Dopant aliovalent

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

In dispersed metal-support systems (Fig. 11.2 right), one can vary pe(M) - M-e(S) by varying the support or by doping the support with aliovalent cations. This is known in the literature as dopant-induced metal-support interactions (DIMSI).8,11,41,42 Thus one can again vary the electrochemical potential and thus the coverage of backspillover O2 on the supported catalyst surface. 

Extrinsic Defects Extrinsic defects occur when an impurity atom or ion is incorporated into the lattice either by substitution onto the normal lattice site or by insertion into interstitial positions. Where the impurity is aliovalent with the host sublattice, a compensating charge must be found within the lattice to pre-serve elec-troneutality. For example, inclusion of Ca in the NaCl crystal lattice results in the creation of an equal number of cation vacancies. These defects therefore alter the composition of the solid. In many systems the concentration of the dopant ion can vary enormously and can be used to tailor specific properties. These systems are termed solid solutions and are discussed in more detail in Section 25.1.2. 

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. 

Aliovalent additives are often called donor dopants, when they tend to provide electrons and enhance intrinsic n-type semiconducting behavior, or acceptor dopants, when they tend to give a population of mobile holes and enhance /j-typc semiconducting behavior. The process of creating electronic defects in a crystal in this way is called valence induction. 

The occurrence of such ion trapping is clearly undesirable since it inevitably leads to a decrease in conductivity. In practice, in materials that contain potential traps such as charged aliovalent impurities/dopants, the conductivity values of a particular sample may actually decrease with time as the mobile ions gradually become trapped. Such ageing effects greatly limit the usefulness of a solid electrolyte in any device that needs to have a long working-life. 

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. 

Point defects are an important part of the work in this paper. There are many reasons for the formation of point defects in minerals and their presence can exert important perturbations on the properties of the material (4). Point defects are formed because of the thermally driven intrinsic disorder in a lattice, the addition of aliovalent impurities or dopants, the presence of metal-nonmetal nonstoichiometry, and the creation of nonideal cation ratios. The first three source of defects are well-known from binary compounds but the last is unique to ternary compounds. Ternary compounds are much more complex than the binary compounds but they also have gained a great deal of attention because of the variety of important behavior they exhibit including now the presence of superconductivity at high temperatures. The point defects can be measured by introducing probe ions into the lattice. 

The effects of deliberately added donors, such as titanium, and acceptors, such as iron and magnesium, on electrical conductivity have been studied. Doping with aliovalent ions affects the concentration of intrinsic defects and, in consequence, the diffusivity of A1 and O. In the case of variable-valency dopants, changes in p0l change the fraction of dopants in the aliovalent state and the nature and concentration of the defects. For example, the dopant Ti substitutes for A1 and, in the fully oxidized state, produces the defect TiA1, compensated by Va", so that... 

Donor dopants, i.e. those of higher charge than that of the ions they replace, are compensated by cation vacancies acceptors, i.e. dopants of lower charge than that of the replaced ions, are compensated by oxygen vacancies. Each dopant type tends to suppress the vacancy type that the other promotes. The common dopants in perovskite-type ceramics are listed in Table 6.1. The effects of aliovalent substituents are discussed in Section 2.6.2. 

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. 

Dopant is also called solute, which is dissolved as solid solution in polycrystalline solids. If there is an interaction potential for the solute to be attracted to or repelled from the grain boundary, the solute atoms or ions will have a nonuniform distribution at the grain boundaries. The interaction could be due to lattice strain energy caused by size mismatch between the solute and host ions and/or electrostatic potential energy for aliovalent solutes. 

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. 

Substitutional dopants Dissolved foreign atoms or ions replacing lattice atoms. If aliovalent they may introduce vacancies 0... 

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