Mixed crystals composition

Proportions of the different hexabromochlororhenates resulting from the Re(7,n) Re reaction as a function of the mixed crystals composition... 

Cadmium/Mercury Sulfides n CdS HgS. Pigment Orange 23 (77201). A series of cadmium/mercury sulfides of mixed crystal composition. Colors vary according to CdS/HgS ratio from deep orange to maroon. Known for high heat stability, solvent insolubility, good chemical resistance, and excellent lightfastness. CdS/HgS ratio, 89.1/10.9. Density 4.8-5.2 g/cm (40-43 Ib/gal) O.A., 33.2 particle size, 0.1-1.0 pm. Syn Mercadium ,... 

Mixed crystal composition and dopant concentration can be held constant or changed in almost any desired manner during growth. 

The relations apply also to the case of a liquid mixture of two substances which is solidifying to a homogeneous solid which contains the two substances in proportions depending on the composition of the melt—a so-called solid solution or mixed crystal ( 138). 

The potassium/caesium phase diagram is an example of a system involving the formation of mixed crystals with a temperature minimum (Fig. 4.4). The right and left halves of the diagram are of the same type as the diagram for antimony/bismuth. The minimum corresponds to a special point for which the compositions of the solid and the liquid are the same. Other systems can have the special point at a temperature maximum. 

Limited formation of mixed crystals occurs when the two components have different structures, as for example in the case of indium and cadmium. Mixed crystals containing much indium and little cadmium have the structure of indium, while those containing little indium and much cadmium have the cadmium structure. At intermediate compositions a gap is observed, i.e. there are no homogeneous mixed crystals, but instead a mixture of crystals rich in indium and crystals rich in cadmium is formed. This situation can even... 

Two metals that are chemically related and that have atoms of nearly the same size form disordered alloys with each other. Silver and gold, both crystallizing with cubic closest-packing, have atoms of nearly equal size (radii 144.4 and 144.2 pm). They form solid solutions (mixed crystals) of arbitrary composition in which the silver and the gold atoms randomly occupy the positions of the sphere packing. Related metals, especially from the same group of the periodic table, generally form solid solutions which have any composition if their atomic radii do not differ by more than approximately 15% for example Mo +W, K + Rb, K + Cs, but not Na + Cs. If the elements are less similar, there may be a limited miscibility as in the case of, for example, Zn in Cu (amount-of-substance fraction of Zn maximally 38.4%) and Cu in Zn (maximally 2.3% Cu) copper and zinc additionally form intermetallic compounds (cf. Section 15.4). 

Even when complete miscibility is possible in the solid state, ordered structures will be favored at suitable compositions if the atoms have different sizes. For example copper atoms are smaller than gold atoms (radii 127.8 and 144.2 pm) copper and gold form mixed crystals of any composition, but ordered alloys are formed with the compositions AuCu and AuCu3 (Fig. 15.1). The degree of order is temperature dependent with increasing temperatures the order decreases continuously. Therefore, there is no phase transition with a well-defined transition temperature. This can be seen in the temperature dependence of the specific heat (Fig. 15.2). Because of the form of the curve, this kind of order-disorder transformation is also called a A type transformation it is observed in many solid-state transformations. 

The topical homochirality problem is presently being investigated in several research laboratories across the world. One new object of study is systems with eutectic mixtures. The addition of chiral dicarboxylic acids that co-crystallise with chiral amino acids to aqueous mixtures of d- and L-amino acids allows tuning of the eutectic composition of the amino acids in several cases, these systems yield new eutectic compositions of 98% ee or higher. Thus, solid mixed crystals with a ratio... 

The isolation of crystalline products having mixed polymorphic compositions (often referred to as concomitant polymorphism) remains a topic of interest, even though the phase rule predicts that a system at equilibrium consisting two components (solvent + solute) and three phases (solution + Form I + Form II) is uni variant. Hence, for crystallizations performed at a fixed pressure (typically atmospheric) the system becomes nonvariant and genuine equilibrium can exist at only one temperature. Therefore, concomitant products must be obtained under nonequilibrium conditions. Flexibility in molecular conformation was attributed to the concomitant polymorphs of a spirobicyclic dione [34] and of 3-acetylcoumarin [35],... 

In order to achieve efficient build-up to heavy depths when dyeing cellulose acetate at 80 °C it is customary, particularly for navy blues, to use a mixture of two or more components of similar hue. If these behave independently, each will give its saturation solubility in the fibre. In practice, certain mixtures of dyes with closely related structures are 20-50% less soluble in cellulose acetate than predicted from the sum of their individual solubilities [87]. Dyes of this kind form mixed crystals in which the components are able to replace one another in the crystal lattice. The melting point depends on composition, varying gradually between those of the components, and the mixed crystals exhibit lower solubility than the sum of solubilities of the component dyes [88]. Dyes of dissimilar molecular shape do not form mixed crystals, the melting point curve of the mixture shows a eutectic point and they behave additively in mixtures with respect to solubility in water and in the fibre. 

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). 

The experimental crystallizer was a 350-mL jacketed glass unit that was provided with sufficient agitation to keep the contents well mixed. The compositions of the charges to the crystallizer were adjusted by adding the amino acids in predetermined ratios concentrations of the amino acid impurities were maintained in ranges comparable to those found in the recovery and purification of L-isoleucine from industrial fermentation reaction masses. The experiments were divided according to the mode of crystallization ... 

This methodology has been applied for determining the relative composition of alloys [225], amalgams [226], and mixed crystals [227], among others [74-78], based on peak current measurements. The essential requisite is that both electroactive components behave independently—i.e., that the components of a mechanical mixture do not influence each other with respect to their thermodynamic activities in electrochemical reactions [77]. In Fig. 4.4, theoretical calibration plots for the absolute peak current, when the amount of mixture is constant for each measurement (left) and for the percentage peak current (right), are shown [77, 228]. 

The redox chemistries of di-, tri and tetra-nuclear Mo—S cyanide complexes have been discussed in relation to their electronic structures.135 Passage of oxygen into an aqueous solution of [Mo2S2(CN)s]6 leads to the formation of a dark violet mixed-crystal compound of the composition K4+J,[Mo2(S02)(S2)(CN)8] [Mo2(S02)(S2)(CN)8]1 -4H20 (x = 0.3). In the crystal the two anions, whilst structurally similar, are located at crystallographically independent positions each involves both molybdenum atoms surrounded by an approximately... 

Each crystal may be made up of both salts but in no molecular ratio. In this case, the composition will vary with the concentration of the two salts in the original solution, and repeated recrystallization will, in general, change the composition in a regular variation. Such crystals are called mixed crystals and are regarded as a solution of one solid in another (Exercise 3). 

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