Migrating charged vacancy

In this diagram, we have shown both charged ions involved in the migrational diffusion process as well as charged vacancies which also add to the overall diffusion process. We must have electroneutrality in diffusion, i.e.- pairs of defects, and using the above example, we can write nine equations, of which the following is just one case ... 

In the absence of an electric field the charged vacancy migrates randomly, and its mobility depends on temperature since this determines the ease with which the Na+ surmounts the energy barrier to movement. Because the crystal is highly ionic in character the barrier is electrostatic in origin, and the ion in its normal lattice position is in an electrostatic potential energy well (Fig. 2.17). 

The mobile electrons frotn the lattice renew the surface ferrous sites, which are now eliminated from the lattice, but, under these conditions of acidity,- no ferric ion appears in the solution. Migration of Fe(ll) (more probably as Fe(lll)+e) towards the surface and the formation of vacancies enable the excess positive charge to be resorbed in the solid. Hence, there is an inteiface transfer of electrons and ions towards the solution, combined with migration of vacancies towards the core of the particles. The net reaction is... 

An important practical way of increasing the value of c, is by means of doping with aliovalent (or heterovalent) ions. This involves partial replacement of ions of one type by ions of different formal charge. In order to retain charge balance, either interstitial ions or vacancies must be generated at the same time. If the interstitials or vacancies are able to migrate, dramatic increases in conductivity can result. 

Hematite forms by a combination of aggregation-dehydration-rearrangement process for which the presence of water appears essential. Structural details about this process at 92 °C were obtained from EXAFS (Combes et al. 1989 1990) face-sharing between Fe octahedra developed before XRD showed any evidence for hematite. It is followed by internal redistribution of vacancies in the anion framework and by further dehydration. The dehydration process involves removal of a proton from an OH group and this in turn leads to elimination of a water molecule and formation of an 0X0 linkage. The local charge inbalance caused by proton loss is compensated for by migration and redistribution of Fe " within the cation sublattice. 

Figure 15-17 Migration of F through LaF3 doped with EuF2. Because Eu2 has less charge than La3, an anion vacancy occurs for every Eu2. A neighboring F can jump into the vacancy, thereby moving the vacancy to another site. Repetition of this process moves F through the lattice.

Figure 42 shows the basic elementary ion migration processes in a low defective isotropic ion conductor with a mobility in the A-sublattice. The vacancy mechanism (Fig. 42 top) can be described by a transport process (Zv= effective charge of the A-vacancy) such as... 

As Figure 4.3 illustrates, when thermal energy promotes a bonding electron from the valence band to the conduction band, the released electrons are free to migrate throughout the lattice. However, the vacancies (i.e., holes) left behind are also free to move - in the opposite direction as electrons. One may consider these holes as positively charged species formed from loss of an electron. Thus, electrons and holes represent the two types of carriers that correspond to electrical conductivity in semiconductors. 

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