Isomorphous displacement

Barium reacts with metal oxides and hydroxides in soil and is subsequently adsorbed onto soil particulates (Hem 1959 Rai et al. 1984). Adsorption onto metal oxides in soils and sediments probably acts as a control over the concentration of barium in natural waters (Bodek et al. 1988). Under typical environmental conditions, barium displaces other adsorbed alkaline earth metals from MnO2, SiO2, and TiO2 (Rai et al. 1984). However, barium is displaced from Al203 by other alkaline earth metals (Rai et al. 1984). The ionic radius of the barium ion in its typical valence state (Ba+) makes isomorphous substitution possible only with strontium and generally not with the other members of the alkaline earth elements (Kirkpatrick 1978). Among the other elements that occur with barium in nature, substitution is common only with potassium but not with the smaller ions of sodium, iron, manganese, aluminum, and silicon (Kirkpatrick 1978). 

Figure 5. Displacements associated with the Q coordinates which transform as irreducible representations of the group S3 (which is isomorphic with the symmetry point group C3r, itself a subgroup of D J. Q, represents the totally symmetric breathing mode of the equilateral triangle, and Q2 represents the antisymmetric distortion coordinate, which is degenerate with the Q3 bending mode for Dih geometries.

A slight deficiency of CsF mixed with [Xe,F,i]+ -AuF,]-was heated under dry N, to 110 °C, at which temperature XeF, evolved. The XeF, displacement CsF -)- [Xe,F ]- [AuF,] ->- 2XeF, + CsAuF, was completed under vacuum, (the slight excess of xenon complex sublimed out of the reactor) and a pale yellow solid remained. X-Ray powder photographs established that CsAuF, is isomorphous with the other CsMF, noble metal salts. Crystal data CsAuF,i M = 443-9 rhombohedral, a — 5-24 i 0-01 A a = 96-5 0-3°, V = 141 A, 2=1, Z c = 5-22 g cm-. Space... 

V- a,b Adducts 157 with X = Cl, Br are isomorphic of die tetragonal space group, where P—Ag—X is slightly nonlinear, with die Ag displaced toward one of die MeO oxygen atoms. No structural assignment could be found for XRD when X = I231. 

The tetrachlorides and tetrabromides are all brown-black or black solids, obtainable by reduction of the pentahalides with H2, Al, Nb or Ta at elevated temperatures. The four compounds are apparently isomorphous. The structure of NbCl4 has been studied in detail the metal atoms occur in pairs, displaced from the centers of their octahedra towards a common edge so that the Nb—Nb distance is 3.06 A. The formation of weak Nb—Nb bonds accounts for the diamagnetism. 

In our discussions of the internal structure of crystals, we have shown that each atom (or molecule) has a precise location in a repeating structure. If this structure is disrupted in some way the crystal is said to have imperfections. There are a number of different kinds of imperfections that can occur. If a foreign atom (or molecule in a molecular crystal) is present in the crystal lattice, this is known as a chemical imperfection. The foreign atom can be present at a lattice site having substituted for an atom in the structure as we saw in our brief discussion of isomorphism and solid solutions. This is called a substitutional impurity. The foreign atom can also be present in the crystal by fitting between the atoms in the lattice. This is called a interstitial impurity. Both of these types of impurities can cause the atoms in the crystal to be slightly displaced since the impurity atoms do not really fit in the perfect lattice structure. The displacement of the atoms causes a strain in the crystal. 

It is possible to remove ions at tracer level concentrations from solutions by precipitation using adsorption or coprecipitation. Coprecipitation occurs if the compound of the tracer and the oppositely charged ion of the precipitate is isomorphous with the precipitate. In these cases the active ion may be included in the crystal lattice of the precipitate at a lattice point, particularly if the tracer ion is close in size to the ion which it displaces. However, at trace level concentrations exceptions are found to this requirement of similarity in size as well as to the requirement of isomorphism. When the distribution of the tracer is foimd to be uniform throughout the precipitate it can be described by the Berthelot-Nernst homogeneous distribution law which is expressed as... 

The chemical interpretation of o-in measured by the Schofield method depends sensitively on the type and concentration of probe electrolyte used. If these properties are chosen so that the cation in the reacting electrolyte neutralizes precisely the exposed functional group charge associated with isomorphic substitutions and dissociated hydroxyls and so that the anion neutralizes only the exposed protonated functional groups, then q+ and q. will have optimal magnitude for the chosen pH value and CTjn will be truly an intrinsic surface charge density. On the other hand, if the cation in the probe electrolyte is not able to displace all of the native adsorbed cations in, e.g., inner-sphere surface complexes, or if the anion cannot displace all of the native anions bound to protonated functional groups, or if either ion does not form only neutral surface complexes in the soil clay, then Ojn will differ from its optimal value. 

---