Solution isovalent cations

Let us consider first isovalent cations. MgO can dilute ions of similar size, as for instance Ni or Co forming NiO-MgO and CoO-MgO solid solutions with an infinite range of composition. The effect of progressively replacing Mg by Ni or similar cations (Co ", Cu ) on the surface properites has been investigated both experimentally [83,177,178] and theoretically [179,180]. The presence of Ni cations diluted in the MgO matrix results in an efficient catalyst for nitrous oxide, N2O, decomposition this has been attributed to the different bond strength of the Ni-0 and Mg-0 bonds at the surface [177]. Plane wave calculations on Ni-doped MgO have shown that the presence of Ni atoms on the... 

Solid solutions are very common among structurally related compounds. Just as metallic elements of similar structure and atomic properties form alloys, certain chemical compounds can be combined to produce derivative solid solutions, which may permit realization of properties not found in either of the precursors. The combinations of binary compounds with common anion or common cation element, such as the isovalent alloys of IV-VI, III-V, II-VI, or I-VII members, are of considerable scientific and technological interest as their solid-state properties (e.g., electric and optical such as type of conductivity, current carrier density, band gap) modulate regularly over a wide range through variations in composition. A general descriptive scheme for such alloys is as follows [41]. 

NiO is a cation deficient semiconductor. The fraction of its cation vacancies and compensating electron holes depends on the oxygen potential as discussed in Section 2.3. The isovalent Ca2+ ions can replace Ni2+ ions in the cationic sublattice of the fee matrix by chemical interdiffusion. TiOz and NiO form NiTi03 which dissolves to some extent in the fee matrix of NiO as Ti and Vmc. The counterdiffusion of Ti02 and CaO in the NiO solvent leads to the encounter of the different solute cations (Fig. 9-12a). With increasing overlap of their concentration profiles, the concentration of the product will eventually surpass the solubility limit (and the nucleation barrier). Precipitation of the rather stable CaTi03 compound as an internal reaction product in the NiO matrix is the result. 

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. 

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