Oxide Fluorites

W. Liu, and M. Flytzani-Stephanopoulos, Transition metal-promoted oxidation catalysis by fluorite oxides A study of CO oxidation over Cu—Ce02, Chem. Eng. J. 64, 283—294 (1996). 

Anion-deficient fluorite oxides are also present, for example, U02- c, Ce02-x The presence of anion vacancies in reduced fluorites has been confirmed by diffraction studies. In reduced ceria for example, some well-ordered phases has been reported (Sharma et al 1999). The defective compounds show very high anion mobilities and are useful as conductors and as catalytic materials as will be described later. However, the structures of many anion-deficient fluorite oxides remain unknown because of the shear complexity of the disordered phases. There are, therefore, many opportunities for EM studies to obtain a better understanding of the defect structures and properties of these complex materials which are used in catalysis. 

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... 

In this structure there are perovskite layers of ABO3 separated by AO rock salt layers. It is this layered structure that allows great flexibility in the oxygen stoichiometry of these materials. It is possible to incorporate excess oxygen (5 > 0) in the unusual form of interstitial oxygens, which provide an alternative to the vacancy-based conduction mechanism present in the perovskite and fluorite oxides, where the dopant-vacancy interactions can limit the observed conductivity. The mobility of the oxide ions in these materials occurs mainly through an interstitialcy mechanism in the aZ)-plane, although evidence of low Ea for the conduction in the c-direction via a Frenkel mechanism has also been reported. ... 

Wachsman, E.D., Effect of oxygen sublattice order on conductivity in highly defective fluorite oxides. Journal of the European Ceramic Society, 2004, 24, 1281-1285. 

Analyzing published data for D and kg, Kilner [73] noted that the two parameters seem to be correlated. A square root dependence of kg with D is found for perovskite oxides ABO3, albeit that the data show substantial scatter, yielding an average value for D /kg of about 100 pm. For fluorite oxides MO2 the two parameters are correlated almost linearly, while the corresponding value of D /kg ranges between mm and cm. The results suggest that related... 

J. Kilner and B.C.H. Steele, Mass transport in anion-deficient fluorite oxides, in O. Sorenson (Ed.), Nonstoichiometric Oxides. Academic Press, New York, 1981, pp. 233-269. 

Liu W. and Flytzani-Stephanopoulos M. 1995. Total oxidation of carbon monoxide and methane over transition metal-fluorite oxide composite catalysts, J. Catal., 153, 304-316. Sedmak G., HoCevar S. and Levee J. 2004. Transient kinetic model of CO oxidation over a... 

Fluorite oxides are the most common and classical oxygen ionconducting materials. The crystal structure consists of a cubic oxygen lattice with alternate body centers occupied by 8-coordinated cations. The cations are arranged into a face-centered cubic structure with the anions occupying the tetrahedral sites. This leaves a rather open architecture, with large octahedral interstitial voids, as shown in Figure 1-6. 

LANTHANIDES ( u AND GdJ MOSSBAUER SPECTROSCOPIC STUDY OF DEFECT-FLUORITE OXIDES COUPLED WITH NEW DEFECT CRYSTAL CHEMISTRY MODEL... 

Total oxidation of carbon monoxide and methane over transition metal fluorite oxide composite catalysts I. Catalyst composition and activity. 

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