The pyrochlore structure

This structure (Fig. 7.4 (between pp. 268 and 269), forms a link between the defect fluorite structures we have been describing and a topic discussed at the end of this chapter. With its complete complement of atoms this (cubic) structure contains eight A2B2X6X in the unit cell, and there is only one variable parameter, the x parameter of the 48 X atoms in the position (x, ) etc. The structure may be 

The coordination groups of the A atoms for (i) and (ii) are illustrated in Fig. 6.12. In fact no examples of this structure are known with x as high as 0-375, the maximum observed value being 0-355, but x is less than 0-3125 in some cases (Cd2Nb207, X = 0-305), when the BXg octahedron is elongated. The relation of the limiting (unknown) structure (i) to fluorite is interesting because certain compounds show a transition from the pyrochlore structure to a defect fluorite structure. 

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

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 57 Part of the comer-sharing B2O6 network in the pyrochlore structure. The ions reside in the hexagonal channels...

Nickel Trifluoride Syntheses. As a dry solid, NiFa decomposed to produce a dark brown (nearly black) material which gave an X-ray powder diffraction pattern which showed it to have a rhombohedral variant of the pyrochlore structure (see below and Table 1) ... 

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