Anion conduction, fluorites

Anion conduction, particularly oxide and fluoride ion conduction, is found in materials with the fluorite structure. Examples are Cap2 and Zr02 which, when doped with aliovalent impurities. Fig. 2.2, schemes 2 and 4, are F and 0 ion conductors, respectively, at high temperature. The 3 polymorph of 61303 has a fluorite-related structure with a large number of oxide vacancies. It has the highest oxide ion conductivity found to date at high temperatures, > 660 °C. 

Unlike fluorides, pure oxides with the fluorite structure (MO2) show high anion conduction only at elevated temperatures, above about 2300 K. Suggest a reason for this. 

Ionic Conductivity (see Ionic Conductors). Pluoride anionic conductivity is observed mainly in derivatives of fluorite (Cap2) and tysonite (Lap3). If Cap2 is doped by a tervalent rare-earth metal ion, the additional fluoride ions are positioned in interstitials where they become mobile by a hopping mechanism. 

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. 

Fast anionic conduction is found mainly in sohds of the fluorite (CaF2) and fluorite-related structures. It is also observed in sohds with the perovskite, YF3, tysonate (LaFs), and simple cubic structures (for these structures, see Oxides Solid-state Chemistry aoA Fluorides Solid-state Chemistry). The smaller anions (r 1.4 A) and F (r 1.2 A) show the fastest conduction however, good anionic conductivity is also found for Cl (r 1.7 A), Br (r 1.8 A), I (r 2.1 A), and for (r 1.7 A). 

Cerium oxide, ceria, has a fluorite structure and shows oxide anion conducting behavior differ from other rare earth oxides. However, the O ionic conductivity of pure ceria is low because of a lack of oxide anion vacancies. For ion conduction, especially for anion, it is important to have such an enough vacancy in the crystal lattice for ion conduction. Therefore, the substitution of tetravalent Ce" by a lower valent cation is applied in order to introduce the anion vacancies. For the dopant cation, divalent alkaline earth metal ions and some rare earth ions which stably hold trivalent state are usually selected. Figure 9-28 shows the dopant ionic radius dependencies of the oxide ionic conductivity for the doped ceria at 800°C. In the case of rare earth doped Ce02, the highest O ion conductivity was obtained for... 

Ravez J. The inorganic fluoride and oxyfluoride faroelectrics. J. Phys. HI 1997 7 1129-1144 Reau J.M., Hagenmuller P. Fast ionic conductivity of fluorine anions with fluorite- or tysonite-type... 

In this review we will consider anion-conductive fluorides and oxyfluorides because the occurrence of oxygen in these phases is an additional parameter that controls the defect structure of fluorite- and tysonite-like solid solutions by increasing the anion vacancies or... 

V. A. Prituzhalov, E. V. Khomyakova, E. I. Ardashnikova, V. A. Dolgikh, Connection between electronic structure of cation and anionic conductivity of the fluorite-type oxyfluorides, 9-th International Meeting Fundamental Problems of Solid State Ionics Chemogolovka, 125 (2008). 

The isomorphous diiodides of Ce, Pr and Gd stand apart from all the other, salt-like, dihalides. These three, like LaH, are notable for their metallic lustre and very high conductivities and are best formulated as (Ln ,2I",e", the electron being in a delocalized conduction band. Besides the dihalides, other reduced species have been obtained such as LnsCln (Ln = Sm, Gd, Ho). They have fluorite-related structures (p. 118) in which the anionic sublattice is partially rearranged to accommodate additional anions. 

Substitution Variants. The fluorite-type structure is maintained in principle when alkaline earth elements are replaced partially by rare-earth elements. Charge compensation is achieved by occupation of additional interstitial anionic sites.The coordination of the metal atoms may increase from 8 to 9 or even 10 by this. Another way of charge balance is the partial replacement of fluorine by oxygen to form oxyfluorides. Since the possible interstitial positions provide pathways for anion disorder and movement, this class of materials shows fluoride ionic conductivity. 

The fluorite oxides are the classical oxygen ion conducting oxide materials the study of these materials as electrolytes derives from the early investigations of Walther Nemst 1900. The fluorite structure illustrated in Figure 13 is best described for the purposes of the present discussion as a primitive cubic array of anions (O ) with half the cube centers occupied by cations the latter form a face-centered-cubic (fee) arrangement of the cation sublattice. The unoccupied cube centers play a central role in the defect physics and ionic conductivity of fluorites, because they... 

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