Aliovalent substitutions

Substitution of an ion with one valence by another with a different valence, aliovalent substitution, will create a charged defect. For example, a divalent ion such as Ca2+ substituted for a monovalent Na+ on a sodium site in NaCl gives a local electronic charge augmented by one extra positive charge ... 

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. 

Whereas the defect chemistry of pure stoichiometric compounds is largely of academic interest, the effects of the introduction of foreign ions are of crucial significance to electroceramics. The defect chemistry of barium titanate itself and, in particular, the effect of lanthanum doping are of such importance that they are discussed in detail in Section 2.6.2. It is for these reasons that the system is chosen here to illustrate basic ideas relating to the aliovalent substitution of one ion for another. 

For any particular solid, the relative activation barriers for the available mechanisms determine whether the anions or cations are responsible for the ionic conduction. For example, in a yttria-stabilized Zr02, with the formula Zri Y ,.02-(x/2). aliovalent substitution of Zr by Y generates a large number of oxygen vacancies, giving rise to a mechanism for oxide ion conduction. Indeed, it is found that the anions diffuse about six orders of magnitude faster than the cations. 

Solid oxide electrolytes with both ionic and electronic conductivities are very attractive as membrane materials as no external circuit is needed [see Figure 4(b)]. The work of Teraoka et alJ showed the potential of perovskite materials with aliovalent substitutions. A typical perovskite has the formula ABO3 (e.g. 

For RAIO3 compounds, simultaneous substitution of R- and Al-cations, including compensative aliovalent substitution, is also t) ical. In particular, the solid... 

The ternary and quaternary oxides with CGG structure provide ample opportunities for isoelectronic (aliovalent) substitution of cations. Hence, it is not surprising that to date some sixty different compounds have been idenhfied, all of which belong to the CGG structural family. 

An alternative approach to generate oxygen vacancies is aliovalent substitution... 

Vasala S, Lehtimaki M, Haw SC, Chen JM, Liu RS, Yamauchi H, Karppinen M (2010) Isovalent and aliovalent substitution effects on redox chemistry of Sr2MgMo06.a SOFC-anode material. Solid State Ionics 181 754—759... 

MO3 arrays by aliovalent substitutions on the A sites. In all cases, the crossover is marked by lattice instabilities. The Mott-Hubbard transition occurs on the itinerant-electron side, and this transition is also marked by lattice instabilities. 

Aliovalent substitution, single-phase systems. The electronic conductivity of ceria can be increased by suitable substitution. Substitution of Tb and Pr will lead to significant p-type electronic conductivity at high pOg (pOs > 10 atm at... 

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