Interstitial mixed crystals

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. 

Mixed crystals of type II have been used in the form of thin films on electrodes as well as in the form of chemically synthesized powders immobilized on electrodes. Depending on the radii of the ions involved in the synthesis, solid solutions can also be formed as single phases. In the case of K CuCo[Fe(CN)(5] films, XRD results indicated that a single phase with a cubic face-centered symmetry was formed [31]. The situation is more complex in the case of K NiPd[Fe(CN)6] deposited as a thin film on electrodes [32]. Kulesza etal. have pointed out that there is a critical concentration of Pd + below which Pd + was taken as the countercation at interstitial position, while above that value a solid solution is formed in which both Ni " " and Pd + are nitrogen coordinated. 

Mixed crystals of type III, V, VI, and VII are not stable, because the interstitial countercations can be easily exchanged. 

The metallic carbides and nitrides are for the most part interstitial compounds and form mixed crystals with one another... 

An iron deficiency could be accommodated by a defect structure in two ways either iron vacancies, giving the formula Fe] /D, or alternatively, there could be an excess of oxygen in interstitial positions, with the formula FeOi+ f. A comparison of the theoretical and measured densities of the crystal distinguishes between the alternatives. The easiest method of measuring the density of a crystal is the flotation method. Liquids of differing densities which dissolve in each other, are mixed together until a mixture is found that will just suspend the crystal so that it neither floats nor sinks. The density of that liquid mixture must then be the same as that of the crystal, and it can be found by weighing an accurately measured volume. 

A variety of defect formation mechanisms (lattice disorder) are known. Classical cases include the - Schottky and -> Frenkel mechanisms. For the Schottky defects, an anion vacancy and a cation vacancy are formed in an ionic crystal due to replacing two atoms at the surface. The Frenkel defect involves one atom displaced from its lattice site into an interstitial position, which is normally empty. The Schottky and Frenkel defects are both stoichiometric, i.e., can be formed without a change in the crystal composition. The structural disorder, characteristic of -> superionics (fast -> ion conductors), relates to crystals where the stoichiometric number of mobile ions is significantly lower than the number of positions available for these ions. Examples of structurally disordered solids are -> f-alumina, -> NASICON, and d-phase of - bismuth oxide. The antistructural disorder, typical for - intermetallic and essentially covalent phases, appears due to mixing of atoms between their regular sites. In many cases important for practice, the defects are formed to compensate charge of dopant ions due to the crystal electroneutrality rule (doping-induced disorder) (see also -> electroneutrality condition). 

Prussian blue — iron(III) hexacyanoferrate(II) is the archetype of sparingly soluble mixed valence polymeric metal hexacyanometalates with the formula Me Me(N) [Me c (CN)6] with (i), (N), and (C) indicating the position in the crystal lattice, where (i) means interstitial sites, (N) means metal coordinated to the nitrogen of the cyanides, and (C) means metal ions coordinated to the carbon of the cyanides. It is one of the oldest synthetically produced coordination compounds and was widely used as pigment in paints because of the intensive blue color. The compound has been studied extensively by electrochemical and other methods. The importance of Prussian blue in electrochemistry is related to the fact that it has two redox-active metal centers and that it has an open structure that allows small cations to... 

Precipitates can develop in parenteral nutrition admixtures because of a number of factors such as the concentration, pH, and phosphate content of the amino acid solutions, the calcium and phosphorus additives, the order of mixing, or the mixing process. The consequences can be serious. In one cohort study of hospitalized patients who received peripheral parenteral nutrition, a subgroup developed unexplained chest pain, dyspnea, cardiopulmonary arrest, or new interstitial infiltrates on chest radiograph. A change in the amino acid source of a parenteral nutrition mixture was associated with respiratory adverse events that ranged from interstitial infiltrates to sudden death. The events apparently resulted from infusion of calcium phosphate precipitate in an opaque admixture, and the deposition of the crystals in the pulmonary microvasculature (147). 

Codeposition, which represents the concurrent processes of colloidal crystal template formation and simultaneous filling of the interstitial sites with the desired framework material, is usually achieved by the deposition of a mixture of the templating colloids with the matrix material precursor (for example, a sol-gel precursor or nanoparticles). For this purpose, a dispersion of large particles, which will constitute the template, is mixed with nanoparticles of the framework material, which have to be small enough to easily fit into the interstitial space without interfering with crystal formation. By this method porous silica [25,32,35] and titania [32] were fabricated. 

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