Aliovalent doping

It is a common practice to dope aliovalent ions (non-stoichiometric doping) to create multivalence and/or vacancies in the material so that domain walls could interact with them. The charged defects created by doping can either pin the domain walls or make the walls more mobile. This method has proven effective to enhance the mesoscale functionality in some ferro-... 

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. 

Nonstoichiometric Compounds Intrinsic defects are stoichiometric defects (i.e., they do not involve any change in overall composition). Defects can also be nonstoichiometric. In the case of extrinsic defects where the host crystal is doped with aliovalent impurities, the solid so formed is a nonstoichiometric compound because the ratio of the atomic components is no longer the simple integer. There is also... 

Bismuth sesquioxide, BijOj, exhibits a high oxide ion conductivity at high temperature without doping of aliovalent cations. The oxide transforms from the monoclinic... 

The composition variation described in the previous chapter has a considerable impact upon the electronic properties of the solid. However, it is often difficult to alter the composition of a phase to order, and stoichiometry ranges arc frequently too narrow to allow desired electronic properties to be achieved. Traditionally, the problem has been circumvented by using selective doping by aliovalent impurities, that is, impurities with a different nominal valence to those present in the parent material. However, it is important to remember that all the effects described in the previous chapter still apply to the materials below. The division into two chapters is a matter of convenience only. 

There are several ways in which a solid doped with an aliovalent impurity can maintain charge balance. It is by no means simple to be sure which compensation mechanism will hold, or even if one mechanism will hold over all of the doping range. However, there are some quantitative guidelines that apply, especially for oxides. The principal mechanism will depend upon how easily the host cation that is being replaced is oxidized or reduced. 

In this section we are concerned with the properties of intrinsic Schottky and Frenkel disorder in pure ionic conducting crystals and with the same systems doped with aliovalent cations. As already remarked in Section I, the properties of uni-univalent crystals, e.g. sodium choride and silver bromide which contain Schottky and cationic Frenkel disorder respectively, doped with divalent cation impurities are of particular interest. At low concentrations the impurity is incorporated substitutionally together with an additional cation vacancy to preserve electrical neutrality. At sufficiently low temperatures the concentration of intrinsic defects in a doped crystal is negligible compared with the concentration of added defects. We shall first mention briefly the theoretical methods used for such systems and then review the use of the cluster formalism. 

An important practical way of increasing the value of c, is by means of doping with aliovalent (or heterovalent) ions. This involves partial replacement of ions of one type by ions of different formal charge. In order to retain charge balance, either interstitial ions or vacancies must be generated at the same time. If the interstitials or vacancies are able to migrate, dramatic increases in conductivity can result. 

Fig. 2.2 Solid solution formation by doping with aliovalent ions.

Anion conduction, particularly oxide and fluoride ion conduction, is found in materials with the fluorite structure. Examples are Cap2 and Zr02 which, when doped with aliovalent impurities. Fig. 2.2, schemes 2 and 4, are F and 0 ion conductors, respectively, at high temperature. The 3 polymorph of 61303 has a fluorite-related structure with a large number of oxide vacancies. It has the highest oxide ion conductivity found to date at high temperatures, > 660 °C. 

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. 

Aliovalent doping can also be used to enhance low temperature conductivities. Both interstitial F ions and F vacancies can be generated in, for example, CaF2 by doping with Na and La, respectively (Kudo and Fueki, 1990). 

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. 

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. 

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... 

The defect concentrations in ionic solids can be enhanced by doping with aliovalent ions if, for example, Cd2+ ions replace Ag+ ions in AgCl, additional positive charges are introduced that are compensated by negative silver vacancies (Fig. lb). In terms of a defect chemical reaction the doping can be written as ... 

Bogicevic, A., Wolverton, C., Crosbie, G.M., and Stechel, E.B., Defect ordering in aliovalently doped cubic zirconia from first principles. Physical Review B, 2001, 64, 014106. 

Effects of Aliovalent Doping on Thermal and Phase Stability ... 

Figure 4 Influence of aliovalent cation doping on specific electrical conductivity and activation energy of electron conduction of 0.5% Pt/Ti02(D) at 333 K, in vacuum. (From Ref. 82.)...

The influence of aliovalent cation doping of the support (Ti02) on the catalytic properties of supported Pt and Rh crystallites was also investigated under other reaction conditions, among which the photocatalytic cleavage of water [116] and the reduction of NO by propylene [117] in the presence or absence of oxygen. In the case... 

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

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