Doping heterovalent

In discussing AO-BO interdiffusion, we saw that the two independent fluxes of this ternary system can lead to different chemical diffusion coefficients D. They depend upon the constraints which define the physical situation (e.g., VjuG = 0 or Vy/v = 0). The analysis of this relatively simple and fundamental situation is already rather complex. The complexity increases further if diffusion occurs between heterovalent components of compound crystals. This diffusion process is important in practice (e.g., heterovalent doping) and its treatment in the literature is not always adequate. We therefore add a brief outline of the relevant ideas for a proper evaluation of D. 

The anion vacancy mechanism of conductivity for RE metal fluorides was confirmed by means of heterovalent doping of Lap3. A sharp increase of conductivity has been observed by doping LaF3 by MF and MF2 fluorides but its doping by TI1F4 leads to a decrease of the conductivity [97]. The vacancy mechanism of conductivity was also confirmed by F NMR [98]. 

Similarly to fluorite-like solid solutions, maxima on the concentration dependence curve of conductivity were found for heterovalent doping tysonite-like solid solutions with 1 and 11 structures (Figure 14.38 [115] and 14.39 [34]). 

The heterovalent anion doping of fluoride phases by oxygen ions or simultaneous heterovalent replacement of both anions and cations is an effective method for additional influence on the defect structure and transport properties. The joint effect of easily deformable cations with a lone electron pair (Bip3 matrix) and heterovalent doping of fluorides by oxygen ions is similar to the effect of an increase in temperature and pressure. 

Generally, if the radius of a doping metal cation is smaller than that of Fe% it can easily substitute the Li+-ion sites. In the so obtained Lij j.M3.FeP04, the valency of Li is low, viz., +1, and this kind of doping therefore is classified as heterovalent doping. This causes the problem of charge neutrality, which... 

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. 

Defect thermodynamics provide the guidelines for the solution of this practical problem. In Chapter 2, the basic ideas on how to influence point defect concentrations by doping with (heterovalent) additions were presented. Due to the electroneutrality condition and the laws of mass action, we can control the point defect... 

In generalizing these results, we can apply them to other solid electrolytes as well, for example, to other fluorite type oxides (e.g., Hf02, CeOz) that have been doped with heterovalent cations (e.g., SrO, BaO, Y203, La203). 

Effect of titania modification. TiC>2 was modified by deposition of platinum or by p-and n-type doping with heterovalent cations. 

These calculations for the formation of associates in KCl doped with heterovalent CaCl2 are illustrations and can be modified without difficulty for other cases of interaction between oppositely charged defects. 

Ceria doped with heterovalent cations, such as alkaline earth and rare earth ions, has been considered one of the most promising electrolyte materials for intermediate-temperature solid oxide fuel cells. It was found that doped ceria materials exhibit relatively high ionic conductivity under nonreducing conditions and relatively lower temperatures in comparison to that of YSZ electrolyte. Among the various dopants studied, Gd " and singly doped Sm ceria have been reported to have a high conductivity [8] and to be relatively stable in a reducing environment [49]. 

Cation doping of fluorides always leads to a crystal structure distortion. The degree of distortion depends on the size ratio and properties (in the first place, electronic structure and electronegativity) of dopant and matrix cations as well as on the replacement type isovalent or heterovalent replacement. 

Heterovalent replacement in the cation sublattice leads to an increase in the number of anion defects, for example vacancies <-> M " " + Vp) in Ri xMxF3 x solid solutions or interstitial ions <-> R + Fj) in Mi xRxF2+x solids solutions. Of course, this replacement greatly increases the conductivity of doped phases and will be discussed in detail hereinafter. 

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