Particular fluorite structure, oxides

Other refractory oxides that can be deposited by CVD have excellent thermal stability and oxidation resistance. Some, like alumina and yttria, are also good barriers to oxygen diffusion providing that they are free of pores and cracks. Many however are not, such as zirconia, hafnia, thoria, and ceria. These oxides have a fluorite structure, which is a simple open cubic structure and is particularly susceptible to oxygen diffusion through ionic conductivity. The diffusion rate of oxygen in these materials can be considerable. 

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. 

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

For the materials with fluorite structure, the only Raman active vibrational mode of T2g symmetry is observable. This mode arises from symmetric shift of the anions relatively to cations. Therefore the ions shifts owing to the thermal effects or impurities insertion influence Raman spectra. This is important for oxide materials and for Zr02iY203 in particular, where oxygen vacancies play an important role in ionic conductivity. 

After this chapter. Part 11 is dedicated to zeolite, ceramic and carbon membranes and catalysts used in membrane reactors. In Chapter 6 (Algieri, Comite and Capannelli) the remarkable properties of zeolite membranes are illustrated. Moreover, the key role of zeolite membrane reactors to improve the yield and the selectivity of reactions is particularly emphasised. Furthermore, the possibility of using zeolite membranes as micro-reactors and sensors is also discussed. Chapter 7 (Tan and Li) deals with dense ceramic membrane reactors, which are made from composite oxides usually having perovskite or fluorite structures with appreciable mixed ionic (oxygen ion and/or proton) and electronic conductivity. This chapter mainly describes the principles of various configurations (disc/flat-sheet, tubular and hollow fibre membranes) of dense ceramic membrane reactors and the... 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

Although the dioxides of zirconium and hafnium crystallize at room temperature in the monoclinic baddeleyite-type structure, this is closely related to fluorite, and they do form fluorite-type solid solutions with rare earth oxides which have important ceramic properties. Compared with Th, U", Ce etc., Zr " and Hf have considerably smaller ionic radii which are close to that of the smallest ion (Sc ), and this fact has an important influence on the phase relationships in these systems, leading to the appearance of intermediate phases not encountered in other MO2-R2O3 systems. They have been studied extensively, particularly in the first instance by Collongues et al. (1965 and references therein). Work published prior to 1964 has been reviewed by Mobius (1964). 

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