Conduction plane defects

Both the oxides (3- and (3"-alumina show extremely high Na+ ion conductivity. As the structure suggests, the conductivity is anisotropic, and rapid sodium ion transport is limited to the two-dimensional conduction plane. There is almost unimpeded motion in the Na+ layers, especially in (3"-alumina, which lacks interstitial oxygen ion defects in the conduction plane, and the conductivity is of the same order of magnitude as in a strong solution of a sodium salt in water. The conductivity is a... 

The high conductivity of (3-alumina is attributed to the correlated diffusion of pairs of ions in the conduction plane. The sodium excess is accommodated by the displacement of pairs of ions onto mO sites, and these can be considered to be associated defects consisting of pairs of Na+ ions on mO sites plus a V N l on a BR site (Fig. 6.12a and 6.12b). A series of atom jumps will then allow the defect to reorient and diffuse through the crystal (Fig. 6.12c and 6.12d). Calculations suggest that this diffusion mechanism has a low activation energy, which would lead to high Na+ ion conductivity. A similar, but not identical, mechanism can be described for (3"-alumina. 

Figure 6.12 Correlated diffusion in (3-alumina (a) the Na+ positions in the conduction plane of ideal NaAlnOi7 (b) the creation of associated defects by location of pairs of Na+ ions on mO sites (c) the ionic jumps involved in diffusion of an associated defect and (d) the final position of the defect.

Divalent ion-stabilized "-alumina is represented by the general composition Nai + jMjAll 1 -yOi7, where M can be Mg, Ni, or Zn. When y = 1, all the sodium ion sites in the conduction plane are filled. For optimum sodium ion conductivity, y k 0.67, which corresponds to NaaO 0.8 MgO 6.I9AI2O3 with approximately 12.5 mol % NaaO and 10 mol% MgO. MgO is incorporated into the /0"-alumina crystal structure by the following defect reaction... 

Fig. 23.2. Structure and infrared spectra of hydrogen / -alumina. (a) Silver -alumina conduction plane with an AI-OIT defect the filled circle represents the oxygen of the AI-O-AI bridge between spinel blocks, (b) Conduction plane of fi/P"-alumina. (c)...

Point defects are zero-dimensional (Figure 10.6) and they are the only defects that are thermodynamically stable. Line and plane defects are not thermodynamically stable and do not occur in equilibrium states. Point defects determine the extrinsic physical properties of solids such as electrical conductivity, work function, and color as well as the chemical properties such as dififusivity, stoichiometry, and sinter rate. Some examples of point defects are (a) vacancies, where atoms or ions that should be on lattice sites are missing (b) interstitials which are atoms or ions between the regular lattice sites of a solid (c) foreign atoms or... 

The Seebeck coefficient of the layered structure Bi2Sr2Co06+s is —33 p,V K-1 at 250 K. The defects are confined to the Co02 planes in the structure, (a) Is the conductivity by way of holes or electrons (b) What are the ionic states of the Co ions in the phase (c) What is the ionic formula of the compound, (d) What is the value of 8 [Note the structure is very similar to that of Bi2Sr2CuC>6, Section 8.6. Data adapted from Y. Nagao and I. Terasaki, Phys. Rev., B76, 144203-1-144203-4 (2007).]... 

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