ni-based family. Neilson et al. reported that with potassium as the spacing layer in Ni selenide and sulphide: KNi2Se2 and KNi2S2 show superconductivity with Tc = 0.8 and 0.46 K, respectively.34,109 The superconductivity was enhanced to be 3.7 K with Tl intercalated instead of K, which is the highest Tc reported in Ni-based system so far.43 These Ni-based compounds also adopt the 122-type structure (I4/mmm) with the lattice parameter a = 3.79-3.91 Å and c = 12.77-13.42 Å . Both two parameters are smaller than the isostructural Fe-based 122-type chalcogenides, which could be resulted from the different size between Ni2+

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and Fe2+. Because of the high thermostability, single crystal (TlNi2Se2) has been easily grown by self-flux method (Figure 13a).43 Similar with FeSe-122 materials, the intercalated NiSe superconductors also show the multiband behavior.34,43 Meanwhile, the specific-heat measurement suggested the feature of a d-wave superconductors in this system, since the normal-state electronic coefficient γN exhibits as the H1/2 behavior (Figure 13b).43 However, the superconductivity in NiSe-based system has not attracted so much attention like FeSe-based materials since the low Tc. The reported spacing layer in the NiCh systems is still limited to the alkali metal and Tl up to now.

Figure 13. (a) Photograph of TlNi2Se2 single crystals before and after cleaving. (b) Specific heat of TlNi2Se2 crystal in low-temperature range. The left inset shows the C/T vs T2 below 1.7 K. The right inset shows the magnetic field H dependence on specific heat coefficient γ N. Reproduced from ref.43. Copyright 2013 American Physical Society. It should be mentioned that the binary NiCh commonly shows the NiAs-type structure with the face-sharing NiCh6 octahedrons, although the edge-sharing NiSe4 tetrahedrons was obtained in NiCh 122-type compounds. An interesting work has been done by Neilson et al., who demonstrated that with gradually de-intercalation of potassium, the [NiSe] conducting layer undergoes a structural transformation from anti-PbO-type KNi2Se2 to NiAs-type K1-yNi2-zSe2 (Figure 14).11 It is indicated that the transformation between edge-sharing NiSe4 tetrahedron and face-sharing NiSe6 octahedron can be reversibly achieved, by intercalating and de-intercalating the alkali metal between the [NiSe] layers. Meanwhile, the Ni vacancy was also found to emerge when the structure switched to NiAs-type, implying the effect of the intercalation on the defect chemistry. The mechanisms


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Table 1. Typical layered FeChs with different intercalations, and their syntheses, structures and properties. Functional layer

Chemical formula

Synthetic method

Space group

Lattice parameter (Å ) a

Main property

Ref.

[FeSe]

K0.8Fe1.6Se2

Solid state

I4/m

a = 8.74536(8) c = 14.10024(18)

AFM TN = 540 K

81

(K2Fe4Se5) Li0.9(NH3)0.5Fe2Se2

Ammonothermal

I4/mmm

a = 3.8273(6) c = 16.518(3)

SC Tc = 44 K

86

Li0.6(ND2)0.2(ND3)0.8Fe2Se2

Ammonothermal

I4/mmm

a = 3.8148(2) c = 16.4820(9)

SC Tc = 43 K

46

Li0.56N1.72D4.63Fe2Se2

Ammonothermal

P4/nmm

a = 3.79756(2) c = 10.30704(8)

SC Tc = 39 K

47

K0.3(NH3)0.47Fe2Se2

Ammonothermal

I4/mmm

a = 3.843(1) c = 15.56(1)

SC Tc = 44 K

88

K0.3Fe2Se2

Ammonothermal

I4/mmm

a = 3.87(1) c = 14.28(4)

SC Tc = 44 K

88

Lix(C5H5N)yFe2-zSe2 (annealed)

Solvothermal

P4/mmm

a = 8.00283 c = 23.09648

SC Tc = 40 K

45

Lix(C2H8N2)yFe2-zSe2

Solvothermal

I4/mmm

a = 3.458(6) c = 20.74(7)

SC Tc = 45 K

44

Lix(C6H12N2)yFe2-zSe2

Solvothermal

I4/mmm

a = 3.453(2) c = 32.450(9)

SC Tc = 38 K

96

(C2N2H8)Fe2Se2

Solvothermal

I4/m

a = 3.8565(2) c = 21.4257(6)

