Crystal stoichiometry

As in molecular chemistry, an alternative path to compensate for electron deficiency is the formation of multiple bonds, through 7r-interactions, as in unsaturated and aromatic molecular systems. Our work in Houston focuses on probing the efficacy of the ZintI concept in rationaUzing stoichiometries, crystal structures and chemical bonding of complex electron-poof ZintI phases that exhibit novel i-systems. Their chemical bonding is reflected by their unusual crystal structures related to unsaturated hydrocarbons [53]. 

The factors which influence the rate of dissolution of iron oxides are the properties of the overall system (e. g. temperature, UV light), the composition of the solution phase (e.g. pH, redox potential, concentration of acids, reductants and complexing agents) and the properties of the oxide (e. g. specific surface area, stoichiometry, crystal chemistry, crystal habit and presence of defects or guest ions). Models which take all of these factors into account are not available. In general, only the specific surface area, the composition of the solution and in some cases the tendency of ions in solution to form surface complexes are considered. 

Acridine and phenothiazine cocrystallized to give two kinds of hydrogen-bonded CT crystals. Both crystals showed some photoreactivity and appear to have given many photoproducts (Scheme 18) [44]. Although this crystalline complex is complicated in terms of stoichiometry, crystal structure, and photoreaction, a transient study by femtosecond diffuse reflectance spectroscopy was carried out, as had been done for durene-pyromellitic dianhydride cocrystal [45]. For the yellow cocrystal, a transient absorption spectrum with maxima around 600 and 520 nm was obtained, which decayed biexponentially with lifetimes of 2 and 50 ps. The two absorption maxima were ascribed to the acridine anion radical and the phenothiazine cation radical, respectively. 

LTS materials of technological interest generally crystallize in one of two structure types A 15 or B1 (Strukturbericht designation). Many LTS phases with the AjB stoichiometry crystallize with the A15 or [Cr Si] structure which is shown in Figure 2-5. 

Up to now several hundred CTCs and RISs have been synthesized with a wide range of stoichiometries, crystal structures and physical properties, and some common features have been identified [331] ... 

An additional problem is encountered when the isolated solid is non-stoichiometric. For example, precipitating Mn + as Mn(OH)2, followed by heating to produce the oxide, frequently produces a solid with a stoichiometry of MnO ) where x varies between 1 and 2. In this case the nonstoichiometric product results from the formation of a mixture of several oxides that differ in the oxidation state of manganese. Other nonstoichiometric compounds form as a result of lattice defects in the crystal structure. ... 

The requirements of thin-film ferroelectrics are stoichiometry, phase formation, crystallization, and microstmctural development for the various device appHcations. As of this writing multimagnetron sputtering (MMS) (56), multiion beam-reactive sputter (MIBERS) deposition (57), uv-excimer laser ablation (58), and electron cyclotron resonance (ECR) plasma-assisted growth (59) are the latest ferroelectric thin-film growth processes to satisfy the requirements. 

The crystal stmeture and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, < 0.59. 

Table 3 is arranged by crystal class (14). The crystal class of a given pigment is determined almost solely by the ratio of the ionic sizes of the cation and the anion and thek respective valences. Hence, for any given stoichiometry and ionic size ratio, only one or two stmctures will be possible. In some classes (spkiel, zkcon), a wide range of colors is possible within the confines of that class. Pigments within a given class usually have excellent chemical and... 

It is no exaggeration to claim that it was the extensive worldwide body of research on semiconductors from the late 1930s onwards that converted physicists to the recognition that scrupulous control of purity, stoichiometry and crystal perfection, together with characterisation methods that could check on these features, are a precondition of understanding the nature of semiconductors and thus also a precondition of exploiting them successfully - indeed, not only semiconductors but, by extension, many kinds of materials. 

The crystal chemistry of metal nitrides has been reviewed and there have recently been some intriguing developments in our understanding of the stoichiometries and structures of ternary and quaternary metal nitrides. 

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