Pyrochlore type structure

The compounds of the MMe205F type, where Me = Nb or Ta M = Rb, Cs, Tl, crystallize in cubic symmetry and correspond to a pyrochlore-type structure [235-237]. This structure can be obtained from a fluorite structure by replacing half of the calcium-containing cubic polyhedrons with oxyfluoride octahedrons. 

Oxygen ion resistivity of polycrystalline material 18, 17.3.7 Pyrochlore-type structure 18, 17.3.7... 

Omataz T, Otsuka-Yao-Matsuo S. Electrical properties of proton-conducting Ca + Doped La2Zr207 with a pyrochlore-type structure. J Electrochem Soc. 2001 148(6) E252. 

The stability of the perovskite structure depends on Goldschmidt s tolerance factor (rA +ro)/> (rB +ro), with 0.7 < t < 1.2. For tolerance factors outside this range nonperovskite structure are stable, including pyrochlore-type structures A2B2X6Z, AuriviUius phases (AuriviUius, 1949), or tetragonal tungsten bronze (TTB)-type structures Ba2MTi2X30i5 (M = Ln,Bi X = Nb,Ta) (Stennett et al., 2005). 

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... 

Xie et al. have synthesized a new tin-titanium oxy-hydroxy-fluoride Sni 24Tii 9403.66(0H)i 5qFi 42 exhibiting a pyrochlore-type structure. The synthesis... 

Scandium ternary nitrides 64 4.3.2. Pyrochlore-type structure 86... 

The compounds characterized by X Me = 3.5 have a common formula of M2Me205F2 and crystallize either in a pyrochlore [192] or a veberite [229] type structure. According to X-ray powder diffraction patterns, the structure of Na2Nb205F2 can be regarded as a super-structure of pyrochlore, which is made up of octahedrons connected in layers and arranged in the (111) direction. The layers are linked via octahedrons so that each octahedron in one layer shares three vertexes with an octahedron in the adjacent layer. 

The crystal structure of cadmium rhenium(V) oxide, as determined by single-crystal technique,1 is of the face-centered cubic pyrochlore type (a = 10.219 A.). The only positional parameter for the 48 (/) oxygens is x = 0.309 0.007 when rhenium is at the origin. The density, determined pycnometrically, is 8.82 0.03 g./cc., compared with the theoretical value of 8.83 g./cc. for Z = 8. The resistivity between 4.2 K and room temperature is very low (10-3-10-4 J2-cm.) and has a positive temperature coefficient. Over the same temperature range the magnetic susceptibility is low and temperature-independent. These properties indicate that cadmium rhenium(V) oxide exhibits metallic conductivity. 

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],... 

Another simplification consists in the fact that among the various structural types of fluorides, the involved d-transition elements generally possess the coordination number 6. The crystallographic features can be deduced from the arrangement of (MF6) octahedra5,6. Besides in three-dimensional (3-D) networks such as found in perovskite, rutile or pyrochlore types for instance, fluorides crystallyze in two-dimensional (2-D) layer structures, one-dimensional (1-D) chain structures and isolated unit arrangements. 

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. 

Crystalline pyr-FeFs , a metastable cubic form of FeFs with pyrochlore-related structure, was obtained in a topotactical oxidation reaction of NH4Fe2F6 with Br2 in acetonitrile. At this example, the designation "chimie douce (soft chemistry) for this type of solid-state reaction under low-temperature conditions was propagated in the 1980s. ... 

Pyrochlore- and RbNiCrFg-Type Structures. Closely related to the weberite variants are pyrochlores AA M2Fv (A = Na, A = Ca, Cd M = Mg, Co, Ni, Cu, Zn). Here, all octahedra are equivalent and the cubic 3D network can be described as intersecting systems of Kagome nets with planes parallel to the faces of a tetrahedron. Another approach is the description as cross-linked octahedral chains where at the crossing points typical tetrahedra of comer-connected octahedra are generated (Figure 31). 

RU2EU2O7. — Rare earth compounds of the pyrochlore type, Ru , belong to the space group FdSm (Oj) and show analogy with the structures of U02 and U4O8. The lattice parameter [333] of Ru2Eu2C>7 is a = 10.252 A. 

FIGURE 4.14. Intergrowth tungsten bronze structure of the pyrochlore type (single tunnel) [4.30],... 

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