Heterovalent

According to X-ray data, 2,5-diamino-l,3,4-triazole retains its diamino form in the heterovalent cobalt complex 375 (91IC4858) and in the polymeric complex with manganese thiocyanate (93ICA53). 

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. 

In contrast with formation of three types of bpz-substituted RU3 cluster species, reactions of 2 with pyq induced isolation of monomer 45 and trimer 46 containing 6>rf/ 6>-metallated pyq depending on the reaction conditions [30]. Reduction of the 3+ trimeric complex 46 by addition of aqueous hydrazine gave neutral species 46a. Oxidation of 46a by addition of two equivalents of ferrocenium hexaflu-orophosphate afforded 2+ intercluster heterovalent complex46b containing two Ru30(0Ac)6(py)2II,III,m and one Ru30(OAc)5(py)2II,m,n moieties. 

As shown in Fig. 7, the electronic spectrum of 2+ intercluster heterovalent trimeric complex 46b includes IC absorption envelopes characteristic of both [RU3O]0 and [Ru30]+ moieties, which are only slightly perturbed compared with those in the corresponding neutral (46a) and 3+ complexes (46). Three reversible redox waves occur at +0.89, —0.29, and —1.57V in the CV... 

Both approaches lead to identical standard thermodynamic values of exchange (9-10). Such a difference in the choice of the surface concentration scale is of course only important for heterovalent exchange equilibria. For the heterovalent case the numerical value for both selectivity coefficients, Kg (Gaines Thomas) and (Vanselov) differ and, consequently, their variation with surface composition also differs. 

Rather small selectivity differences are observed for homovalent-and heterovalent exchanges involving alkali, alkaline earth, bivalent transition metal ions, aluminium and rare earth cations, as is amply evidenced from the extensive compilation by Bruggenwert and Kamphorst (16). This compilation includes various clay minerals illite, montmorillonite, vermiculite and kaolinlte. 

Heterovalent exchange. Although double layer theory predicts increasing AG of mono-bivalent exchange with charge density increase, such a relationship was not always obvious (20,48). 

Standard free energy changes of heterovalent exchange equilibria among complexes increase with charge density agreemen +with double layer expectations as shown for the Ag(en) ... 

Na - K - Co exchange in Y zeolite. Heterovalent exchange reactions in zeolites generally show an even more pronounced dependency on loading (116-118). Rees (116) observed variations of the selectivity coefficient by a factor 1000 for the Na-Ca and Na-llg exchange in zeolite A at 25 °C. An+exajnple of e treipe variations is shown in fig. 9 for the K -Co and Na -Co selectivities in zeolite Y at 45 °C (117). The exchange... 

An important practical way of increasing the value of c, is by means of doping with aliovalent (or heterovalent) ions. This involves partial replacement of ions of one type by ions of different formal charge. In order to retain charge balance, either interstitial ions or vacancies must be generated at the same time. If the interstitials or vacancies are able to migrate, dramatic increases in conductivity can result. 

Let us also consider the pairing reaction B A -t-V A = [B, V] in an ionic crystal AX, where the dopant BA is a heterovalent cation and V A is the compensating cation vacancy. We define the degree of pairing to be NP = A[B>V T/VB. From the mass balance equation A B = AB+AjB Vj and the condition of electroneutrality jVv + A b = NyA, one finds for the case that the undoped AX crystal exhibits Schottky type disorder (which means that = Ks)... 

In discussing AO-BO interdiffusion, we saw that the two independent fluxes of this ternary system can lead to different chemical diffusion coefficients D. They depend upon the constraints which define the physical situation (e.g., VjuG = 0 or Vy/v = 0). The analysis of this relatively simple and fundamental situation is already rather complex. The complexity increases further if diffusion occurs between heterovalent components of compound crystals. This diffusion process is important in practice (e.g., heterovalent doping) and its treatment in the literature is not always adequate. We therefore add a brief outline of the relevant ideas for a proper evaluation of D. 

Defect thermodynamics provide the guidelines for the solution of this practical problem. In Chapter 2, the basic ideas on how to influence point defect concentrations by doping with (heterovalent) additions were presented. Due to the electroneutrality condition and the laws of mass action, we can control the point defect... 

Most of the irregular SE s formed by irradiation interact with impurities that are the native irregular SE s of the crystal. Impurities interact with the irradiation products either by their stress field or, if heterovalent, by the electrostatic (Coulomb) field. Photolysis (radiolysis) is found in other than halide crystals as well. In oxides, the production of Frenkel pairs under photon irradiation is negligible. This has been ascribed to the fact that the reaction O2- +0" = 02 is endothermic, whereas the reaction X- +X = is exothermic. 

In generalizing these results, we can apply them to other solid electrolytes as well, for example, to other fluorite type oxides (e.g., Hf02, CeOz) that have been doped with heterovalent cations (e.g., SrO, BaO, Y203, La203). 

Effect of titania modification. TiC>2 was modified by deposition of platinum or by p-and n-type doping with heterovalent cations. 

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