Conduction plane 1-aluminas

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)...

Figure 7. Ionic conductivities for various monovalent (a) and multivalent (b) ions in f) and / " -alumina single crystals in the direction of the conduction plane [4, 19J.

The ionic conductivity for various ions in the piP"-alumina structure along the conduction planes shows a maximum for an optimum size of the ions. It should be neither too small nor too big to fit the available pathways in the lattice [8]. 

The spinel blocks in (3-alumina are related by mirror planes that mn through the conduction planes that is, the orientation of one block relative to another is derived by a rotation of 180°. A second form of this compound, called (3"-alumina, has similar spinel blocks. However, these are related to each other by a rotation of 120°, so that three spinel block layers are found in the unit cell, not two. The ideal composition of this phase is identical to that of (3-alumina, but the unit cell is now rhombohedral. Referred to a hexagonal unit cell, the lattice parameters are a = 0.614 nm, c = 3.385 nm. The thickness of the spinel blocks and the conduction planes is similar in both structures.3... 

The disposition of the atoms in the conduction plane itself is identical to that in (3-alumina (Fig. 6.10b). However, the local environment of the conduction planes is different in the two phases because the oxygen ions on both sides of the conduction plane are superimposed in (3-alumina and staggered in (3"-alumina (Fig. 6.KM). This means that Beevers-Ross and anti-Beevers-Ross sites are not present. Instead the sodium sites, which have a tetrahedral geometry, are called Beevers-Ross-type (BR-type). The mid-oxygen (mO) sites, located midway between the BR-type sites, also have a different geometry in the two phases. 

In both of these materials the distribution of the ions in the conduction planes changes with temperature. At high temperatures the large cations tend to occupy all suitable sites in a random manner. Thus in (3-alumina the BR, aBR, and mO sites, and in (3"-alumina the BR-type and mO sites, are occupied statistically. 

Figure 6.10 Structure of (3-alumina (a) idealized structure projected down [110] drawn as Al3+ centered polyhedra (h) conduction plane, containing one Na+ and one bridging O2-ion per unit cell, this forming the in-conduction-plane vertex of the tetrahedra in (a).

The open nature of the conduction planes allows the Na+ ions to be easily replaced in all these phases. In general, the (3-alumina phase is less flexible to replacement the 3"-alumina more so, and the sodium can be replaced by almost any monovalent, divalent, or trivalent ion. The idealized formulas of these exchanged solids are A+A1h017, Ao A1iiOi7, or Aoj AlnOi . 

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.

The phase Na2Sx is sodium polysulfide, a material with a sulfur content of between 3 and 5. The anode reaction takes place at the liquid sodium - (3"-alununa interface. Here sodium atoms lose an electron and the Na+ ions formed enter the conduction planes in the electrolyte. The cathode reaction, which occurs at the interface between the (3"-alumina and the liquid sulfur forms sodium polysulfides. Despite the desirable properties of the cell, technical and economic considerations have acted so as to curtail large-scale commercial production. 

The above two mechanisms may be regarded as isolated ion hops. Sometimes, especially in solid electrolytes, cooperative ion migration occurs. An example is shown in Fig. 2.1(c) for the so-called interstitialcy or knock-on mechanism. A Na" ion. A, in an interstitial site in the conduction plane of j -alumina (see later) cannot move unless it persuades one of the three surrounding Na ions, B, C or D, to move first. Ion A is shown moving in direction 1 and, at the same time, ion B hops out of its lattice site in either of the directions, 2 or 2. It is believed that interstitial Ag" ions in AgCl also migrate by an interstitialcy mechanism, rather than by a direct interstitial hop. 

The beta-alumina structures show a strong resemblance to the spinel structure. They are layered structures in which densely packed blocks with spinel-like structure alternate with open conduction planes containing the mobile Na ions. The and /S" structures differ in the detailed stacking arrangement of the spinel blocks and conduction planes. Fig. 2.9. 

