Disordered materials

The oxide Ba2In205 is another well-studied phase that adopts the brownmillerite structure. This material disorders above 930°C to a perovskite-type structure containing oxygen vacancies. Both the Sr-Fe and Ba-In oxides are of interest for electrochemical applications in fuel cells and similar devices (Section 6.10). 

Fig. 10 Crystal packing of tpcb bta (34 33) cocrystal material disordered water and DMSO molecules have been omitted from supramolecular cavity [57]...

The colloidal chemistry of very diluted clay suspensions is usually described by the double layer theory (3). In more concentrated suspensions or thick pastes, where the average distance between sheets is shorter, the Van der Waals forces have to be considered. The combination of all these effects influences the distribution of the individual sheets and, thus, the viscosity of the slurry. From the viewpoint of this review, these spatial distributions of the individual sheets affect the availability of the surface area with the important consequence that the surface area is no longer an extensive property of the material. Disordered... 

We mentioned above that in real materials, disorder of any kind localizes the excitations over a limited length on the chain, generating effective conjugation lengths so that, for instance, the optical properties are those of a finite, often quite small, molecule. It may well be that excitons are very important in such cases. This has been proposed, for instance, for substituted PPV and polythiophene [83]. This is not by itself a proof that were it possible to prepare the corresponding perfect chain, the excitons would still be important there, although this is precisely what is found in PDAs, because electron correlations have different intensities on different CPs. 

Figure 11.6. NMR spectra of ordered N13AI (sharp resonance) and the same material disordered by ball milling. The broad spectrum of the disordered material was determined by a point-by-point spin-echo determination. From Scherrer et al. (1998) by permission of the...

Nayak PK, Garcia-Behnonte G, Kahn A, Bisquert J, Cahen D (2012) Photovoltaic efficiency limits and material disorder. Energ Environ Sci 5 6022-6039... 

The ideal chain-structures employed in theoretical models bear little resemblance to real materials. Disorder has been modelled by either a distribution of conjugation lengths or electron-lattice interactions. Bond length and substitutional disorder has been shown to lead to the disappearance of bond alternation and closure of the band gap, though with a low density of states at the Fermi energy. A simple model has been developed for the influence of defects on the electrical conductivity of doped samples. Pinning of solitons by defects has been discussed, but it is clear that the role of disorder requires further study. ... 

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials. 

Nearly all experimental eoexistenee eurves, whether from liquid-gas equilibrium, liquid mixtures, order-disorder in alloys, or in ferromagnetie materials, are far from parabolie, and more nearly eubie, even far below the eritieal temperature. This was known for fluid systems, at least to some experimentalists, more than one hundred years ago. Versehaflfelt (1900), from a eareflil analysis of data (pressure-volume and densities) on isopentane, eoneluded that the best fit was with p = 0.34 and 8 = 4.26, far from the elassieal values. Van Laar apparently rejeeted this eonelusion, believing that, at least very elose to the eritieal temperature, the eoexistenee eurve must beeome parabolie. Even earlier, van der Waals, who had derived a elassieal theory of eapillarity with a surfaee-tension exponent of 3/2, found (1893)... 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

Unwanted stmctures in the film plane—often found within LB films fonned from simple rodlike molecules or from molecules polymerized after deposition—can be problematic, since many possible applications of such films require a unifonn stmcture within the plane. On the other hand, however, the production of a system in which the stmcture within the plane is so disordered that there exist no stmctural features large enough to cause problems would also render applications possible. In tliree-dimensional materials, for example, both inorganic glasses and many polymers are capable of transmitting light without any appreciable scattering for substantial distances. 

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

Two point defects may aggregate to give a defect pair (such as when the two vacanc that constitute a Schottky defect come from neighbouring sites). Ousters of defects ( also form. These defect clusters may ultimately give rise to a new periodic structure oi an extended defect such as a dislocation. Increasing disorder may alternatively give j to a random, amorphous solid. As the properties of a material may be dramatically alte by the presence of defects it is obviously of great interest to be able to imderstand th relationships and ultimately predict them. However, we will restrict our discussion small concentrations of defects. 

Prostaglandins arise from unsaturated C20 carboxylic acids such as arachidonic acid (see Table 26 1) Mammals cannot biosynthesize arachidonic acid directly They obtain Imoleic acid (Table 26 1) from vegetable oils m their diet and extend the car bon chain of Imoleic acid from 18 to 20 carbons while introducing two more double bonds Lmoleic acid is said to be an essential fatty acid, forming part of the dietary requirement of mammals Animals fed on diets that are deficient m Imoleic acid grow poorly and suffer a number of other disorders some of which are reversed on feed mg them vegetable oils rich m Imoleic acid and other polyunsaturated fatty acids One function of these substances is to provide the raw materials for prostaglandin biosynthesis... 

The EXAFS technique is used primarily for investigations of disordered materials and amorphous solids. Figure 8.35(b) shows how interference occurs between the wave associated with a photoelectron generated on atom A and the waves scattered by nearest-neighbour atoms B in a crystalline material. 

The tetrahedrally bonded materials, such as Si and Ge, possess only positional disorder however, materials of this type exhibit high density of defect states (DOS). It is only with the addition of elements such as hydrogen and/or a halogen, typically fluorine, that the DOS is reduced to a point such that electronic device appHcations emerge. These materials contain up to - 10 atomic % hydrogen, commonly called hydrogenated amorphous siHcon (i -Si H). 

Global AMI.5 sun illumination of intensity 100 mW/cm ). The DOS (or defect) is found to be low with a dangling bond (DB) density, as measured by electron spin resonance (esr) of - 10 cm . The inherent disorder possessed by these materials manifests itself as band tails which emanate from the conduction and valence bands and are characterized by exponential tails with an energy of 25 and 45 meV, respectively the broader tail from the valence band provides for dispersive transport (shallow defect controlled) for holes with alow drift mobiUty of 10 cm /(s-V), whereas electrons exhibit nondispersive transport behavior with a higher mobiUty of - 1 cm /(s-V). Hence the material exhibits poor minority (hole) carrier transport with a diffusion length <0.5 //m, which puts a design limitation on electronic devices such as solar cells. 

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