Explanations of cation ordering

The X-ray structure refinements and spectroscopic measurements described in 6.7 demonstrate that transition metal ions are ordered to varying degrees in the crystal structures of numerous ferromagnesian and aluminosilicate minerals. The enrichment of a transition metal ion relative to Mg2+ or Al3+ in a specific coordination site results from the interplay of several crystal chemical and bond energy factors, including the crystal field stabilization energy. 

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The difference in these patterns probably reflects that the hydrate entropies are related simply to the net positive charge on the cationic species (i.e., +2 for Pu022) while the hydrolysis reaction is the result of interaction of a water molecule with the metal atom itself — i.e., Pu in Pu022. If this is a valid explanation, the hydrolysis order indicates that the charge on Pu in Pu022 is actually between +3 and +4 and probably about +3.3. 

Arrange the cations Rb" ", Be2+, and Sr23 in order of increasing polarizing power. Give an explanation of your arrangement. 

It is found in this study that an adjustment of pH value of solution by acid (HF or HC1) to 10.5 is very important for the effective formation of uniform mesopores. However, the acid should be added into the mixture solution after the addition of surfactant otherwise, the formation of the ordered mesoporous structure would be affected. The explanation is that when acid is added to a mixture solution without surfactant, the pH value of system will reduce and subsequently influence the interaction between cationic surfactant and anionic silicate species in the mixture, leading to the poor polymerization of inorganic silicate species. In addition, when HF is used prior to the addition of surfactant, the formation of stable NajSiFg can deactivate the polymerization of silicate species, further terminating the growth of mesoporous framework. 

Natta et al. (167,188,287,298,312) have built a strong case in favor of a coordinated anionic mechanism in which an electropositive metal complexes and polarizes the monomer and a polymer anion adds to the positively polarized carbon of the monomer. One of the points which was used to support the anionic mechanism was that the order of reactivity for ethylene, propylene and butene is opposite to that of cationic catalysts. The lower reactivity of propylene and butene versus ethylene was attributed to the electron releasing alkyl groups (287), but steric hindrance is believed to be a better explanation. Support for the steric effect is indicated by the influence of bulk placed at some distance from the double bond (116). For example, reactivity decreases sharply in the order pentene-1 > 4-methylpentene-l > 4,4-dimethylpentene-l, although basicity of the double bonds must change only very slightly. 

Statistical thermodynamic treatments of defect populations have lead to an explanation of existence of grossly nonstoichio-metric crystals in terms of microdomains of ordered structure. The model considers that the nonstoichiometric matrix is made up of a mosaic of small regions of ordered defect-free structures, the microdomains. To account for stoichiometric variation, one can postulate that at least two microdomains with different compositions occur. However, compositional change might simply arise at the surface of the domain. For example, if there are compositionally identical microdomains, one of which is bounded by an anion surface and one by a cation surface, variation in the two populations can give rise to compositional variation. In a strict sense, as each microdomain is ordered, the concept of a defect is redundant, except for... 

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