Oxides pyrochlore structure

Some pyrochlore, A2B2O7, phases are moderately good oxide ion conductors. The pyrochlore structure may be regarded as a fluorite derivative in which g of the oxygens are missing but since the oxygen sublattice, ideally, is fully ordered, it is necessary to introduce defects to achieve high conductivity. 

A2Pt207, similar to those reported for tin, ruthenium, titanium, and several other tetravalent ions. Trivalent ions which form cubic platinum pyrochlores range from Sc(III) at 0.87 A to Pr(III) at X.14 A. Distorted pyrochlore structures are formed by lanthanum (1.18 A) and by bismuth (1.11 A). Platinum dioxide oxidizes Sb203 to Sb2(>4 at high pressure. The infrared spectra and thermal stability of the rare earth platinates have been reported previously and will not be repeated here, except to point out the rather remarkable thermal stability of these compounds decomposition to the rare earth sesquioxide and platinum requires temperatures in excess of 1200 °C. 

Other three-dimensionaUy connected oxide structures show ion-exchange chemistry and sorption of small molecules such as water or ammonia. These inclnde oxides with the A2B2O7 pyrochlore structure, KSb03 related phases, and LiNb03. Compositions with these structure types can be exchanged in mineral acids or in molten salts. 

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. 

M.P. Dijk, K.J. de Vries, A.J. Burggraaf, A.N. Cormack and C.R.A. Catlow, Defect structures and migration mechanisms in oxide pyrochlores. Solid State Ionics, 17 (1985) 159-167. 

While metal oxides and mixed metal oxides have often been considered for various electrocatalytic applications, they are often limited by low electronic conductivity and/or low surface area. One series of mixed metal oxides with the pyrochlore structure has been discovered ( ) that has demonstrated high catalytic activity for electroreduction and electroevolution of oxygen 2) and the selective electrooxidation of certain organics 3). These materials, which are characterized by high electronic conductivity and can be prepared in high surface area form, are described by the general formula ... 

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