Misfit compounds

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. 

The issue of stability of these phases was not answered in the early studies. Only after a large number of misfit compounds was synthesized and studied did it become apparent that the key factor for the stability of these phases is a mechanism of charge transfer between the Q and H sublattices. The Q-part plays the role of electron donor and the H-part that of acceptor, i i s xhis hypothesis was of course... 

O. Gourdon, V. Petricek and M. Evain, A new structure type in the hexagonal perovskite family structure determination of the modulated misfit compound SiyigTiSj, Acta Crystallogr., B56, 409-418 (2000). 

I thank Dr. Alain Meerschaut for providing material regarding the misfit compounds. I also thank the National Science Foundation for generous support. 

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.

One of the most important questions to optimise the figure of merit of these materials is to understand the evolution of S vs T, correlate the value of S at 300 K with doping, and compare this value with the Heikes formula. To investigate the doping effect, several misfit compounds have been synthesised and characterised. 

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). 

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. 

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. 

Alternatively, response additivity (RA) for independently acting chemicals as a mathematical null model for testing observed responses (associated with the pharmacological concept of independent joint action) and with an assumed correlation of sensitivities of 0 also often fits the data well. Again, misfits occur (e.g., when the test mixture consists of compounds with the same MOA at concentrations below the individual compound s no-effect concentrations), and when they occur, they are often in the tails of the response curves. 

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