Heterovalent cations

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. 

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 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. 

H. Laudelout, R. van Bladel, G. H. Bolt, and A. L. Page, Thermodynamics of heterovalent cation exchange reactions in a montmorillonite clay, Trans. Faraday Soc. 64 1477 (1968). 

Cation exchange in soils or clay minerals involves replacement of a given cation on a given mineral surface by another cation. Exchange equations are commonly used to evaluate ion availability to plant roots and/or release of metals to soil water (e.g., heavy metals to groundwater or surface water). There are two major types of cation-exchange reactions in soil systems—homovalent and heterovalent cation exchange. 

An equation that is most commonly used to describe heterovalent cation exchange, such as Na+-Ca2+ exchange, is the Gapon exchange equation. For example, for the Na+-Ca2+ system,... 

Equation 4.54 shows that even if Ky is constant across the entire exchange isotherm, is exchangeable Na-load dependent. By taking the limit of Equation 4.57 at the Na of 0.60, it can be shown that Ky - KG, and when the monovalent cation approaches an equivalent fraction of 1, KG = °° (Figs. 4.30 and 4.31). In summary, similar conclusions would be reached on the behavior of a heterovalent cation exchange up to an equivalent monovalent fraction load of 0.20 employing Ky or KG. 

Purton J. A., Allan N. L., and Blundy J. D. (1997) Calculated solution energies of heterovalent cations in forsterite and diopside implications for trace element partitioning. Geochim. Cosmochim. Acta 61, 3927-3936. 

Heterovalent cations, such as transition metal cataions will be liable to catalyze Redox reactions by virtue of their own ability to swing easily between oxidized and reduced states. 

Ceria doped with heterovalent cations, such as alkaline earth and rare earth ions, has been considered one of the most promising electrolyte materials for intermediate-temperature solid oxide fuel cells. It was found that doped ceria materials exhibit relatively high ionic conductivity under nonreducing conditions and relatively lower temperatures in comparison to that of YSZ electrolyte. Among the various dopants studied, Gd " and singly doped Sm ceria have been reported to have a high conductivity [8] and to be relatively stable in a reducing environment [49]. 

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