Mixed oxides, structure types pyrochlore

The redox properties of ceria-zirconia mixed oxides are interesting, because these materials find applications as electrolytes for solid oxide fuel cells, supports for catalysts for H2 production, and components in three-way automobile exhaust conversion catalysts. The group of Kaspar and Fornasiero (Montini et al., 2004, 2005) used TPR/TPO-Raman spectroscopy to identify the structural features of more easily reducible zirconia-ceria oxides and the best method for their preparation by suitable treatments. TPR/TPO experiments and Raman spectra recorded during redox cycles demonstrated that a pyrochlore-type cation ordering in Ce2Zr2Og facilitates low temperature reduction. 

The most often found types of mixed oxides are perovskites (RBO3), K2NiF4-type oxides (R2BO4), R 1 B C>2 +1 ( = 2 or 3), lamellar perovskites, pyrochlores, spinels, and oxide solid solutions. Perovskite oxides are, by far, the most commonly used oxides. Therefore, this chapter will reflect this situation by putting more emphasis on this type of materials. Within this section the structure, preparation methods and general characteristics of the mixed oxides will be discussed. Note that R stands for rare-earth elements while A includes all types of elements. 

Taking all this nanostructural information into account, it becomes tempting to establish a direct correlation between the improved redox performance and the presence of a pyrochlore-type structure in the bulk of the metastable SR-MO oxide. Thus, on the basis of DFT studies in which the tetragonal and pyrochlore phases of a Ceo sZro 5O2 mixed oxide were compared, Wang et al recently proposed that the difference in OSC between the two structures is due to a higher localization of the structural relaxation effects... 

As described in Section 8.2.6, along with YSZ, mixed oxygen-ion, and electron-conducting oxides with a perovskite-type structure, the so-called Aurivillius phase and pyrochlore materials are fundamentally used for the production of a variety of high-temperature electrochemical devices [50-58],... 

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]. 

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