Cation substitutions

The existence of these isostructural compounds suggests that solid solutions could be formed between two end members via isomorphous substitution for Fe by other cations. The likelihood of substitution depends on the similarity of the ionic radii and the valency of the cations (Goldschmidt, 1937). m is the most suitable cationic species and a radius about 18% higher or lower than that of high-spin Fe in sixfold coordination can be tolerated. Isomorphous replacement of Fe in Fe oxides by a number of cations has been observed in nature and, more frequently, in the laboratory. As far as is known, however, almost all these solid solutions have broad miscibility gaps, possibly induced by development of structural strain as substitution rises. 

For the size of the unit cell of M substituted Fe oxides, a linear relationship between the Fe oxide and the appropriate structural analogue has been postulated. This is the so-called Vegard rule (Vegard, 1921). In practice, the change in unit cell parameters often deviates from the Vegard line, with the extent and direction of the deviation being different for different substituents and different unit cell parameters 

The Iron Oxides Structure, Properties, Reactions, Occurences and Uses. R. M. Cornell, U. Schwertmann Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30274-3 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides 

Various properties of Fe oxides change regularly with increasing substitution. Among these are the positions of the visible and infrared absorption bands, mag- 

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If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Simple ABO compounds in addition to BaTiO are cadmium titanate [12014-14-17, CdTiO lead titanate [12060-00-3] PbTiO potassium niobate [12030-85-2] KNbO sodium niobate [12034-09-2], NaNbO silver niobate [12309-96-5], AgNbO potassium iodate [7758-05-6], KIO bismuth ferrate [12010-42-3], BiFeO sodium tantalate, NaTaO and lead zirconate [12060-01 -4], PbZrO. The perovskite stmcture is also tolerant of a very wide range of multiple cation substitution on both A and B sites. Thus many more complex compounds have been found (16,17), eg, (K 2 i/2) 3 ... 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

The intrinsic ion-exchange properties of p-doped polymers have been used to bind electrocatalytic anions such as porphyrins238 and phthalocy-anines.239 Cation-substituted polymers have been used to bind anions such as [Fe(CN)6]3-/4-,235 oxometallates,240 and porphyrins241 and anion-substituted polymers have been used to bind a variety of cationic species such as [Co(2,2/-bipyridine)3]3+/2+)58 [Ru(NH3)6]3+/ 2+,183 and porphyrins.242... 

A close analysis of the chlorine-centered ClA6.nMn octahedral SBU (secondary building unit) has revealed that the acentricity of the bulk lattice may be introduced via cation substitution. The di-cation substitution (n=2) in ClNas... 

The kinds of substitution mechanisms that may be relevant to super-low concentration elements such as Pa involve intrinsic defects, such as lattice vacancies or interstitials. Vacancy defects can potentially provide a low energy mechanism for heterovalent cation substitution, in that they remove or minimise the need for additional charge balancing substitutions. Formation of a vacancy per se is energetically unfavourable (e.g., Purton et al. 1997), and the trace element must rely instead on the thermal defect concentration in the mineral of interest, at the conditions of interest. Extended defects, such as dislocations or grain boundaries, may also play a key role, but as these are essentially non-equilibrium features, they will not be considered further here. 

The oxides with general formulas A20, AO, A203, A02, and A2Os are doped into MgO so that the cation substitutes for Mg. Write general defect equations for the reactions, assuming cation vacancies rather than electronic compensation occurs. 

The point defects present are Al3+ cations substituted on Mg2+ sites, Al g and Mg2+ vacancies, V g. 

This strategy has lead to the development of anion-substituted compounds such as Ag3SI and cation-substituted compounds such as RbAg4I5, as well as materials with... 

Weber WJ, Griffin CW, and Bates JL. Effects of cation substitution on electrical and thermal transport properties of Y0rO3 and LaCr03. J. Am. Ceram. Soc. 1987 70 265-270. 

Ding X, Liu Y, Gao L, and Guo L. Effects of cation substitution on thermal expansion and electrical properties of lanthanum chromites. J. Alloys Compounds 2006 425 318-3 22. 

Near-infrared (NIR) absorption spectroscopy has been used to characterize the delocalized re-stacks on electronic conducting dendrimers by Miller and coworkers [47-49]. These dendrimers were prepared by peripherally modifying PAMAM dendrimers with cationically substituted naphthalene diimides, and then reduced with one electron per imide group to convert each imide into its anion radical. The re-stacking of these radical anions on these dendrimer surfaces was indicated by an absorbance band beyond 2000 nm in the NIR spectra. 

The surface charge of aluminosilicate minerals may arise either from isomorphous cation substitution within the structure,... 

I0M.S. Kaba, I.K. Song, and M.A. Barteau, Investigation of Framework and Cation Substitutions in Keggin-type Heteropoly acids Probed by Scanning Tunneling Microscopy and Tunneling Spectroscopy,/ Vac. Sci. Technol. A, 15,1299-1304(1997). 

In the fluorides, chlorides and oxides of the Group-A main-group metals and the transition metals zirconium and hafnium, aliovalent cation substitutions are generally charge-compensated by the introduction of native defects (e.g. an oxygen vacancy in Zr, ,Ca 02 x) because the intrinsic is large however, in some oxides neutral oxygen or water may... 

Hazen R. M. and Wones D. R. (1972). The effect of cation substitutions on the physical properties of trioctahedral micas. Amer. Mineral, 57 103-129. 

The pyroxene group of minerals, where cations substitute one for another at interchain sites, also have a tetrahedral Si site that may contain Al. Some of these substitutions, especially those at M2, can distort the ideal crystal structures, as is depicted in Fig. 2.10. The small distortions are detected as variations in bond lengths between adjacent atoms during crystal structure analyses. These data, combined with accurate composition analyses, could indicate some of the conditions present during growth of the crystal and contribute to our understanding of why some mineral species have formed as fibers. Unfortunately, few detailed crystal structural analyses have been performed on pyroxene minerals with fibrous habits. 

Amphibole group Amphiboles are hydrated silicates with Mg, Fc, Ca, and Na as the dominant cation species. These, together with other cations, substitute at specific... 

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