Lattice substitution

Barium titanate is usually produced by the soHd-state reaction of barium carbonate and titanium dioxide. Dielectric and pie2oelectric properties of BaTiO can be affected by stoichiometry, micro stmcture, and additive ions that can enter into soHd solution. In the perovskite lattice, substitutions of Pb ", Sr ", Ca ", and Cd " can be made for part of the barium ions, maintaining the ferroelectric characteristics. Similarly, the TP" ion can partially be replaced with Sn +, Zr +, Ce +, and Th +. The possibihties for forming solution alloys in all these stmctures offer a range of compositions, which present a... 

In addition to the characteristic XRD patterns and photoluminescence, UV-visible and X-ray absorption spectra, another fingerprint thought to indicate lattice substitution of titanium sites was the vibrational band at 960 cm-1, which has been recorded by infrared and Raman spectroscopy (33,34). Although there is some controversy about the origin of this band, its presence is usually characteristic of a good TS-1 catalyst, although it turned out to be experimentally extremely difficult to establish quantitative correlations between the intensity of the 960 cm-1 band and the Ti content of a Ti silicate and/or its catalytic activity. 

The scatter of points in Figure 1, with the value of k 3 ranging from 0.03 to 0.05, may reflect the more random behavior of coprecipitation by adsorption/trapping as compared to the more reproducible behavior of lattice substitution. Sensitivity of the partition coefficient to experimental conditions is, in fact, one of the tests for distinguishing the former from the latter (21,34). Attempts to refine the experimental procedure to achieve greater consistency therefore are not warranted any resultant more precise value of the partition coefficient would be applicable only to a more limited set of conditions. 

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

To find the distribution of iron within the nanotube walls an energy dispersive x-ray spectroscopy (EDS) line scan was performed via scanning transmission electron microscopy (STEM), see Fig. 5. 55. The intensity of both the TiK and FeKa lines are maximum at the center of the wall due to its torus shape. Despite the presence of isolated hematite crystallites, a more or less uniform distribution of iron relative to the titanium can be seen across the wall. STEM line scans were performed across a number of walls, and while the average relative intensity of the TiK and FeKa lines varied from wall to wall the relative distribution across a single wall remained uniform. It appears that some of the iron goes into the titanium lattice substituting titanium ions, and the rest either forms hematite crystallites or remains in the amorphous state. 

Solution There are actually two ways that CaCl2 can be placed in the KCl lattice substitutionally and interstitially. The defect reaction equation for substitution is... 

For higher amounts of selenium dioxide in the feed (Z > 0.02), while the surface concentration of adsorbed oxygen ions gradually decreases, the surface concentration of adsorbed products becomes significant, and possibly results in the modifier s entering the catalyst lattice substitutionally, rather than interstitially, Cu2+, Cu+, or O" is replaced by selenium, forming a small unit of a covalent compound. Thus, both free... 

It is unfortunate that the word "doping" has crept into the language of polymeric organic conductors. This is a misnomer since it does not mean lattice substitution as it applies to current semiconductors science but oxidation or reduction of an electron-rich or electron-deficient chain of atoms. That the atom-chain does not need to consist of ir-bonded elements was shown recently by West. West, R. David, L.D., Djurovich, P.I., Sterley, K.L., Srinivasan, K.S.V., Yu, H.J. Amer. Chem. Soc. 1981, 103, 7352. 

The coupons were contacted for a second 48 h period with KTOX solution to remove some of the radionuclides associated with lattice substitution into iron-bearing minerals. 

In the coupon experiment, 60 to 75% of the sorbed Ce was found to be associated with oxyhydroxides and "fixed" by lattice substitution (Figure 4). However, selective extraction data from the mixing-cell tests is ambiguous with respect to the relative importance of exchangeable inventory and that potentially associated with oxyhydroxides. Total surface inventories show a decreasing trend with increasing alteration. No clear trend with temperature is evident. 

Both types of charge compensation are considered present in each case, but the value of u is low when the apatites are prepared in an aqueous medium poor in F- (in which the second type of lattice substitution is dominant) (Table II) and higher for the apatites prepared in an aqueous medium rich in F ions (in which the first type of substitution dominates) (Table I)... 

Ce enters the CaF2 lattice substitutionally for the host Ca ions. Several site symmetries are possible depending upon the charge compensation. Non-locally compensated Ce + produces a center of 0 symmetry, whereas C4v symmetry results when F" ions in interstitial positions take up nn positions to the impurities along <110> directions. In addition many higher-order cluster configurations of these impurity-interstitial pairs exist (23). 

The purity of a crystalline product depends on the nature of the other species in the mother liquor from which the crystals are produced, the physical properties of the mother liquor, and the processing that occurs between crystallization and the final product (downstream processing). Impurities can find their way into the final product through a number of mechanisms the formation of occlusions, trapping of mother liquor in physical imperfections of the crystals or agglomerates, adsorption of species onto crystal surfaces, as part of chemical complexes (hydrates or solvates), or through lattice substitution. 

Lattice substitution requires that the incorporated impurity be of similar size and function to the primary crystallizing species. In other words, the impurity must fit into the lattice without causing significant dislocations. An example of such a system is found in the crystallization of L-isoleucine in the presence of trace quantities of L-leucine. The two species have similar molecular structures, differing only by one carbon atom in the position of a methyl side group. In this system, the incorporation of L-leucine in L-isoleucine crystals is proportional to the concentration of L-leucine in the mother liquor. Moreover, the shape of the recovered crystals changes as the content of L-leucine in recovered crystal increases. 

Fe-substituted ZSM-5 type zeolites, allowing the hydroxylation of benzene with N20 with the help of the so-called redox properties of lattice substituted a-iron [34]. 

A2N, where N is the total number of atoms in the linear lattice. Substituting for the complex conjugate from Equation (4.12), with lattice site label f, gives ... 

The direct synthesis of CoZSMS results not only in a lattice substitution of Si with Co, but also in the formation of small Co-oxide clusters. The preparation of CoZSMS by impregnation led to the material with the highest Co concentration, but also the lowest dispersion and the least defined clusters. 

Pure titania has the rutile structure and therefore has limited solubility in YSZ. The observed linear decrease in lattice parameter with increasing titania concentration in these solid solutions suggests that titanium cations enter the lattice substitutionally for zirconium. Concordant with the data from XRD measurements [29,30,123] the cubic fluorite structure is retained upon addition of 12-20 mol% titania, above which a second phase appears, claimed to be ZrTi04 [123]. The spread in data of the solubility limit produced by different authors may be due to slight differences in, e.g., yttria concentration, sample processing, sintering temperature and impurity content in the cited studies. Microstructural investigations based on SEM and TEM indicated that precipitates of the second phase actually may appear already at lower titania contents [123,125]. 

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