Simultaneous crystallization

In addition to the methods already outlined for platinum(II) complexes, simultaneous crystallization can be used to prepare mixed complex halides (Figure 3.105). 

Eutectic point (Tc) A single point on a temperature concentration phase (or state) diagram for a binary solution (e.g., water and sugars or salts) where the solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at Te results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred (based on Fennema, 1996). 

Figure 2.11. The Au-Si diagram is an example of a simple eutectic system with complete mutual solubility in the liquid state and no (or negligible) solubility in the solid state at a temperature of 363°C the liquid having the composition of 18.6 at.% Si solidifies with the simultaneous crystallization of the practically pure gold and silicon mechanically mixed. In the Cr-U system a slightly more complex situation due to the solid-state transformations of uranium is shown.

An interesting feature of the conduction mechanism in these materials and the LISICONS is that it is, at least partially, an interstitialcy mechanism. Both structure types contain examples of face-sharing tetrahedral sites. Fig. 2.13. Such sites are much too close together for both to be occupied simultaneously. Crystal structure refinements show that often, on average, one site of each pair contains a Li ion but the occupancy appears to be random. This means that, during conduction, one site of each pair may contain a Li ion but this is ejected when an incoming... 

Particular Cases (II) Simultaneous Crystallization of Several Phases... 

Figure 1.16 shows the relative complex viscosity as a function of conversion at 120 and 150°C. The 150°C curve shows a dramatic rise due to the simultaneous crystallization during polymerization. In addition, notice in Figure 1.13 that the complex viscosity-time curves of 150 and 160°C polymerization tend to converge. Both these effects—the nonlinear rise in relative complex viscosity in Figure 1.16 and the convergence of 150 and 160°C curves in... 

Figure 1.13—occur because of simultaneous crystallization and polymerization at 150°C. This temperature is near the maximum crystallization rate temperature ( 145°C) of nylon 6 homopolymer [66]. The presence of solid crystallites increases the complex viscosity of the polymerizing system because of a filler effect. 

By a similar usage in petrology, a euleeltc is a discrete mixture of two or more minerals, in definite proportions, which have simultaneously crystallized front the mutual solution of their constituents The eutectic point is the lowest temperature at any given pressure at which the above physical-chemical process may lake pluce, The eutectic ratio is the ratio hy weight of two minerals dial originate bv the above process. 

Thin-Film Stage. The mechanism of thin-film formation is characterized by three simultaneous crystal building processes nucleation (formation), growth, and coalescence of three-dimensional crystallites (TDCs). 

The topochemical differences in the physical environment of dissolved and crystallized active centers explain easily the difference in reactivity ratios between the two phases. The gain in free energy arising from immediate crystallization of growing chain ends enhances the incorporation of trioxane into the crystalline copolymer. Simultaneous crystallization is considered an important driving force in copolymerization as well as in the homopolymerization of trioxane. On the other hand, dioxolane units do not fit the crystal lattice of polyoxymethylene and reduce the crystallinity of the polymer. This impedes the incorporation of dioxolane units into the crystalline copolymer. 

Intergrowth simultaneous crystallization of two minerals Intumescence visible loss of water with heating Isotypic having the same structure... 

In view of the many experiments carried out in achieving the crystallization of proteins and subsequently refining those conditions to maximize crystal size and quality for X-ray structure determination, it is not surprising that examples of concomitant crystallization are found among proteins. We cite two here. Fu et al. (1994) reported the simultaneous crystallization of three polymorphs of an m-class glutathione... 

The behavior of aqueous solutions during freezing is much more complicated. At low cooling rates (1 to 5 K/min), crystallization in the water-salt system is thermodynamically controlled and can be satisfactorily described by the rules of eutectic crystallization, as most of these systems have eutectic-type phase diagrams (Figure 4.6). If the solution concentration is below the eutectic point (which is the usual case for most of the solutions used), crystallization of ice will be observed at T < T, followed by a systematic increase in concentration of the remaining solution until T, and simultaneous crystallization of ice and salt at T < Tg (Figure 4.6, line 1). For a solution with a eutectic concentration, only the last process will be observed (Une 2). 

M = Li, Na, K, Cs). Previous literature data (discussed below) suggest that there is scope for a range of structure types to be produced in this system, based both on possible variability in the relative proportions of octahedra and tetrahedra, and a possible diversity in the manners in which they can be interlinked. Broad scope in the possible product structures, however, implies that the simultaneous crystallization of more than one phase might be common. Initial scoping experiments are therefore likely to be complicated by problems associated with product multiphasic behavior. 

The eutectically solidified ceramics represent a large group of ceramic systems with microstructures that can be tailored during cooling. Since the eutectic solidification is based on the simultaneous crystallization of two solids from one liquid, many complex arrangements of the eutectic phases have been observed [93]. A schematic representation of a eutectic reaction is as follows ... 

Such a solution is said to be saturated. Saturation occurs at very low concentrations of dissolved species for slightly soluble substances and at high concentrations for very soluble substances. When imperfect crystals are placed in saturated solutions of their ions, surface defects on the crystals are slowly patched with no net increase in mass of the solid. Often, after some time has passed, we see fewer but larger crystals. These observations provide evidence of the dynamic nature of the solubility equilibrium. After equilibrium is established, no more solid dissolves without the simultaneous crystallization of an equal mass of dissolved ions. 

An intimate intergrowth of two different compounds or crystal structures as a consequence of their simultaneous crystallization from a melt. 

Increasing solubility because of increased concentration of impurities will result in a similar equilibrium change, although in some cases, the effect could be much greater. In extreme cases, when the residual solvent concentration is reduced to less than a critical value, the substrate could melt or solidify, depending on the melting point and the impurity effect. This condition is often used in laboratory preparations for convenience in changing solvents and is referred to as concentration to dryness. It is obviously not a scalable operation in a stirred vessel. Specialized tubular evaporators with close-clearance or scraped-surface rotors are available for these applications and have been successfully used by the authors for concentration but not for simultaneous crystallization. 

---