Pyrochlores, crystal structure

Other oxygen ion conductors that have potential use as solid electrolytes in electrochemical devices are stabilized bismuth and cerium oxides and oxide compounds with the perovskite and pyrochlore crystal structures. The ionic conductivity and related properties of these compounds in comparison with those of the standard yttria-stabilized zirconia (YSZ) electrolyte are briefly described in this section. Many of the powder preparation and ceramic fabrication techniques described above for zirconia-based electrolytes can be adapted to these alternative conductors and are not discussed further. 

A new family of high conductivity, mixed metal oxides having the pyrochlore crystal structure has been discovered. These compounds display a variable cation stoichiometry, as given by Equation 1. The ability to synthesize these materials is highly dependent upon the low temperature, alkaline solution preparative technique that has been described the relatively low thermal stability of those phases where an appreciable fraction of the B-sites are occupied by post transition element cations precludes their synthesis in pure form by conventional solid state reaction techniques. 

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. 

For our present purposes, simple crystal structures might be segregated into three groups. The first of these is comprised of those few structures with even fewer parameters than independent bond lengths examples already encountered above include the (cubic) perovskite, pyrochlore and Na3Pt04 structures. 

Mazzi, F. Munno, R. 1983. Calciobetafite (new mineral of the Pyrochlore Group) and related minerals from Campi Flegrei, Italy crystal structures of polymignite and zirkelite comparison with pyrochlore and zirconolite. American Mineralogist, 68, 262-276. 

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. 

Ceria may be dissolved in YSZ, which also has the fluorite structure. Then a driving force for reduction of Ce to Ce exist because the radius of Ce is big enough to facilitate the formation of the pyrochlore compound, CejZrjO- , which has a crystal structure similar to the fluorite but with fully ordered oxide vacancies. This tends to segregate to the grain boundaries and induces electronic conductivity into the YSZ. ... 

The pyrochlore-type compounds, where the crystal structure is usually considered as a cation-ordered fluorite derivative with % vacant oxygen site per fluorite formula unit, constitute another large family of oxygen anion conductors [9, 33, 41—43, 84—88]. The unoccupied sites provide pathways for oxygen migration furthermore, the pyrochlore structure may tolerate formation of cation and anion vacancies, doping in both cation sublattices, and antistructural cation disorder. Regardless of these factors. 

M.A. Subramanian and A.W. Sleight, Rare earth pyrochlores 225 R. MiyawaM and I. Nakai, Crystal structures curare earth minerals 249 D.R. Chopra, Appearance potential spectroscopy of lanthanides and their intermetallics 519 Author index 547 Subject index 579... 

Brannerite, when fully reduced, is brown in colour but, like uraninite, it darkens with oxidation to a pitchy black colour. Its crystal structure is related to the perovskites, pyrochlores and columbites in that it is based on a framework of linked octahedra of (Ti,Ta,Nb)06 units with interstitial U, Ca, Th and rare earths. These ions substitute rather freely for one another. The brannerite structure is shown in Fig. 4. The structural unit is a sheet of corner and edge-shared TiOe octahedra with UOe octahedra cross-linking these sheets. The sheet structure is closely related to the anatase form of Ti02. The monoclinic structure results from the nature of the sheet, which steps one-... 

For Ln = Sm to Lu, LnaTiaOy crystallizes in the pyrochlore/cubic structure in bulk. In this structure, the Ln cations occupy the 16d site and are coordinated with 8 oxygen ions, while Ti occupy the 16c site and are located at the center of the distorted octahedra (using Wyckoff notation). The pyrochlore/cubic structure is a superstructure of the ideal fluorite with 1/8 anion defect. The lattice parameter evolves from = 10.231(1) A [59,60] to 10.024 A [61] for Sm to Lu, respectively. 

The addition of phosphoric acid to freshly made antimonic acid (see Section 4.1.3) prevents precipitation of the pyrochlore phase and allows stabilization of phosphatoantimonic high polymers as sols [28]. Incorporation of phosphates in the high polymers alters their crystal structure [29] and leads to a significant reduction in their size and quantity, to the benefit of phosphatoantimonic polyanions. The ratio P/Sb = 0.5 is the threshold for the existence of high polymers in solution. The composition and degree of condensation of the polyanions are functions of the pH... 

Fig. 11 Crystal structure of an A2B2O6O pyrochlore. B cations (blue) are located in a distorted octahedral coordination and A cations (yellow) are in an eight coordinate environments. Anions, in four coordinate environments, are represented by red spheres...

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