Structural defects, solid electrolytes

Cul) is not due to point defects but to partial occupation of crystallographic sites. The defective structure is sometimes called structural disorder to distinguish it from point defects. There are a large number of vacant sites for the cations to move into. Thus, ionic conductivity is enabled without use of aliovalent dopants. A common feature of both compounds is that they are composed of extremely polarizable ions. This means that the electron cloud surrounding the ions is easily distorted. This makes the passage of a cation past an anion easier. Due to their high ionic conductivity, silver and copper ion conductors can be used as solid electrolytes in solid-state batteries. 

One of the most important aspects of point defects is that they make it possible for atoms or ions to move through the structure. If a crystal structure were perfect, it would be difficult to envisage how the movement of atoms, either diffusion through the lattice or ionic conductivity (ion transport under the influence of an external electric field) could take place. Setting up equations to describe either diffusion or conductivity in solids is a very similar process, and so we have chosen to concentrate here on conductivity, because many of the examples later in the chapter are of solid electrolytes. 

Often this term is used for - solid electrolytes and/or for solids with structural disorder (see -> defects in solids), although all these designations are not synonyms. The high concentration of defects, necessary for fast ionic conduction, may be induced by external factors such as - doping, electromagnetic forces, radiation, etc. Creation of these defects may lead to the generation of - electron - charge carriers and, thus, induce electronic - conductivity. 

Perovskite-structured oxides with high electronic and oxygen ion conductivities could be used as a membrane alternative to solid electrolytes for oxygen separation. In such materials, both oxygen ions and electronic defects are transported in an internal circuit in the membrane material. 

One of the most important practical applications of lithium compounds is as fast ion conductors with potential electronic applications such as solid electrolytes for lithium batteries. Li20 is a fast ion conductor in which the Li ions occupy a simple cubic sublattice with the antifluorite structure. Both MAS and static Li NMR spectra of Li20 have been reported, the former recorded as a function of temperature up to 1000 K (Xie et al. 1995). The effect of introducing vacancies on the Li sites by doping with LiF has been studied by high-temperature static Li NMR, which reveals the interaction of the Li defects > 600 K and the appearance of 2 distinct quadrupolar interactions at about 900 K. Measurements of the relative intensities of the satellite peaks as a function of temperature have provided evidence of thermal dissociation of an impurity-vacancy complex (Xie et al. 1995). 

Based on the Wagner method [16], the required character of conductivity can be achieved by the introduction of admixture into the basis oxide, which has a common anion with the basis oxide, and the cation has less valence. Type and quantity of the defects are stipulated by the admixture and its concentration. As a rule, the majority of well-known solid electrolytes with pure oxygen-ionic conductivity have a fluorite CaFj crystalUne structure [17]. 

Only particles in the close vicinity of the solid electrolyte surface take place in the electrochemical reaction. According to this fact, the surface layer possessing the defect structure in the double electrical layer can be... 

Given that mullite is a defect structure, one would expect high ionic conductivity. Rommerskirchen et al. have found that mullite has ionic conductivity superior to that of the usual CaO-stabilized Zr02 solid electrolytes at temperatures from 1,400 to 1,600°C [52], The oxygen self diffusion coefficient in the range 1,100 single crystal of 3 2 mullite has been given by [53] ... 

This is accepted for liquid electrolytes (33), but it may seem surprising that the use of ionic cind electronic imperfections (3 ) cein be avoided when treating solid media indeed, defects are undoubtedly responsible for transport in solid electrolytes, By resisting this temptation, however,(as Wagner did origineilly) we actually arrive at formulas of a more general nature vrtiich ceui be specialized later in accordance with whatever defect structure may seem most appropriate. 

The crystallographic structure in solids with fixed positions for atoms or ions restricts the free mobility of the ions. The ionic conductivity observed in solid electrolytes is based on defects and disorder in the crystallographic structure. One can distinguish... 

Once considered rare among sohds, fast ionic conduction has been found characteristic of hundreds of compounds. R.V Kumar and H. Iwahara discuss the science and application of these rare-earth superionic conductors as solid electrolytes. Conduction by oxygen and fluorine anions as well as hydrogen and other cations associated with these electrolytes are emphasized. They deal with extrinsic and intrinsic types together with their associated structures and structural types including structural defects. They conclude by outlining the many phcations of these solid electrolytes. 

Baumard, J. F., and Abelard, P. (1984). Defect structure and transport properties of ZrOj-based solid electrolytes. Advances in Ceramics, Vol. 12, pp. 555-571, Claussen, N., Ruble, M., and Heuer, A. H., eds., Columbus, OH The American Ceramic Society. 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

HEYNE As you stated a lot of confusion exists due to different points of view from which solid electrolyte problems are approached. I should like to emphasize that such confusions could be considerably reduced if thermodynamic arguments and model considerations would always be clearly separated. For instance splitting up of a components chemical potential into either an electrochemical potential plus an electrostatic potential [usual in normal electrochemistry) or into an ion electrochemical pot. plus an electron chem. pot. (= Fermi level), is completely arbitrary [and unnecessary) from a purely thermodynamic point of view. As soon as we split in one way or another we must be aware, and clearly state, that we use a certain model such as for instance the band picture of a semi-conductor, or the defect structure of a solid electrolyte. 

As mentioned, the solid electrolytes are sintered metal oxides with mobility of ions where the ionic conductivity is influenced by both the microstructure and geometry. The effects of composition, structure, microstructure, and strain on ionic transport at grain boundary provided complementary tools for futiu-e developments in solid electrolyte materials. Among these, a particular attention was given to the impact on ionic transport of defects in various types of structures, dislocations, grain boundaries, and heterostructure interfaces. The design of such structural properties also considered the achievements of the development in nanotechnologies. 

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