PM

98

SC & AFM Tc = 40 K; TN = 8.5 K

99

[FeS]

Li0.8Fe0.2OHFeSe

Hydrothermal

P4/nmm

a = 3.77871(4) c = 9.1604(1)

Li0.85Fe0.15OHFeS

Hydrothermal

P4/nmm

a = 3.6886(3) c = 8.915(1)

FM TC = 50 K

107

(NH3)Fe0.25Fe2S2

Hydrothermal

I4/mmm

a = 3.68795(12) c = 13.1134(8)

FM TC = 60 K

48

“SC”: Superconducting; “AFM”: Antiferromagnetic; “PM”: Paramagnetic; “FM”: Ferromagnetic; “TN”: the Neel temperature; “Tc”: the critical temperature of superconductivity; “TC”: the Curie temperature. a All the lattice parameters are taken from the patterns obtained at around the room temperature (298 K). of these changes are suggested to be the electron count from K content and the Ni-Ni bonding.43

Magnetic interaction in Co-based family Quite similar with the Ni-based system, the Co-based family also crystallizes into the 122-type structure based on samples from solid state reaction. Eight compounds with the chemical formula ACo2Ch2 (A = K, Rb, Cs and Tl; Ch = Se, S)

have been synthesized and all of them exhibit long-range magnetic ordering below the critical temperature.12 This is very different from the Fe- and Ni-based 122-type compounds which commonly exhibit no magnetic ordering when TM-site is fully occupied. The reason could be from the difference of 3d valence electrons from TM, where the Co2+ cation has an odd number (7) of 3d electrons but the Fe2+ and Ni2+ have even one (6 and 8). All the members in this family exhibit the magnetic moments ferromagnetically



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Figure 14. PXRD illustrating a phase transformation from edge-sharing NiSe4 tetrahedron and face-sharing NiSe6 octahedron with K de-intercalation. Reproduced from ref. 11. Copyright 2012 American Chemical Society. arranged in the [CoCh] layers. The spontaneous magnetic momonets are quite small (0.72, 0.58 and 0.52 µ B for KCo2Se2, RbCo2Se2 and CsCo2Se2, respectively), suggesting the itinerant ferromagnetism in these compounds.39 However, although the magnetic ordering along [CoCh] layers is the same in this family, TlCo2Se2 and CsCo2Se2 particularly show antiferromagnetic interaction between the layers, while the other six compounds still exhibit ferromagnetic interlayer interaction.12,110,111 This difference suggests that the spacing layers have the power to control the magnetic interaction of the adjacent [TMCh] conducting layers. To understand the origin of these two different magnetic interactions, Guo et al. investigated the KCo2Se2-xSx (0≤ x ≤2) solid solution, which has quite similar structural parameters with the antiferromagnetic TlCo2Se2-xSx but exhibits as the ferromagnetic metal for the whole x range.112 Electronic structures of these two systems were calculated and the results showed that the density of electronic states (DOS) near Fermi level has been clearly influenced by changing the spacing layer from K to Tl (Figure 15a and b). DOS is highly localized in the K-intercalated system compared to the Tl-intercalated one and this difference was suggested to come from the 6p orbitals of Tl. The distinction on DOS leads to the different interlayer interaction in these two systems based on the interaction coefficient calculated from the Ruderman-Kittel-Kasuya-Yosida (RKKY) model. The chemical origin was supposed to be the different bonding types between K-Ch and Tl-Ch: K-Ch bonds are almost purely ionic while the Tl-Ch bonds are partially covalent due to the large electronegativity difference between potassium and thallium.112 This work indicated that the operation on spacing layer could also determine the type of the magnetic coupling of functional layers by adjust the DOS.

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Figure 1.5 Total and partial DOS of (a) KCo2Se1.4S0.6 and (b) TlCo2Se1.75S0.25. The inset shows the different interlayer interactions between [CoCh] layers in the two compounds. Reproduced from ref. 112. Copyright 2016 Royal Society of Chemistry. As mentioned, the binary CoCh adopts the NiAs-type structure, same with the NiCh systems. However, the scenario after the K de-intercalated is very different from the NiSe-based system. Recently, Zhou et al., de-intercalated K from the KCo2Se2 and KCo2S2, and the obtained binary CoSe and CoS showed the anti-PbO-type structure (Figure 16).32 It suggested that the tetragonal CoSe and CoS as the intercalating parents can be synthesized although they are metastable phase with the poor thermal stability. Both these two binary cobalt chalcogenides show weak ferromagnets with Curie temperature TC close to 10 K, which is much lower than that of KCo2Se2 and KCo2S2.112 Hence, it can be concluded that the K intercalation could significantly enhance the ferromagnetism of [CoCh] layer. This enhancement could also be due to the electrons doping from the K intercalation, similar with the adjustment from the spacing layer in superconducting FeCh system we discussed above.

