Isomorphous material

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. 

Solid solutions do not all result from the substitution of isomorphous materials in the lattice sites, other types of solid solutions are possible and are described in Vainshtein (1981). 

Although we will examine only a few materials here, each one has the same structure as other important materials we will list a few of these isomorphous materials. The examples chosen are also important because other crystal structures can be related to them with only a small distortion added to change the symmetry. 

We could have chosen to stack the cations and then fill the interstices with anions, but the anions are usually larger. Other isomorphous materials include InP, InSb, GaP (known collectively as the III-Vs), and cubic SiC. 

Following the success of PRF-25, Loeb and coworkers extended this series by utilizing the same axle with differently sized macrocycles, 22C6 and B24C6, to give isomorphous materials PRF-25(22C6) PRF-... 

However, in rare cases, crystallisation is not a satisfactory method of purification, especially if the impurity forms crystals that are isomorphous with the material being purified. In fact, the impurity content may even be greater in such recrystallised material. For this reason, it still remains necessary to test for impurities and to remove or adequately lower their concentrations by suitable chemical manipulation prior to recrystallisation. 

Commerical grades of EVOH typically have vinyl alcohol contents in the range 56-71%, but in contrast to the corresponding EVA materials these copolymers are crystalline. Furthermore, an increase in the vinyl alcohol content results in an increase in such properties as crystalline melting point, tensile strength and tensile modulus together with a decrease in oxygen permeability. This is a reflection of the fact that the ethylene and vinyl alcohol units in the chain are essentially isomorphous (see Sections 4.4 and 14.3.1). 

Copolymerisation. Except in those rare cases where monomer segments are isomorphous (see Figure 4.11), copolymerisation, as usual, leads to less crystalline and frequently amorphous materials. As might be expected, these materials are tough, leather-like, flexible and, when unfilled, reasonably transparent. 

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

At room temperature, trans-8 is formed, whereas at -78 °C mixtures of cis-8 and trans-8 are obtained. Pure cis-8, which rearranges to trans-8 at 20°C, is the product when Si2Cl6 is the starting material. Crystalline trans-8 is isomorphous with the respective Ge- and Sn- complexes (d Si-Cl 2.27, d Si-P 2.35 A). 

The oxidation by electrocrystallization of the d2 anionic complexes [Cp M (dmit)2] with M = Mo or W afforded in both cases the neutral radical species [Cp M(dmit)2] on the electrode [42, 43] as a crystalline material. The two compounds are isomorphous. As shown in Fig. 17, the absence of any counter ion... 

The problem of a priory assessment of stable structure formation is one of the main problems of chemical physics and material science. Its solution, in turn, is directly linked with the regularities of isomorphism, solubility and phase-formation in general. Surely, such problems can be cardinally solved only based on fundamental principles defining the system of physical and chemical criteria of a substance and quantum-mechanical concepts of physics and chemistry of a solid suit it. 

Kirkova E., Djarova M., Donkova B. Inclusion of isomorphous impurities during crystallization lfom solutions, Progress in Crystal Growth and Characterization of Materials, Volume 32, Issues 1-3, 1996, Pages 111-134. 

The surfaces of colloidal particles are often charged and these changes can arise from a number of sources. Chemically bound ionogenic species may be found on the surface of particles such as rubber or paint latex particles. Charged species may be physically adsorbed if ionic surface active materials, for example, have been added. A charged surface may occur on a crystal lattice. An example is the isomorphous substitution of lower valency cations such as aluminium for silicon in the lattice structure of clays. A further example is the adsorption of lattice ions... 

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). 

A new microporous solid material has been obtained made of Ti02 and Si02 (TS-1) which has a silicalite-1 structure modified by isomorphous substitution of Si(IV) with Ti(IV), Its synthesis takes place in the presence of tetraalkylammonium bases under carefully controlled conditions,... 

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