Defect pyrochlores

The quantum efficiency data for the defect pyrochlore Bag, -SrQ 5 2 6 iS Presente< i-n Figure 3 (10). It shows an indirect band gap at 3.4 eV with a "tail" extending to nearly 2.6 eV. The higher-energy transition at 4.4 eV shows some curvature of the data, and indeed, corresponds to a direct transition when replotted as (nhv) versus energy (10). 

Exactly as for MgSiOs, high pressure stabihzes a variety of compositions in the compact perovskite structure. Competition with other structures, such as Umenite, defect pyrochlore, and various polytypes of perovskite, as a function of ionic radius, pressure, and temperature has been reviewed. ... 

PbRu03 andBiScOs, which have defect pyrochlore stmctures at atmospheric pressure, adopt the denser perovskite structure at 90 and 70 kbars respectively. 

The nature of M7O12 phases involving tungsten and the lighter rare earths appears to depend substantially on the preparative temperature. At 1400°C such phases appear to be defective pyrochlores of formula R2(RW]/2)07-i (Chang and Phillips, 1964), whereas at lower temperatures (1250°C) lower symmetry phases, described as pseudo-cubic, have been obtained (McCarthy 1972 Trunov et al., 1968). 

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. 

Alpha-decay event damage. Pyrochlore and zirconolite are often metamict (Lumpkin and Ewing 1988). Sinclair and Ringwood (1981) investigated natural zirconolite from several localities. They found that zirconolite transforms to a defect fluorite stmcture after a cumulative dose of 8 x 10 a-decays/g. Specimens with more than 1.1 x 10 a-decays/g were generally metamict, although some crystalline domains were identified... 

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. 

M.P. Dijk, K.J. de Vries, A.J. Burggraaf, A.N. Cormack and C.R.A. Catlow, Defect structures and migration mechanisms in oxide pyrochlores. Solid State Ionics, 17 (1985) 159-167. 

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