Figure 16. Schematic for synthesizing the metastable CoSe and CoS via K de-intercalation from 122 phase. Reproduced from ref. 32. Copyright 2016 American Chemical Society.


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Thermoelectric property in Cu-based family Different from the Fe-, Co- and Ni-based metallic system, the layered Cu-based chalcogenides are commonly bad metals or semiconductor with obvious band-gaps.13,15 The adjustment of band gap in layered Cu chalcogenides will be discussed below. From the view of crystal structure, the layered Cu chalcogenides also include CuCh4 edge-sharing tetrahedron after the chemical intercalation, where both 1111- and 122-type structures could be obtained.13,113 However, the layered Cu-based chalcogenides may show distinct features on phase formation from the other three systems, because the chemical valence of Cu in [TMCh] layer is +1 instead of +2, resulting in the strong Coulomb attraction between conducting layer [CuCh]- and spacing layer [LnO]+ or An+ (Ln = rare earth and An = alkaline-earth metals). The synthetic method used on layered Cu-based family is commonly the solid state reaction due to the relatively high thermostability. Among them, the Cu selenide with bismuth oxide as spacing layer has caused great attention in the material field since it was reported to be a potential thermoelectric material in year 2010.18 The crystal structure of BiOCuSe (Figure 17a) belongs to the 1111-type, similar with (Li1-xFexOH)FeSe superconductor mentioned before. In this layered compound, the [CuSe]- layer acts as the conducting layer while [BiO]+ can be considered as the spacing layer. Single crystal of BiOCuSe could be grown by flux method with Bi2O2Se (reacted from Bi2O3 and Bi2Se3) and Cu2Se as the starting materials and NaCl/KCl mixture as the flux (Figure 17b).41 Interestingly, it has been found that by increasing the reaction temperature from 690 to 730 ℃, the electrical property switched from semiconductor to metal, where the reason was suggested to be the Bi vacancy in the higher-temperature crystal sample.41 As we know that the performance of thermoelectric material is evaluated by the dimensionless figure of merit ZT (defined by ZT = (S2σ/κ)T, where S, σ, κ, T are Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively).4 It can be seen that the increased electrical conductivity is clearly associated with the enlarged ZT. Hence, doping on the carrier reservoir layer to increase carrier concentration in the structure would be an effective way to obtain large ZT. Since BiOCuSe is a p-type semiconductor, several works have been done to partially substitute Bi3+ with M2+ (M = Mg, Sr, Ca and Ba), which could provide excess hole-carriers.18,25,27,114,115 A remarkable increase of carrier concentration with Ba doping was observed according to the Hall measurement (Figure 17c).25 As expected, the ZT values were improved in different extent from 0.5 for pristine BiCuSeO to 0.67, 0.9, 0.76, and 1.1 for Mg, Ca, Sr, and Ba doped samples, respectively (Figure 17d).4 Undoubtedly, this enhancement is resulted from the

charge doping rather than the lattice adjustment, which is quite in line with the regulation on superconductivity in Fe-based chalcogenides. Besides adjusting the chemical composition of spacing layer [BiO], other routes can also enhance the ZT of 1111-type BiOCuSe, and the details are given in the review work by Zhao et al.4

Figure 17. (a) Crystal structure of tetragonal BiOCuSe with 1111-type; Reprinted from ref. 18. Copyright 2010 American Institute of Physics. (b) XRD pattern of BiOCuSe single crystal with photograph of the sample inset. Reprinted from ref.41. Copyright 2015 Royal Society of Chemistry. (c) Carrier mobility and carrier concentration as a function of Ba doping content in Bi1-xBaxOCuSe. Reprinted from ref. 25. Copyright 2012 Royal Society of Chemistry. (d) ZTmax for BiOCuSe with different metal (Mg, Ca, Sr, and Ba) doped into the structure. Reprinted from ref. 4. Copyright 2014 Royal Society of Chemistry. There is a special dopant that needs to be mentioned: Pb2+. It shows the same valence with the alkaline-earth metals, but the Pb doping on Bi-sites seems to have a more complex mechanism in promoting the thermoelectric property.116,117 Lan et al. found that besides the increased carrier concentration and carrier mobility as expected after Pb doping, the calculated DOS close to the valence-band maximum is also increased, which was provided by the 6s orbitals of Pb (Figure 18).118 With this multifunctional doping, the Pb dopant enhanced the electrical conductivity more effectively than other dopants. This phenomenon is quite similar with the influence of guest in Co-based magnetic compounds mentioned in the previous section, where the substitution on guest layer could noticeably adjust the DOS near the Fermi level. However, Pan et al., did not find the change of DOS in Pb doping samples from the Sommerfield coefficient.119 Further investigation needs



