Mixed oxides, structure types fluorite

It was rather surprising when Hund and Durrwachter [312] found that La20s is miscible with TI1O2 to a great extent (52 mole per cent) whilst still preserving the cubic fluorite structure. The lattice constant of the mixed oxide has an a value 5.645 A compared to 5.592 A for TI1O2. The lattice constants of some orthorhombic perovskite and cubic garnet-type europium compounds are listed in Table 22. 

Praseodymium is one of the possible "new" additives, which today attracts more and more attention. In fact, it was demonstrated that the oxygen exchange occurs at lower temperature on cerium-praseodymium mixed oxides than on ceria [7]. Furthermore, high temperature pretreatment does not affect the oxygen exchange capacity (OSC) of the mixed oxides. Moreover, high surface area PrOy-Zr02 materials with a fluorite-type structure were also prepared by the sol-gel way [7, 8]. 

We prepared by sol-gel some thermally stable Zro.io(Cei-xPrx)o.9o02 mixed oxides (x between 0 and 0.75) with a fluorite-type structure. This structure was confirmed by the presence, in the Raman spectrum, of a single band ca. 460 cm characteristic of the M-0 vibration in the fluorite-type structure. Moreover, the band position and the shoulder at 570 cm indicate the presence of oxygen vacancies probably associated with praseodymium cations. Consequently, high OSCs appears to be the result of the presence of both cerium and praseodymium atoms. Thus, addition of praseodymium atoms into zirconia-ceria oxides appears to be very promising for the design of new automotive catalysts. 

The other oxides listed in Table 9.1 are included because their mechanical properties have received some attention (especially quartz). Quartz is by no means close-packed because of the stability of the [Si04] tetrahedron. Yttria and the rare-earth sesquioxides and the mixed oxide garnet structures are compact, in spite of their complexity, but cannot be described as close-packed in the traditional sense. In fact, the C-type M2O3 structure of yttria can be thought of as having the fluorite structure of urania with one-quarter of the anion sites vacant. 

Additional attempts have been presented to render hosts with the fluorite and the related pyrochlore structure electronically conductive by doping with mixed-valence and/or shallow dopants. The list of dopant materials examined includes oxides of elements of, for example, Ti, Cr, Mn, Fe, Zn, Fe, Sn, Ce, Pr, Gd, Tb and U. In general, however, the extent of mixed conductivity that can be obtained in fluorite-type ceramics is rather limited, by comparison with the corresponding values found in some of the perovskite and perovskite-related oxides considered in the next section. 

Pyrochlores and Other Fluorite-Type Oxides (Y, Nb, Zr)02S Pyrochlore-type oxides have the general formula A2 B2 07 in which A is a rare-earth element such as Gd or Y, and B is Ti or Zr. Gd2Zr207 is the typical composition of pyrochlore-type oxides and can be considered as fluorite-type in which ionic defects are regularly arranged. The defect structure and the mixed conductivity can be controlled by the value of x in, for example, Gd2(ZrxTii x)20 . which is abbreviated as GZT [69]. When the ionic radius of rare-earth elements for the A site is larger than that of Gd, the structure changes from highly defective fluorite to pyrochlore [70, 71]. 

Abstract Dense ceramic membrane reactors are made from composite oxides, usually having perovskite or fluorite structure with appreciable mixed ionic (oxygen ion and/or proton) and electronic conductivity. They combine the oxygen or hydrogen separation process with the catalytic reactions into a single step at elevated temperatures (>700°C), leading to significantly improved yields, simplified production processes and reduced capital costs. This chapter mainly describes the principles of various types of dense ceramic membrane reactors, and the fabrication of the membranes and membrane reactors. 

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