Conduction plane oxide ions

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... 

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. 

The different oxide stacking sequences in j8 and j8". Fig. 2.9, and in particular, the presence of mirror symmetry in the conduction plane of j8 but not P", lead to differences in detail in the nature of the sites for Na" ions in the conduction plane. Such differences together with the different charge compensation mechanisms cause the electrical properties of p and P" to differ somewhat, and in particular lead to rather different conduction mechanisms. 

In the previous section, /1-aluminas were discussed. If one replaces Na+ by H30+ or NH4 in these oxide compounds by, for example, electrolysis, then proton conducting materials can be obtained. Their ionic conductivity, however, is relatively low (10—5 Q-1 cm-1) because of the strong interactions between the small H+ ions and O2- ions in the conduction plane (OH ). The electrical conductivity is markedly higher in crystals of NH4 -/ -Ga203. 

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. ... 

Tl2Ba2Ca -iCu 02n+4. There are alternating conduction layers where the flow of supercurrent takes place, and binding layers that support and hold together the conduction layers. The conduction layers contain copper oxide (CUO2) layers of the type sketched in Figure 12, where each copper ion Cu + is surrounded by four oxygen ions These planes are... 

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