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to be done to confirm this effect of carrier reservoir layer via DOS near Femi level in Cu-based thermoelectric materials.

Figure 18. Orbital-projected density of states in Bi0.875M0.125 OCuSe [M = Pb, Mg, Sr, Ba] system. It is clear that only Pb 6s electrons significantly contribute to the states close to Fermi surface. Reprinted from ref. 118. Copyright 2013 WILEY. The enhancement of ZT from charge doping was also observed in 122-tpye Cu selenide: BaCu2Se2. Na and K have been used as the dopants on Ba-site to provide hole-carrier to enhance the thermoelectric property of BaCu2Se2.120,121 It is worth noting that the pristine BaCu2Se2 adopts an orthorhombic lattice (Pnma) at room temperature instead of the tetragonal one, which consists of the CuSe4 tetrahedra connected by sharing both vertices and edges.120 But the K doping on Ba-site switched the structure from orthorhombic to the 122-type with 30% K substitution.121 Optical properties in Cu-based family Before the study on CuSe-based thermoelectric materials, layered Cu chalcogenides have already received a lot of attention since Ueda et al. have discovered that an intercalated Cu sulfide: LaOCuS is a transparent p-type semiconductor in year 2000.14 The as-prepared LaOCuS thin films showed a large band gap of 3.1 eV but relatively high electrical conductivity (1.2×10-2 S·cm-1) with the positive Seebeck coefficients. Extensive investigations have been done on these potential transparent p-type semiconducting systems: LnOCuCh and BaFCuCh (Ln = rare-earth and Ch = S, Se and Te) with 1111-type structure.13,122 Meanwhile, more complex structures were also synthesized, such as the one with a perovskite oxide as the spacing layer which belongs to I4/mmm space group (2322-type).123-125 For the syntheses, conventional solid state reaction is usually carried out to prepare these Cu chalcogenides under anaerobic conditions, while Sheets et al. also reported the synthesis of BiOCuS by hydrothermal method.126 First principle calculations have suggested that in LnOCuCh, the maximum of valence band (VBM) consists of 3d orbital from Cu and p orbital from Ch, while the 4s

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orbital from Cu mainly contributes to the minimum of the conduction band (CBM).13 This is quite in line with the expectation, since the conducting [CuCh] layer should play the dominant role in determining the electricity properties. Fixing the chalcogenide ion, the band gap tends to decrease with decreasing size of the Ln ion, but the change is inapparent (sulphide from 2.9 to 3.1 eV; selenide from 2.58 to 2.8 eV and telluride from 2.2 to 2.31 eV).15 Only the substitution of Ln ions by trivalent Bi ions could significantly decrease the band gap (~1.1 eV for BiOCuS and ~0.8 eV for BiOCuSe), which is due to the reduction of the CBM by adding the Bi 6p orbitals.13 Hence, regardless of the same functional layer, one can expect the obvious change on band gap if the dopant in spacing layer shows a different electronic configuration from that of the original component, such as the Bi3+ substitution on Ln3+. Further decrease of the band gap has been observed in 2322-type Cu chalcogenides recently. Jin et al. have synthesized two new layered Cu selenides: Sr2CoO2Cu2Se2 and Sr2MnO2Cu2Se2, and found that the band gaps of these two compounds to be 0.068 eV (Sr2CoO2Cu2Se2) and 0.073 eV (Sr2MnO2Cu2Se2) from middle IR absorbance spectra.127 These band gaps are much narrower than that of the quaternary oxyselenides reported before. Although further electronic structure calculation is absent in this work, it can be speculated that this dramatic decrease of band gap is mainly attributed to the magnetic elements: Co and Mn, which could provide the unpaired 3d orbitals that may play the same role as 6p orbitals of Bi in 1111-type structure. This phenomenon was further confirmed by Zhang et al., who reported another band gap narrowing in Ba3Fe2O5Cu2S2, where the magnetic trivalent Fe was demonstrated to be responsible for the smaller band gap (1.03 eV) compared to LaOCuS (Fig. 19).128 A summary of the syntheses, structures and related properties of Ni-based, Co-based and Cu-based TMChs are shown in Table 2.

