Bulk solid, impurities

Contaminant precipitation involves accumulation of a substance to form a new bulk solid phase. Sposito (1984) noted that both adsorption and precipitation imply a loss of material from the aqueous phase, but adsorption is inherently two-dimensional (occurring on the solid phase surface) while precipitation is inherently three-dimensional (occurring within pores and along solid phase boundaries). The chemical bonds that develop due to formation of the solid phase in both cases can be very similar. Moreover, mixtures of precipitates can result in heterogeneous solids with one component restricted to a thin outer layer, because of poor diffusion. Precipitate formation takes place when solubility limits are reached and occurs on a microscale between and within aggregates that constitute the subsurface solid phase. In the presence of lamellar charged particles with impurities, precipitation of cationic pollutants, for example, might occur even at concentrations below saturation (with respect to the theoretical solubility coefficient of the solvent). 

Very broadly speaking, two situations have to be considered in explaining devices such as those we have mentioned. In the first, which is relevant to the ruby laser and to phosphors for fluorescent lights, the light is emitted by an impurity ion in a host lattice. We are concerned here with what is essentially an atomic spectrum modified by the lattice. In the second case, which applies to LEDs and the gallium arsenide laser, the optical properties of the delocalised electrons in the bulk solid are important. 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the application of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the application of an electric field is possible. Electromigration, the migration of minority elements (ie, impurities, defects) through the bulk solid under the influence of current flow, is another process by which an atom maybe moved between the surface and the tip of an STM. 

In this chapter, the question of small amounts of polymorphic and solvatomorphic impurities in a bulk solid will be addressed. The most useful methods for performing such work will be outlined, and illustrated with appropriate examples. 

Each crystalline substance has a unique structure. Groups of compounds classified as isomorphous have similarities of lattice symmetry, but dimensions, and hence interionic forces, are different. Moreover, a particular substance can adopt alternative structures under changed conditions of temperature, pressure, crystallization conditions, presence of impurities, etc. Ordered packing, with symmetrical intracrystalline forces, appears to confer enhanced stability within the bulk solid so that decomposition processes usually occur at surfaces within a restricted reaction zone. Interfaces can be regarded variously as complex imperfections, zones of destabilizing strain, or (product) sites of catalytic activity. 

Precipitation should not be confused with crystallization (see p. 65). A precipitate is produced instantaneously, its particles are amorphous (that is they have no particular shape or form) and they do not grow in size like crystals. Precipitated solids are often of a lesser purity than crystals since sometimes spongy aggregates of particles are formed which trap (occlude) liquid or solid impurities within the bulk. For this reason, vessels used for precipitation are usually provided with very efficient means of agitation to ensure that the particles of the precipitate are properly dispersed in the liquid. In most cases, only limited cooling is necessary, and sometimes it is not required at all. 

Of the three categories of liquid-solid equilibria considered in Section 3.3.7.S and briefly considered above, a solid solution in equilibrium with a molten mixture has special importance in the purification of semiconductor materials such as silicon. Here, the bulk solid phase, as well as the molten mixture, consists essentially of silicon the concentrations of impurities are at a very low level. Therefore the area of focus is very close to either end of the type of phase diagram in Figures 3.3.6A for a given impurity-silicon system. Usually the solution being extremely dilute, the distribution coefficient Kb for the impurity i between the solid phase and the melt (see Example 1.4.3),... 

Fig. 13.11. A schematic representation of the complex situation near a crystal-solution interface. Shows solute molecnles with variable conformation in the bulk solution, partial structuring of solute molecules near the interface, and solute, solvent and impurity particles adsorbed on the solid surface. The structure of the solid at the surface may be different from the bulk solid structure.

An X-ray structure is often represented in a diagram showing the positions of all the atoms in the molecule (e.g.. Fig. 5.7). These have a deceptively persuasive appearance so we have to be aware of the potential pitfalls. Is the crystal representative of the bulk Minor impurities can crystallize while the major species do not. As a check, an IR spectrum of the specific crystal used for the structure can be compared with the bulk sample. The more difficult question is whether the structure in the solid state is really the same as the structure of the same material in solution, to which the solution reactivity and NMR data correspond. Some organometallics exist as one isomer in... 

Segregation of impurities to surfaces may be driven by a desire to lower surface energy or because the impurity is much larger than the site into which it would need to incorporate in the bulk solid or both. 

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... 

Most metals in commercial use contain quite large quantities of impurity (e.g. as alloying elements, or in contaminated scrap). Solid-state transformations in impure metals are usually limited by the diffusion of these impurities through the bulk of the material. 

Unlike high-resolution NMR spectra of bulk solutions where NMR linewidths well below 1 Hz can be obtained routinely, NMR spectra of liquids permeating porous solids in most cases will not exhibit such a high spectral resolution. First of all, the interaction of liquid phase molecules with pore walls of the catalyst and rapid diffusion-driven intrapore transport will lead to a pronounced homogeneous broadening of the observed NMR lines. Smaller pore sizes and the presence of paramagnetic impurities in the solid material usually aggravate the situation and thus should be avoided. Another reason why NMR spectra of liquids in porous... 

Movement through the body of a solid is called volume, lattice, or bulk diffusion. In a gas or liquid, bulk diffusion is usually the same in all directions and the material is described as isotropic. This is also true in amorphous or glassy solids and in cubic crystals. In all other crystals, the rate of bulk diffusion depends upon the direction taken and is anisotropic. Bulk diffusion through a perfect single crystal is dominated by point defects, with both impurity and intrinsic defect populations playing a part. 

In most ordinary solids, bulk diffusion is dominated by the impurity content, the number of impurity defects present. Any variation in D0 from one sample of a material to another is accounted for by the variation of the impurity content. However, the impurity concentration does not affect the activation energy of migration, Ea, so that Arrhenius plots for such crystals will consist of a series of parallel lines (Fig. 5.21a). The value of the preexponential factor D0 increases as the impurity content increases, in accord with Eq. (5.13). 

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