Vacancies conduction planes

In both P- and j9"-alumina, the conduction planes contain a nonintegral number of Na ions and there are considerably more sites available than Na ions to fill them. The structures are often described in terms of the supposedly ideal 1 11 stoichiometry with the formula NaAliiOi7. In such a case, of the three out of four oxide ions that are missing from the conduction planes, only one half of their vacant sites would contain a Na ion, as indicated schematically in Fig. 2.10. In practice, excess Na" ions are almost always present (e.g. ion A in Fig. 2.1(c)) but rarely in suflBcient quantities to fill all the available vacancies this then gives rise to the high carrier concentrations in these phases. 

Na6Al32VAi051, respectively. This suggests that compared to //-alumina the fi" modification contains an aluminium vacancy compensated by three extra sodium ions in the conduction plane. The consequent higher conductivity of the ft" modification makes it favoured for battery electrolytes. The conductivity of polycrystalline //-alumina at 350 °C (the temperature appropriate to battery operation) is about 5Sm-1 and for polycrystalline //"-alumina about 50 8 m-1. 

In this structure there are perovskite layers of ABO3 separated by AO rock salt layers. It is this layered structure that allows great flexibility in the oxygen stoichiometry of these materials. It is possible to incorporate excess oxygen (5 > 0) in the unusual form of interstitial oxygens, which provide an alternative to the vacancy-based conduction mechanism present in the perovskite and fluorite oxides, where the dopant-vacancy interactions can limit the observed conductivity. The mobility of the oxide ions in these materials occurs mainly through an interstitialcy mechanism in the aZ)-plane, although evidence of low Ea for the conduction in the c-direction via a Frenkel mechanism has also been reported. ... 

Crystallochemically, it is to be expected that Zn+2 can readily substitute for Cu+2, although some local lattice contraction is to be expected. Currently, it is considered that the square pyramidal sites make a negligible contribution to the superconductivity on the grounds that the substitution of large moment rare earths in their proximity has no effect upon Tc other than that which can be ascribed to size (carrier density) effects (8). These zinc substitution effects support this point of view. Zinc substitutions will act as insulators in the two dimensional network due to the closed shell nature of Zn+2. Even if we assume that there is mixed valence on the plane, the introduction of 5% resistive elements would not be expected to significantly influence the percolation conductivity in this layer, since the percolation threshold is ten times this amount, or 50% in two dimensions (9). In the one dimensional chains, substitution of Zn+2 for the oxygen vacancy induced Cu+2 would result in a loss of charge carriers (carrier density) as well as a loss of delocalization in the chain. The former effect would lead to... 

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... 

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