Solution aliovalent cations

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. 

Extrinsic Crystal Self-Diffusion. Charged point defects can be induced to form in an ionic solid by the addition of substitutional cations or anions with charges that differ from those in the host crystal. Electrical neutrality demands that each addition results in the formation of defects of opposite charge that can contribute to the diffusivity or electronic conductivity. The addition of aliovalent solute (impurity) atoms to an initially pure ionic solid therefore creates extrinsic defects.10... 

The intrinsic case applies at small doping levels or at high temperatures where the thermal equilibrium site fraction of the intrinsic cation vacancy population exceeds that due to the aliovalent solute atoms. In this case, the effect of the added solute atoms is negligible. The activation energy for cation self-diffusion is therefore the same as in the pure material and is given by Eq. 8.44. 

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. 

It is well known that the ionic conductivity of aliovalent-doped ceria solutions show a maximum at a certain dopant concentration and cation radius. Compared to divalent cation doping, however, trivalent dopants are observed to contribute higher conductivity values in ceria. Among trivalent rare earth metals, samarium and gadolinium are accepted as the most effective dopants. Therefore, we have selected a cermet made up of nickel and 20 at% samarium-doped ceria (SDC) as the anode material in our standard cells [12-14]. 

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