Figure 19. Schematic band structures of LaOCuS, Sr3Sc2O5Cu2S2 and Ba3Fe2O5Cu2S2 based on the first principle calculations. Reproduced from ref. 128. Copyright 2016 Chinese Physical Society and IOP Publishing Ltd.


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Table 2. Typical layered NiChs, CoChs and CuChs with different intercalations, and their syntheses, structures and related properties. Functional layer

Chemical formula

Synthetic method

Space group

Lattice parameter (Å ) a

Main property

Ref.

[NiSe]

KNi2Se2

Solid state

I4/mmm

a = 3.90979(2) c = 13.4145(2)

SC Tc = 0.8 K

34

TlNi2Se2

Solid state

I4/mmm

a = 3.870(1) c = 13.435(1)

SC Tc = 3.7 K

43

KCo2Se2

Solid state

I4/mmm

a = 3.8672(7) c = 13.6950(14)

FM TC = 80 K

112

TlCo2Se2

Solid state

I4/mmm

a = 3.8416(7) c = 13.594(1)

AFM TN = 86 K

12

CsCo2Se2

Solid state

I4/mmm

a = 3.824(2) c = 15.314(8)

AFM TN = 34 K

12

BiOCuSe

Solid state

P4/nmm

a = 3.9213(1) c = 8.9133(5)

Semi Eg = 0.8 eV ZTmax = 0.5

4

Bi0.925Sr0.075OCuSe

Solid state

P4/nmm

--

Metallic ZTmax = 0.76

18

Bi0.875Ba0.125OCuSe

Solid state

P4/nmm

--


25

BaCu2Se2

Solid state

Pnma

a = 9.575 b = 4.202 c = 10.755

Semi ZTmax = 0.28 Eg = 1.8 eV

120

Ba0.7K0.3Cu2Se2

Solid state

I4/mmm

--


121

LaOCuSe

Solid state

P4/nmm

a = 4.0670(1) c = 8.8006(8)

Semi Eg = 2.8 eV

13

Sr2CoO2Cu2Se2

Solid state

I4/mmm

a = 4.0488(1) c = 18.3571(2)

Semi Eg=0.068 eV

127

Sr2MnO2Cu1.5Se2

Solid state

I4/mmm

a = 4.0689(1) c = 17.8760(2)

Semi Eg =0.073 eV

127

BiOCuS

Solid state

P4/nmm

a = 3.8691(1) c = 8.5602(4)

Semi Eg = 1.1 eV

13

LaOCuS

Solid state

P4/nmm

a = 3.9938(2) c = 8.5215(4)

Semi Eg = 3.1 eV

13

Ba3Fe2O5Cu2S2

Solid state

I4/mmm

a = 3.9995(1) c = 27.6873(3)

Semi Eg = 1.03 eV

128

[CoSe]

[CuSe]

[CuS]

“SC”: Superconducting; “FM”: Ferromagnetic; “AFM”: Antiferromagnetic; “Semi”: Semiconducting. “Eg”: Band gap; “ZTmax”: The maximum figure of merit of the thermoelectricity. a All the lattice parameters are taken from the patterns obtained at around the room temperature (298 K).



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 The layered TMChs possess many superior properties, including the high-temperature superconductivity, the excellent thermoelectric property, the good transparent conductivity and the strong magnetic interaction. Based on the layered characteristic, chemical intercalation can be operated in these chalcogenides, which effectively tunes the original properties and significantly improves the related performance. The experimental routes to achieve the intercalation include solid phase sintering, solvothermal synthesis, hydrothermal synthesis and electrochemical method, which are highly dependent on the thermostability of the intercalated compounds and the properties of the intercalating guest. By integrating the results in all sections above, the effects of chemical intercalation on the functional [TMCh] layers can be divided into two categories:

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materials need to be effectively controlled. For example, the superconductivity in layered FeS is suppressed since the intercalation inevitably induces the Fe vacancies; Meanwhile, the interstitial Fe atoms in FeTe-based system are always unavoidable, which hinder the chemical intercalation. More novel structure and excellent performances can be expected if we could adjust the stoichiometry of both conducting and spacing layers under suitable synthesis condition during intercalation. Finally, the affecting mechanisms of spacing layer on the electromagnetic properties of functional layer need to be deeply understood. More importantly, we know that once it reaches to its full potential, TMChs will exert the crucial influences. It is absolutely no doubt that with the development of work on chemical intercalation in TMCh systems, more and more novel electromagnetic functional materials will be discovered in the future. 

