Effects of aliovalent substituents

The effect of substituents is a complex matter but, with caution, a number of important generalizations can be made regarding aliovalent substituents in perovskites. 

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. 

Typical concentrations of dopants (0.05-5 at.%) must result in the formation of dipolar pairs between an appreciable fraction of the dopant ions and the vacancies, e.g. 2La A-VA or 2Fel i+ -V( ). Donor-cation vacancy combinations can be assumed to have a stable orientation so that their initially random state is unaffected by spontaneous polarization or applied fields. Acceptor-oxygen vacancy combinations are likely to be less stable and thermally activated reorientation may take place in the presence of local or applied fields. The dipoles, once oriented in a common direction, will provide a field stabilizing the domain structure. A reduction in permittivity, dielectric and mechanical loss and an increase in the coercive field will result from the inhibition of wall movement. Since the compliance is affected by the elastic movement of 90° walls under stress, it will also be reduced by domain stabilization. 

Donor doping in PZT would be expected to reduce the concentration of oxygen vacancies, leading to a reduction in the concentration of domain-stabilizing defect pairs and so to lower ageing rates. The resulting increase in wall mobility causes the observed increases in permittivity, dielectric losses, elastic compliance and coupling coefficients, and reductions in mechanical Q and coercivity. 

The introduction of oxygen vacancies through acceptor doping also leads to a slight reduction in unit-cell size, which tends to reinforce the effects referred to above. 

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The effects of aliovalent substituents in PbTi03 and Pb(Ti,Zr)03 are, broadly speaking, similar to those in BaTi03. 

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