Pyrochlore structures

The nonstoichiometric pyrochlore structure phase has a composition range from Lu2Ti207 to Lu2Tij3L7O5.35 at 1400°C (Fig. 4.2). What point defects might account for this ... 

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. 

The bixbyite structure of e.g. SC2O3 represents a solution to the problem of filling three quarters of the tetrahedral sites of cubic close-packed Sc with O. It is not the simplest solution that would be the structure obtained by filling the tetrahedral sites not filled in CU2O (i.e. the pyrochlore structure of AgSb03, with Ag, Sb replaced by Sc, cf. Sect. 2.1.1.3). With normal Sc-O bond lengths that structure would have short O... O distances, and so an alternative structure that allows adjustment of the O... O and M... M distances is adopted. 

The effect of a-decay on the pyrochlore structure Gd2Ti207 doped with 244Cm has been investigated in detail (Weber et al. 1998). The material was completely amorphized at a dose of 3.1 x 10 8 a-decays/g. Amorphization is accompanied by volume expansion and increase of dissolution rate (by approximately a factor of... 

Chakoumakos, B. C. 1984. Systematics of the pyrochlore structure type, ideal A2B2XeY. Journal of Solid State Chemistry, 53, 120—129. 

Figure 35. The interpenetrating pyrochlore structure in Hg2Nb207. Circles in order of decreasing size represent Hg, Nb, and O. Interpenetrating adamantane units of the [Nb206 ] net (black bonds) and the [Hg202+] net ( open bonds) are shown.

Cs4V ioMo2033 K2Nb2W2012 amwo6 X. pyrochlore structure c... 

FIGURE 8.11 Schematic representation of the eight fluorite unit cells required to represent the pyrochlore structure. 

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. 

Goodenough and co-workers [10] made a detailed study of the solid state chemistry and electrochemistry of ruthenates of general formula Bi(2 2x)P-b2xRu20(7. v) with the pyrochlore structure and reported that the electroreduction of oxygen proceeds at low overpotentials according to the electrokinetic equation... 

MFv(OH)3 v (0.4 < x < 2.07) [83] occurring in the pyrochlore structure. This is more open than the HTB-structure because slightly distorted hexagonal channels are located along all six plane diagonals of the cubic cell [84],... 

Fig. 9. Linking of MF6 octahedral (a) in the structure ofa-AlF3 (b) in the HTB structure of /J-A1F3 in [001] direction and (c) in the pyrochlore structure of A1F2 3(OH)o 7 FLO in [110] direction (one of six channel directions).

At first sight, the catalytic behaviour and the surface properties of pyrochlore Al(OH,F)3 does not fit this model since the pyrochlore structure (cf Fig. 9(c)) is a more open one than the HTB-A1F3 structure. However, since aluminium hydroxofluoride is susceptible to thermal decomposition, it is in fact no longer pyrochlore Al(OH,F)3 under the temperature conditions employed for the catalytic reactions. Thus, the behaviour of this phase in heterogeneous catalytic halogen exchange can be explained by the presence of amorphous alumina which determines the surface characteristics at the initial stage. Consequently, this phase acts in a manner similar to alumina and not until the surface becomes completely fluorinated does it reach its full catalytic activity. 

Doping of CrF, with Fe(III) or Mg(II), would be expected to result in the formation of solid solutions. However, depending on the Fe(III) concentration, there are two distinct regions, a concentration range from 0 to 41% Fe(III) with the pyrochlore structure and a second range from 65 to 100% Fe with the HTB structure [93], The surface areas of the latter samples are about double those of the former. At about 65% Fe(III) a maximum in the acidity of the samples was observed, acidity decreases with further increase of Fe(III). These phases however, exhibit only average catalytic activity. 

Figure 3.21. The A2B2X7 pyrochlore structure. The 62X5 sublattice is a tetrahedral channelforming network of vertex-sharing BX octahedra. The channels are occupied by the anions of the A2X sublattice of vertex-sharing AX4 tetrahedra (not shown).

Li et al. reported pyrochlore-structured La2Zr2Q7 prepared by electrospinning technique from PVP/lanthanum nitrate/zirconium oxychloride as precursors, which was then calcinated at 1000 °C for 12 h, with a diameter of 100-500 run (Figure 56) (Li et al., 2006b). The fiber structure shows a low sintering ability, which could be attributed to the random stacking of fiber, resulting in a structure with low contact area between fibers. 

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