Misfit layered compounds

The interplay of variously capped (and oriented) trigonal coordination prisms with coordination octahedra is essential also for the complex lanthanide sulfides and the layer misfit compounds. Through the entire range of structures and compositions the relative plasticity (ability for minor adjustments) of large coordination polyhedra created around large or lone-electron-pair cations is of primary importance in the interlayer and interblock adjustments. 

Figure 16. Interlayer match of pseudotetragonai (Q) and orthohexagonal (H) submesh in (a) layer misfit compound (LaS)./ 2(pi S2 (i.e., "-LaCrS/) and (b) in the lanthanide sulfide ErgLa.f( 27 match analogous to that in cannizzarfte.

Of special interest to intercalation studies are complex non-stoichiometric systems, such as the so-called misfit layer chalcogenides that were first synthesized in the 1960s [45]. Typically, the misfit compounds present an asymmetry along the c-axis, evidencing an inclination of the unit cell in this direction, due to lattice mismatch in, say, the -axis therefore these solids prefer to fold and/or adopt a hollow-fiber structure, crystallizing in either platelet form or as hollow whiskers. One of the first studied examples of such a misfit compound has been the kaolinite mineral. 

In the book by Hyde and Andersson (1989), the Nowotny phases are presented as a special case of a group of ID, columnar misfit structures which also include compounds such as Bam(Fe2S4) and other complex sulphides. Layer misfit structures, such as those of some oxide-fluorides, arseno-sulphides, etc., are also presented and classified with reference to a concept of structure commensurability based on the recognition that (along one or more axes) the ratios between the different repeat units of various interpenetrating substructures can (or cannot) be represented as ratios between integer numbers. 

Known examples of layered misfit structures are listed in Tables 1 to 5. It is interesting that they have been recognised, and independently described as such, in at least four instances Cowley and Ibers Allmann, Lohse and Hellner Makovicky " and Hyde et al.. (The general absence of rigid, two-dimensional layers in organic structures suggests that layered misfit structures will be improbable for organic compounds. We know of no examples.)... 

Layer misfit was first recognised in the structures of two metal chloride-graphite intercalation compounds. Intercalates of graphite with many other metal halides, including fluorides and bromides, have since been reported, but they seem to be of the CC type and are not discussed here. 

Table 7. Selected compounds transitional between the layer-misfit structures and the CC structures. (Intralayer vectors are underlined.)... 

Wiegers GA (1996) Misfit Layer Compounds Stmctures and Physical Properties. Prog SoUd State Chem 24 1-139... 

Figure 4.31 Idealized structures of misfit-layer compounds (a) (LaS)i.20CrS2 (b) (SnS)i.i7 NbS2 (c) (Ca2CoO3)0.62(CoO2) and (d) (Bi0.87SrO2)2(CoO2)i.s2. The shaded polyhedra represent sheets of MX6 octahedra (a, c, d) or trigonal prisms (b).

G. A. Wiegers, Misfit Layer Compounds Structures and Physical Properties, Prog. Solid State Chem., 24, 1-139 (1996). 

Makovicky Hyde (1981) have reviewed incommensurate misfit structures in graphite intercalation compounds, brucite-type compounds, sulphides and related layered systems. A simple two-dimensional incommensurate system is provided by graphite with adsorbed rare gas monolayers. At low densities and high temperatures. 

Misfit layered compounds of the type, (RX) (TX2) , where R is a rare earth, Pb or Bi, T = Ti, V, Nb or Ta and X = S or Se, are typical incommensurate phases that have been investigated in detail (Rouxel Meerschaut, 1994). The incommensuration here arises from a structural misfit between the rocksalt-like RX and layered TXj dichalcogenide units which are stacked alternately. The interplay of structure and electron-transfer from the RX to TXj units gives rise to novel electronic properties. For... 

Cario, L., Lefond, A., Palvadeau, A., Deudon, P., and Meerschault, C. (1999). Evidence of a mixed valence state for europium in the misfit layer compound [(EuS)i.5]i. i5NbS2 by means of a superspace structural determination, Mossbauer spectroscopy and magnetic measurements. /. Solid State Chem. 147, 58-67. 

After an introductory discussion of such misfit structures, various terms that have previously been applied are reviewed, and degrees of incommensurability are used as the basis for a systematic nomenclature. The known structures of specific examples are then discussed graphite intercalates minerals with brudte-like layers as one component (koenenite, valleriite, tochilinites) silicates heavy metal sulphides (cylindrite, incaite, franckeite, cannizzarite, lengenbachite, lanthanum-chromium sulphide) anion-excess, fluorite-related yttrium oxy-fluorides and related compounds. 

Compound Misfit Component layers Compo- Sym- Subcell Compo-... 

Compound Misfit modulation type Component layers Component sublattices Sym- metry Subcell Component intralayer lattices axial ratio bic Sym- metry Unit cells subcells in terms of Ratio of unit cell areas Ref. 

Figure 52 (a) The MQ and TQ2 parts of a misfit layer compound, (b) Different stacking motifs between the so-called quadratic and hexagonal slabs... 

The misfit layer compounds are typified by materials with a formula MS +fiTS2)m, in which T is a transition metal atom, Ti, V, Cr, Nb, or Ta, and M is a large atom such as Sn, Pb, Bi, with stereochemically active electron lone pairs, or a lanthanide. The structures are built from S-T-S layers, in which the metal T takes trigonal-prismatic coordination. These layers are interleaved with layers of the halite structure, usually two or three atom layers in thickness, with composition MSx. This leads to a chemical formula of [MSx]n(71S 2)m, where n varies from approximately 1.08-1.24, and m takes values of 1-3, depending upon the nature of T and M. A typical example is the compound [(Lni/3Sr2/3S)i.5]i.i5 NbS2. In all of the misfit layer compounds, the lattice parameter of the interpolated halite layers fit one lattice parameter of the rS2 layer but not the other, so that in this direction, the... 

Exfoliation is also facilitated by mechanical methods such as the application of high-intensity ultrasound. High-intensity ultrasound is known to increase the rate of intercalation reactions. However, in recent studies on the exfoliation of the misfit layer compounds PbNb2S5 and SmNb2S5, some degree of dispersion was obtained by ultrasonic treatment in specific solvents such as ethanol and isopropanol. ° The solvents were not intercalated, but acted to stabilize the dispersions. The dispersions contained a substantial fraction of particles with fewer than 10 layers, but only a small fraction was exfoliated. Along with the ion exchange reaction under relatively severe conditions compared with other layered... 

The misfit layer compounds are typified by materials with a formula in which T is a transition metal... 

More recently, a new family of composite-layered chalcogenides with (MX)i f (TX2)2 stoichiometry (M = Sn, Pb, Bi T = Ti, Nb, Ta, X = S, Se) has been prepared and characterized. A common feature of these compounds is a misfit layer structure consisting of a double layer of MX and TX2 sandwiches along the c-axis. From the point of view of intercalation reactions, these hosts have the potential to separate the two adjacent TX2 slabs in order to accommodate new guest molecules. 

Makovicky, E. Hyde, B.G. (1992). Incommensurate, two-layer structures with complex crystal chemistry minerals and related synthetics . In "Incommensurate Misfit Sandwiched Layered Compounds", ed. Meerschaut, A., Trans.Tech Publ. Ltd., in press. 

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