Mixed crystal Substitutional solid

Achiral butadienes 143 and 144 formed chiral mixed crystals (substitutional solid solutions) of space group P2 l lx by cooling the melts of both components or on crystallization from ethanol solution. A single large-sized mixed crystal 143144 was pulverized and irradiated resulting in [2 + 2] photocycloaddition, thereby giving the optically active heterodimers 145 as well as the achiral homodi-... 

Isomorphism. TiC is isomorphous with TiN and TiO. Thus oxygen and nitrogen as impurities, or as deliberate addition, can substitute for carbon to form binary and ternary solid solutions over a wide range of homogeneity. These solutions may be considered as Ti(C,N,0) mixed crystals. TiC forms solid solutions with the other monocarbides of Group IV and V. It is the host lattice for WC.li" ... 

For the calculation of solid-solution mineral behavior, two conceptual models may be used the end-member model (arbitrary mixing of two or more phases) and the site-mixing model (substituting elements can replace certain elements only at certain sites within the crystal structure). 

A necessary and sufficient condition for the formation of substitutional solid solutions of organic molecules is similarity of shape and size of the component molecules, For the formation of a continuous series of solid solutions the crystal structures of the pure components must be isomorphous Due to the rather irregular shape of organic molecules the principle of close packing leads to structures of low symmetry so that the latter requirement is not often fulfilled. Several diacetylenes which were found to form mixed crystals are given in Table 5. A large number of... 

The dimensionally stable anode in this system is composed of an electrically conductive substrate of titanium, having a coating of a defect solid solution containing mixed crystals of precious metal oxides. These substitutional solid solutions are both electrically conductive, electrocatalytic, and dimensionally stable. Within the aforementioned solid-solution host structures the valve metals include titanium, tantalum, niobium, and... 

If crystals of different species are isostructural and have the same type of bonding, they also will have very similar unit-cell dimensions and will macroscopically appear almost identical. This is known as isomorphism. Examples of isomorphic materials include ammonium and potassium sulfate and KH2PO4 and NH4H2PO4. In each of these materials, the potassium and ammonium ions can easily substitute for each other in the lattice since they are of almost the same size. This illustrates one of the properties of isomorphous materials, that is they tend to form solid solutions, or mixed crystals. Crystallization from a solution of two isomorphous materials, therefore, can result in a solid with varying composition of each species with unit-cell dimensions intermediate between the two components. The purification of isomorphous substances can, therefore, be difficult. 

VN, VC, and VO have isotypical structures and form solid solutions where nitrogen or oxygen can substitute for carbon over a wide range of composition. These solutions may be considered as V(C,N,0) mixed crystals. VN forms solid solutions with TiN and NbN, and with TiC, NbC, and TaC (see Fig. 11.6 and Fig. 4.8 of Ch. 4). 

Solid solutions of the impurity and the target compound can be formed via two different mechanisms that are regarded as limiting cases. Either the impurity molecules occupy sites in-between host molecules, forming so-called interstitial mixed crystals, or the impurity (guest) molecules replace the molecules of the target compound (host) forming so-called substitutional mixed crystals. 

If the introduced ions have the same charge as the ions in the solvent, the effect of the dissolution will be very slight. For example, mixed crystals of sodium chloride and potassiiun chloride can be considered as solutions of Na ions in KCl (or of ions in NaCl). The resulting solution has all the characteristics of a perfect solution. Indeed, we know that the difference from perfection in a solution is, primarily, due to the exchange energy Wab- In the case of the substitution of ions by Na ions, this energy is extremely low. This results in large solubility and even in the chosen case, complete miscibility of the two solids. 

It is also possible for solids to form solutions where they mix together in a manner similar to liquids. Consider a solid originally of pure a to which species b is added. A sohd solution forms if the crystal structure stays the same upon addition ofb. There are two ways in which solid solutions form. In a substitutional solid solution, species b occupies the lattice sites where species a once sat. As long as the crystal can accommodate b without altering its basic structure, a solid solution will occur. On the other hand, an interstitial solid solution forms when species b sits in interstitial spaces in between the lattice sites where a sits. These spaces are not part of the crystal structure. In this case, the b will be only sparingly soluble in a. 

Significant non-idealities can occur in the solids with two kinds of mobile cations (or anions). The so-called mixed alkali effect refers to the partly extremely strong depression of the alkali ion conductivity in crystals or glasses if substituted by another alkali ion. This is explained by the individually preferred environments and their interactions.221"228... 

The reason why mixes with AR > 1.7 do not yield any CjjA, on independent crystallization is that the solid phases are not pure CjA, QAF and CjS. For AR = 2.71, the quaternary liquid in equilibrium with C,S, C S and CjA at 1400X contains 55.7% CaO, 27.1% AljOj, 10.0% FcjOj and 7.2% SiOj (S8). This composition can be closely matched by a mixture of aluminate (63%), ferrite (30%) and belite (7%) with the normal compositions given in Table 1.2, the bulk composition of this mixture being 54.4% CaO, 26.4% AI2O3, 9.7% Fe Oj, 5.6% SiO and 1.8% MgO, with <1% each of TiOj, Mn20j, NajO and KjO. Independent crystallization can thus yield a mixture of the three phases. The liquid composition cannot be matched by a mixture of pure CjA, C AF and CjS, which is relatively too high in CaO, so that if no ionic substitutions occurred, some C,2A7 would also be formed. A strict comparison would be with the actual composition of the clinker liquid, which is modified by minor components, but lack of adequate data precludes this. 

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