Pyrochlore series

The elements themselves require little comment. Niobium is 10 to 12 times more abundant in the earth s crust than tantalum. The main commercial sources of both are the columbite-tantalite series of minerals, which have the general composition (Fe/Mn)(Nb/Ta)206, with the ratios Fe/Mn and Nb/Ta continuously variable. Niobium is also obtained from pyrochlore, a mixed calcium-sodium niobate. Separation and production of the metals is complex. Both metals are bright, high melting (Nb, 2468°C Ta, 2996°C), and very resistant to acids. They can be dissolved with vigor in an HN03—HF mixture, and very slowly in fused alkalis. 

A homologous series of cesium tungsten oxides having the formula Cs2 W4 iOi2 has been reported. The general stmcture consists of slabs composed of layers of the hexagonal tungsten bronze and the pyrochlore type. The alkali cations reside in between the layers and in the tunnels formed within the slabs. ... 

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 ... 

General Chemical and Physical Characterization. The x-ray diffraction data, chemical analyses by x-ray fluorescence and the effects of various synthesis parameters explored in this study lead to the conclusion that a new series of pyrochlores represented by formula 1 has been synthesized. The substitution of the larger post transition element cation for the noble metal cation on the octahedrally coordinated B-site leads to a considerable enlargement of the pyrochlore s cubic unit cell dimension. The relationship between lattice parameter (ag) and extent of substitution of ruthenium by either lead or bismuth is linear as shown in Figure 1. 

In the case of the Bi2[Ru2-x x]07-y series, it is possible to explain the bismuth substituted pyrochlore either as resulting from the substitution of ruthenium by Bi ", with subsequent vacancy formation on the anion lattice, or by substitution of ruthenium by pairs of Bi and Bi5+, with no anion vacancy formation necessary. Any of the above valence distributions for the bismuth ruthenates could account for the expanded lattice parameter since they all involve average B-site ionic radii larger than Ru ". ... 

All the cited literature references to the above compounds have described solid-state syntheses at temperatures of 700-1200°. Such synthesis conditions will always lead to pyrochlore structure compounds in which all of the octa-hedrally coordinated sites are occupied by the noble metal cation, thus requiring the post-transition metal to noble metal molar ratio always to be 1.0. This paper focuses on solution medium syntheses at quite low temperatures (<75°), thereby stabilizing a new class of pyrochlore compounds in which a variable fraction of the octahedrally coordinated sites are occupied by post-transition element ca-tions.5,6 The specific example here involves the Pb2[Ru2 Pb4+]06 s series. The synthesis conditions may be simply adapted, however, to accommodate preparation of a wider range of pyrochlores which can be described by the formula A2[B2 xAx]07.3> where A is typically Pb or Bi, B is typically Ru or Ir and 0 < 1, and 0 < 1. 

The present work was focused on the synthesis of nanocrystalline components for electrochemical cells via the cellulose-precursor technique. This method was used to prepare nanostructured intermediate-temperature (IT) SOFC anodes made of a series of cermets comprising gadolinia-doped ceria (CGO), yttria-stabilized zirconia (YSZ), Gdi.86Cao.i4Ti207.5 (GCTO) pyrochlore, metallic nickel and copper. Perovskite-type SrFcojAlo.sOs.s (SFA) powder, also obtained via the cellulose precursor, was applied onto membranes of the same composition to enhance specific surface area and electrocatalytic activity in the reactors for methane conversion [3]. 

In this cross-sectional specimen, it can be seen that the residual pyrochlore phase extends across the entire surface of the film. The presence of this layer results in a degradation of the electric properties of the film due to a series dilution effect that is, the pyrochlore phase has a low dielectric constant compared to the perovskite phase, and in addition, is nonferroelectric. In a planar specimen, prepared by thinning from the backside, it is possible to use selected area electron diffraction (SAED) to identify the two phases. Results are shown in Figure 12.6 and c. 

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