Conduction plane cationic

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. 

The Na+ ions in the conducting plane have been substituted for by many other (mostly monovalent) cations, and Al3+ in the spinel block has also been substituted for by other di- and trivalent cations. This exchange results in a very complex crystal... 

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... 

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. 

The conducting ions, such as Na, populate the planes between the spinel blocks. For optimum two-dimensional ion conduction in these planes, it is preferable that not all the available sites be occupied by the mobile cations. As the temperatures increases, the mobile ions in these conducting planes become disordered and occupy positions at random. The ionic mobility of the Na in these planes is higher in the /J structure than it is in the / " structure because of the particular configuration of the bridging oxygen ions that act as obstacles to ionic motion within the plane. Na ion conduction is anisotropic and two-dimensional within these conduction planes for both the p and structures. No ionic conduction exists in a direction perpendicular to the conduction plane. 

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. 

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... 

Cation-conducting electrolytes (see Chapter 7) are also used in electrochemical sensors, one of the most widely used being 3 alumina [55, 56]. The 3 alumina structure consists of relatively densely packed spinel blocks that are separated by less densely packed planes through which ionic conduction occurs. The most common example is sodium 3 alumina, which is a Na+ ion conductor, although the sodium can be exchanged with other ions to create electrolytes that conduct other cations [57], as well as other species, such as O [58] (see also Chapter 8). 

The sodium ions in the conduction planes of 3- and 3"-alumina type structures are exchangeable with many other monovalent and divalent cations. The main host crystals are p and -alumina, and the Ga, Fe analogues P- and P"-gallate, P- and P"-ferrite. The guest ions which contain protons include (in this review the use of H30 does... 

Frequency data can also provide important information about the potential energy environment of the mobile ionic species. In the previously mentioned study Colomban et al which included NH4" 3-alumina, the motion of the cations in the conduction plane was assumed to occur in the periodic potential... 

Quantitative determination of water reorientational and diffusional modes by NMR and 0 tracer experiments on the analogous HUAs provided no correlation with the data for proton conductivity . Instead a good correlation between molecular diffusion and proton conductivity indicated a mechanism in which H2O and HjO are mobile as a whole in the conduction plane without a significant amount of proton transfer between them. The proton conductivity in HUP and HUAs is somewhat higher than that of other monovalent cations in corresponding compounds. This was one of the reasons why molecular diffusion was not... 

Every ionic crystal can formally be regarded as a mutually interconnected composite of two distinct structures cationic sublattice and anionic sublattice, which may or may not have identical symmetry. Silver iodide exhibits two structures thermodynamically stable below 146°C sphalerite (below 137°C) and wurtzite (137-146°C), with a plane-centred I- sublattice. This changes into a body-centred one at 146°C, and it persists up to the melting point of Agl (555°C). On the other hand, the Ag+ sub-lattice is much less stable it collapses at the phase transition temperature (146°C) into a highly disordered, liquid-like system, in which the Ag+ ions are easily mobile over all the 42 theoretically available interstitial sites in the I-sub-lattice. This system shows an Ag+ conductivity of 1.31 S/cm at 146°C (the regular wurtzite modification of Agl has an ionic conductivity of about 10-3 S/cm at this temperature). 

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