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. 

As long as the /1-alumina sensor remains homogeneous as far as Na+ is concerned (which is achieved by the high fraction of Na20), we see from Eqn. (15.6) that the electron potential varies inversely with the oxygen activity. We have already mentioned that /1-alumina is able to incorporate a number of different cations into the conducting plane. This non-specificity hampers the use of / -alumina as a universal sensor material under ordinary conditions. If more than one mobile component is... 

Single crystals of /S-A1203 are essentially two dimensional conductors. The conducting plane has hexagonal symmetry (honeycomb lattice). This characteristic feature made -alumina a useful model substance for testing atomistic transport theory, for example with the aid of computer simulations. Low dimensionality and high symmetry reduce the computing time of the simulations considerably (e.g., for the calculation of correlation factors of solid solutions). 

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. 

Fig. 4.30 Structure of /f -alumina (a) alternating spinel blocks and conduction planes (b) migration pathway of Na+ ions indicating paths of concerted motion.

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. 

The preparation of pure //"-alumina is not easy and dopants (e.g. Mg and Li) are added to stabilize the modification. The dopants have the added advantage of reducing interstitial oxygen in the conduction plane and therefore facilitating the movement of the sodium ions. 

In the so-called superionic conductors (see Fig. 44) virtually all constituents are carriers and usually the migration thresholds are very small ( 0.1 eV in flf-Agl) leading to very high ion conductivities (cf. Section IV.5.//). The most popular example is a-Agl the crystal structure of which is displayed in Fig. 45.187 Another example is fi-alumina in which there is a 2D superconduction in so-called conduction planes.198 A detailed discussion of individual crystal structures would lead us too far from the scope of this contribution (see, e.g., reference10). 

Correlation between the moving species is another important factor to consider, as all the atoms of a given sublattice (i.e. cationic in a-AgI, anionic in fluorites) or aU the atoms of the conducting plane (jS-alumina) may be involved in the conduction process. 

Figure 6 Crystal structure of /3-alumina and sodium sites in the conduction plane (a) unit cell structure (b) site model of conduction plane. (Reprinted from Ref. 76 1978, with permission from Elsevier)...

There is a need for far more experimental studies on the formation and reactivity of these intergrowth phases, as to date there is a paucity of reliable information. However, the ease with which CS planes and the conduction planes in j8-alumina are able to grow or shrink suggest that they will prove to be the seat of an enhanced chemical reactivity. This aspect of these phases has been hardly studied at all to the present time and further experiments would be of considerable interest. 

In the /i"-alumina structure, the phase is stabilized at high temperatures by small amounts of monovalent (e.g., Li20) or divalent (e.g., MgO, ZnO, NiO) oxidesIn these stabilized structures, the cation dopant substitutes directly for trivalent aluminum ions in the spinel block (i.e., LiXi, MgAi) and is electrically compensated by additional sodium ions (Nai) in the conduction plane. 

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... 

Because of charge compensation effects, Li20 stabilization leads to twice the number of additional sodium ions in the conduction planes (per mole of cation dopant) than is the case for MgO (or ZnO or NiO) stabilization. Li20 and MgO can be intermixed in appropriate combinations to form a 8"-alumina composition with mixed... 

The conductivity data in Table 1 of 17.3.7, show (1) that the Na" ion conductivity is higher in the monocrystalline form for each structural type and (2) that the Na" ion conductivity in the fS" phase is significantly higher than that in the phase. The Na ion conductivity in polycrystalline /8"-alumina is comparable to that in monocrystal j6-alumina. The higher ionic conductivity in the jS" structure compared to the phase is due to the higher concentration and mobility of sodium ions in the conduction planes. 

Figure 7.5 Costal structure of sodium p-alumina [57] one of the two Na + -conducting planes, together with the two adjacent spinel blocks. Na ions = gray AIO4 tetrahedra and AIO, octahedra = white.

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