Pyrochlore fluorite-type

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. 

In this chapter, we present some latest analysis results of lanthanides (Lns Eu and Gd)-M6ssbauer structure and powder X-ray diffraction (XRD) lattice parameter (oq) data of defect-fluorite (DF) oxides with the new defect crystal chemistry (DCC) Oq model [ 1,2] as an upgrade of the former random oxygen coordination number (CN) Oq model [3,4]. This is, thus, the first report of our ongoing efforts to further elaborate the model and extend its applicability to more various systems, especially to pyrochlore (P)-type stabilized zirconias (SZs) and stabilized hafnias (SHs). 

Pyrochlores and Other Fluorite-Type Oxides (Y, Nb, Zr)02S Pyrochlore-type oxides have the general formula A2 B2 07 in which A is a rare-earth element such as Gd or Y, and B is Ti or Zr. Gd2Zr207 is the typical composition of pyrochlore-type oxides and can be considered as fluorite-type in which ionic defects are regularly arranged. The defect structure and the mixed conductivity can be controlled by the value of x in, for example, Gd2(ZrxTii x)20 . which is abbreviated as GZT [69]. When the ionic radius of rare-earth elements for the A site is larger than that of Gd, the structure changes from highly defective fluorite to pyrochlore [70, 71]. 

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. 

The structure of pyrochlore is considered to be an anion deficient derivative of the fluorite structure type. Ca atoms are in eight-fold coordination, while Nb atoms are in six-fold coordination. Steady-state luminescence spectra of pyrochlore revealed emission of REE, such as trivalent Dy and Nd (Gorobets and Rogojine 2001). The natural pyrochlore in our study consisted of four... 

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. 

Among other ion-conducting phases with fluorite-like structures, note should be taken of Y4Nl)Ox s, (Y,Nb,Zr)O2 8 solid solutions, and their derivatives (see Refs. [89-91] and references cited therein). The total conductivity of Y4NbO8.5-8 is essentially independent of the oxygen partial pressure, which may suggest a dominant ionic transport [90]. However, the conductivity level in this system is rather low, and similar to that in pyrochlore-type titanates and zirconates, although some improvements can be achieved by the addition of zirconia. 

Other types of oxides, non-perovskite oxides including pyrochlores and fluorites, have also been investigated. It was found that the ammonia formation rates from pyrochlore-type membrane were close to that obtained from perovskite-type membranes, although in the case of the fluorite membrane, the formation rates were slightly higher (Liu et al., 2006 Xie et al., 2004 Wang et al., 2005). More comprehensive details about characteristics of each material, fabrication, cell components, and the formation rate can be found in a review presented by Amar et al. (2011). 

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