Dopants cation

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

The fabrication of lasers based upon color centers adds a further dimension to the laser wavelengths available. Ordinary F centers do not exhibit laser action, but F centers that have a dopant cation next to the anion vacancy are satisfactory. These are typified by FLi centers, which consist of an F center with a lithium ion neighbor (Fig. 9.26a). Crystals of KC1 or RbCl doped with LiCl, containing FLi centers have been found to be good laser materials yielding emission lines with wavelengths between 2.45 and 3.45 p,m. A unique property of these crystals is that in the excited state an anion adjacent to the FLi center moves into an interstitial position... 

FIGURE 1.14 Ionic conductivity of doped ceria at 1073 K against the radius of dopant cation, rc shown in the horizontal axis is for the critical radius of divalent or trivalent cation, respectively. (Data from Inaba, H. and Tagawa, H., Solid State Ionics, 83, 1, 1996.)... 

For a given composition and a given charge cation, the conductivity increases as the radius of the dopant cation approaches that of zirconium. Thus, scandia-doped zirconia shows the best conductivity (rSc3+ = 0.81 A and zZr4+ = 0.8 A) in this family. 

In this section, a few general statements about solid solutions are given, followed by a discussion of specific solid solutions of cubic crystals. The solid solutions formed from MO and AO oxides are denoted the MO-AO system, with AO being considered as the host matrix and M the dopant cation. The mole fraction of M is indicated by subscript x (e.g., M.VA vO). 

In host matrices with cubic structure (e.g., AO = MgO, CaO, NiO, or SrO), the coordination number of the guest cation is 6 when substituting, for example, for bulk Mg2+ ions but M5C, M4C, and M3c dopant cations are obtained when the substitution occurs with surface Mg2+ or Ca2+ ions located on terraces, steps, or corners (1, 25, 331). As has been mentioned, at low x values M cations are isolated in the matrix, but increasing x leads to pairs, triplets, etc. (25, 332). 

Fig. 4.29 The ideal fluorite structure showing half a unit cell including a dopant cation and a charge-compensating oxygen vacancy.

FIGURE 10.1 Ionic conductivity and diffusion enthalpies as a function of ionic radius of dopant cations to ZrOj. ... 

Empirical calculations carried out for cations show that vacancy compensation is clearly the preferred route, at least for large dopant cations (radius >0.8A). 

These reactions imply that when x moles of dopant oxide are added, Ce sites of CeO are filled with x moles of dopant cation (l ) moles of host... 

Figure 2.13. Relation between ionic conductivity ai I073K and radius of dopant cation. Data relative to Ce, with x= 0.3 (D) and x=0.2 0.+) , On the right the binding energy against ihc...

Furthermore, the redox chemistry of doped ceria is dependent on the ionic radius of the doped cation. The energy due to lattice stresses resulting from mismatch of the host and dopant cation sizes will also go into the free energy balance as briefly described below. 

A doped ceria will have a slightly different TEC dependent on the ion radius and charge (oxidation state) of the dopant cation. Larger radius and lower charge than Ce will increase the TEC and vice versa. 

The crystalline form of interest in Zr-based ceramic compounds is the cubic fluorite structure based on the mineral CaF2. In this structure, consisting of interpenetrating face-centered-cubic and simple cublic arrays of cations (Zr ) and anions (O ), respectively, oxygen ion conductivity is enhanced by replacing zirconium (Zr ) ions on the cation lattice with soluble dopant cations having a valence less than 4, typically divalent (Mg, Ca ) and trivalent (Y, Yb , Sc ) cations. These dopants, which are in solid solution, are incorporated into the zirconia structure by the following types of defect reaction ... 

Y,Zrj -,O2 /2 ceramics, the maximum conductivity is obtained for x 0.10. Higher concentrations of dopant cations decrease conductivity by the effect of their progressive association with oxygen vacancies. The working temperature range extends from 400 to 1600°C for partial pressures of oxygen between 10 ° and 10 bar. 

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