Structural:  Enlarging the interlayer distance, and meanwhile changing the TM-Ch bond length and related angle in [TMCh] layer;  Adjusting the defect chemistry (e.g. the TM vacancies) in the [TMCh] layer;  Turning the structure of [TMCh] layer (e.g. from hexagonal to tetragonal lattice, or from primitive tetragonal lattice to body-centred tetragonal lattice). Electrical:  Increasing the carrier concentration of [TMCh] layer, which could significantly improve the electronic conductivity, and resulting in the enhancement on superconductivity, ferromagnetism and thermoelectric property.  Adjusting DOS near the Fermi surface, which could affect the magnetic interaction between the [TMCh] layers;  Narrowing the band gap in semiconducting system by providing the extra orbitals. After the intense research in the last decade, it is still a challenge for the scientists to purposefully design the functional transition metal chalcogenide materials and better understand the factors that intrinsically promote the property. Firstly, the diversity of spacing layers still needs to be enriched. Compared with layered pnictides, the number of intercalated chalcogenides has been rather limited up to now. Many new structures, such as the oxide intercalation in Fe-based family, or the 1111-type compounds in Co- and Ni-based family, have to be in focus of further synthesis. Especially for some novel metastable structures with potential superior properties, more reasonable synthesis methods need to be developed. Secondly, the stoichiometry and defect chemistry in these layered TMCh functional

Corresponding Author [email protected] (W.X. Yuan). Author Contributions This manuscript has been written through contributions of all authors.  The authors would like to thank Xiaolong Chen and Shifeng Jin in Institute of Physics, Chinese Academy of Sciences for the helpful discussion. The financial support by the National Natural Science Foundation of China (Nos. 51542010, 51172025 and 51402014) and the Fundamental Research Funds for the Central Universities (FRF-TP-16-006A3) is gratefully acknowledged. Notes

The authors declare no competing financial interest.  (1) Johnston, D. C. Adv. Phys., 2010, 59, 803-1061. (2) Stewart, G. R. Rev. Mod. Phys., 2011, 83, 1589-1652. (3) Vivanco, H. K.; Rodriguez, E. E. J. Solid State Chem., 2016, 242, 3-21. (4) Zhao, L. D.; He, J. Q.; Berardan, D.; Lin, Y. H.; Li, J. F.; Nan, C. W.; Dragoe, N. Energ. Environ. Sci., 2014, 7, 2900-2924. (5) Tan, G. J.; Zhao, L. D.; Kanatzidis, M. G. Chem. Rev., 2016, 116, 12123-12149. (6) Hosono, H. Thin Solid Films, 2007, 515, 6000-6014. (7) Ohta, H.; Nomura, K.; Hiramatsu, H.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H. Solid State Electron., 2003, 47, 2261-2267. (8) Tate, J.; Newhouse, P. F.; Kykyneshi, R.; Hersh, P. A.; Kinney, J.; McIntyre, D. H.; Keszler, D. A. Thin Solid Films, 2008, 516, 5795-5799.


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Chemical intercalations in layered transition metal chalcogenides: syntheses, structures and related properties Zhongnan Guo, Fan Sun and Wenxia Yuan*

The latest advances in chemical intercalation of layered transition metal chalcogenides (TMChs) have been reviewed. The role of these intercalated guests on syntheses, structures and relevant properties has been summarized and discussed, which may provide some new insights for design of the novel layered functional materials.


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Figure 1 177x151mm (96 x 96 DPI)



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Figure 10 179x199mm (96 x 96 DPI)


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Figure 13 101x112mm (96 x 96 DPI)



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Figure 14 140x107mm (96 x 96 DPI)


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Figure 15 134x121mm (96 x 96 DPI)



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Figure 17 203x163mm (96 x 96 DPI)



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Figure 18 248x148mm (96 x 96 DPI)


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Chemical Intercalations in Layered Transition Metal Chalcogenides

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