Aliovalent dopants

Cul) is not due to point defects but to partial occupation of crystallographic sites. The defective structure is sometimes called structural disorder to distinguish it from point defects. There are a large number of vacant sites for the cations to move into. Thus, ionic conductivity is enabled without use of aliovalent dopants. A common feature of both compounds is that they are composed of extremely polarizable ions. This means that the electron cloud surrounding the ions is easily distorted. This makes the passage of a cation past an anion easier. Due to their high ionic conductivity, silver and copper ion conductors can be used as solid electrolytes in solid-state batteries. 

The most well-studied and useful materials to date are those with fluorite-related structures, especially ones based on ZrOj, ThOj, CeOj and Bi203 (Steele, 1989). To achieve high oxide ion conductivity in ZrOj, CeOj and ThOj, aliovalent dopants are required that lead to creation of oxide vacancies. Fig. 2.2, scheme 4. The dopants are usually alkaline earth or trivalent rare earth oxides. 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

Fig. 10.7. (a) Defect and (b) conductivity diagram for ceria-doped YSZ at 1000°C. The relevant parameters to construct the diagrams are given in Ref. [119]. The theoretical dependence of ionic transference number tionand oxygai permeability /02 are given in (c). Dashed lines in (c) refer to YSZ. Fm (YzrO represents the aliovalent dopant used. Reproduced (slightly adapted) from Marques et al. 

Defects in ceria can be intrinsic or extrinsic. Intrinsic defects may be present because of thermal disorder or can be created by reaction between the solid and the surrounding atmosphere (i.e. redox processes) whereas extrinsic defects are formed by impurities or by the introduction of aliovalent dopants. We shall draw attention here... 

Dopants, also known as solutes, with low concentrations, can significantly influence the sintering of ceramics. Dopants sometimes are necessary to create functionalities of ceramics. When the cation valence is different from that of the host cation, the dopant is called aliovalent dopant, whereas if the cation has the same valence as that of the host, it is called an isovalent dopant. For aliovalent dopants, when the valence of the solute cation is higher than that of the host cation, the dopant is known as a donor, otherwise, it is called a acceptor. Therefore, if AI2O3 is host, Ti02 and MgO are donor and acceptor, respectively. 

Express the mobility of an oxide grain boundary in which the migration is governed by the diffusion of segregated aliovalent dopant atoms. 

An aliovalent dopant has different charge than the ion that it replaces. 

For doped metal oxides, the aliovalent metal ions influence the overall defect concentrations via what is sometimes called the First Law of Doping. This law simply states that adding an aliovalent dopant increases the concentration of defects with opposite charges, and decreases the concentration of defects with charges of the same sign. 

At this stage solids with trivalent rare-earth ionic conduction should be considered as exceptions. On the other hand, rare-earth elements are effective as a constituent of cationic solid electrolytes, serving either as a large-size framework ion in the lattice or as an aliovalent dopant ion. 

The space charge potential changes when the bulk concentrations of the defects are altered by solutes or nonstoichiometry. Taking the case where an aliovalent dopant such as CaCb is present in NaCl, the defect reaction for the incorporation of the dopant can be written... 

The dopants control the phase formation based on the valence of the dopant. Aliovalent dopants generate vacancies which may be necessary for specific structures. 

We assume that the amount of MIO added is well below the solubility so that [Ml ] = constant. Fig. 4.5 shows a Brouwer diagram of the defect situation as a function of oxygen partial pressure when the level of aliovalent dopant is higher than the level of intrinsic disorder. At the lowest the oxide is oxygen-deficient and oxygen vacancies and electrons predominate. As these defects decrease with increasing pg we hit the level of the acceptor dopant. From here... 

Partially ionised point defects act as combinations of point and electronic defects and generally gives oxygen activity dependent solubilities of aliovalent dopants. 

Let us also briefly recollect the variable solubility of aliovalent dopants treated previously in this chapter. While the acceptors are compensated by oxygen vacancies, their solubility does not vary with anything else than the temperature, as shown before. However, the dissolution of acceptors by simultaneous dissolution of protons. 

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