Solid-state chemistry crystal structures

This kaleidoscope of contemporary research interests reveals that another distinctive feature of supramolecular chemistry is its ability to unite areas with seemingly widely differing perceptions. In keeping with such a feature, structural chemists and crystallographers have had little difficulty in recognizing a molecular crystal as the ultimate example of a supermolecule. Consequently, supramolecular chemistry today encompasses the study of molecular crystals with all the applications and ramifications that such study implies in the fields of solid-state chemistry, crystal engineering and materials science. This then is the theme of this volume. Crystals constitute one end of the supramolecular continuum and may be viewed as hard supermolecules in contrast to the softer supramolecular aggregates which exist in solution. 

As it is currently practiced, supramolecular chemistry, with its emphasis on the interactions between molecules, underpins a very wide variety of chemistry and materials science impinging on molecular host-guest chemistry, solid-state host-guest chemistry, crystal engineering and the understanding and control of the molecular solid state (including crystal structure calculation), supramolecular... 

As for solid-state chemistry, that began in the form of crystal chemistry , the systematic study of the chemical (and physical) factors that govern the structures in which specific chemicals and chemical families crystallise, and many books on this topic were published from the 1930s onwards. The most important addition to straight crystal chemistry from the 1940s onwards was the examination of crystal... 

C. R. A. Catlow, Computational Techniques and Simulation of Crystal Structures, in Solid State Chemistry Techniques, A. K. Cheetham and P. Day, Eds., Oxford University Press, Oxford, United Kingdom, 1987. 

Introductory Remarks. Since the surfaces can be looked at as extending structures of the solid phase, the reader should familiarize himself with the structure chemistry of the solid phase. Since it is not the objective of this book to cover this subject, the reader should consult some of the books on crystal structure and solid state chemistry such as for example Cox (1987), Greenland and Hayes (1987), Newman (1987), Stucki, Goodman and Schwertmann (1985), Wells (1984), West (1984). 

Finally, reference must be made to the important and interesting chiral crystal structures. There are two classes of symmetry elements those, such as inversion centers and mirror planes, that can interrelate. enantiomeric chiral molecules, and those, like rotation axes, that cannot. If the space group of the crystal is one that has only symmetry elements of the latter type, then the structure is a chiral one and all the constituent molecules are homochiral the dissymmetry of the molecules may be difficult to detect but, in principle, it is present. In general, if one enantiomer of a chiral compound is crystallized, it must form a chiral structure. A racemic mixture may crystallize as a racemic compound, or it may spontaneously resolve to give separate crystals of each enantiomer. The chemical consequences of an achiral substance crystallizing in a homochiral molecular assembly are perhaps the most intriguing of the stereochemical aspects of solid-state chemistry. 

Almost all the crystalline materials discussed earlier involve only one molecular species. The ramifications for chemical reactions are thereby limited to intramolecular and homomolecular intermolecular reactions. Clearly the scope of solid-state chemistry would be vastly increased if it were possible to incorporate any desired foreign molecule into the crystal of a given substance. Unfortunately, the mutual solubilities of most pairs of molecules in the solid are severely limited (6), and few well-defined solid solutions or mixed crystals have been studied. Such one-phase systems are characterized by a variable composition and by a more or less random occupation of the crystallographic sites by the two components, and are generally based on the crystal structure of one component (or of both, if they are isomorphous). 

Even starting from achiral molecules it is in some systems possible to achieve crystallization in a chiral structure. Perhaps one of the most striking achievements in organic solid-state chemistry has been the trapping of the chirality of such a crystal as the chirality of the stable product of chemical reactions in the crystal. Such asymmetric synthesis has been reviewed (255), and a recent book (256) also provides a thorough discussion of chirality in crystals. The related and fascinating topic of the chemical consequences of the presence of a polar axis in some organic crystals has also been reviewed (257). 

Because a book of this size could not cover all topics in solid state chemistry, we have chosen to concentrate on structures and bonding in solids, and on the interplay between crystal and electronic stracture in determirring their properties. Examples of sohd state devices are used throughout the book to show how the choice of a partictrlar solid for a particular device is determined by the properties of that solid. 

The most important aspect of electron microscopy in solid state chemistry lies in its ability to elucidate problems that are beyond the capability of X-ray or neutron crystallography. High-resolution electron microscopic (HREM) images show local structures of crystals in remarkable detail in Fig. 2.10 we show the HREM image of Big, W03 obtained with the Cambridge University 500 kV microscope to show the improved resolution compared with the image obtained with a 200 kV microscope... 

The nature of the surface promoter species has been debated for many years. As MoS2 crystals exist as layered structures, two models evolved. One proposed that the promoter was intercalated deep within the bulk of the MoS2 layers (51) and another proposed that bulk intercalation was not thermodynamically stable and a surface-intercalated structure was more likely (52) (see Fig. 16d). Both proposals related the promotion to crystal surface reconstruction and solid-state chemistry and are valid only for multilayered structures. 

Fig. 4.4 Crystal structures of two more 1 2 compounds oxygen is the larger circle in both (a) unit cell of rutile, Ti02l tetragonal, space group Pd1fmmr, (b) unit cell of 0-crKtobaiite, SiO, [From Ladd. M. F C. Structure and Bonding in Solid State Chemistry, Wiley ...

Fig. 4.5 Crystal structures ofjwo forms of ciilcium carbonate (a) unit cell of calcitc. rhombohedral. space group R3c (b) unit cel of aragonite, orthorhombic, space group Pcmn Circles in decreasing order of size are oxygen, calcium, and carbon. [From Ladd. M. F. C Structure and Bonduiy in Solid State Chemistry Wiley New York, 1979. Reproduced with permission.]...

By necessity, the treatment of solid state kinetics has to be selective in view of the myriad processes which can occur in the solid state. This multitude is mainly due to three facts 1) correlation lengths in crystals are often much larger than in fluids and may comprise the whole crystal, 2) a structure element is characterized by three parameters instead of only by two in a liquid (chemical species, electrical charge, type of crystallographic site), and 3) a crystal can be elastically stressed. The stress state is normally inhomogeneous. If the yield strength is exceeded, then plastic deformation and the formation of dislocations will change the structural state of a crystal. What we aim at in this book is a strict treatment of concepts and basic situations in a quantitative way, so far as it is possible. In contrast, the often extremely complex kinetic situations in solid state chemistry and materials science will be analyzed in a rather qualitative manner, but with clearcut thermodynamic and kinetic concepts. 

Structural modification of a molecule usually has profound and complex influence on its crystal s packing and mechanical properties. This type of behavior is so pervasive that one usually despairs of being able to apply the kind of linear free energy relationships that are so useful in solution chemistry to studies of solid-state mechanisms. When structural modification is sufficiently subtle, however, analogous techniques can be helpful. 

The range of inorganic chemistry includes both molecular compounds, which exist as discrete molecules, and crystals, whose structures are described by infinite lattices of regularly-ordered atoms and which are studied by crystallography and solid-state chemistry. Sec also Solid-State Chemistry. 

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