Oxides based on fluorite (MO₂) and pyrochlore (A₂B₂O₇) have potential uses in devices such as solid-oxide fuel cells and oxygen sensors, arising from their oxide ion conductivity.  The high temperature Bi₂O₃ polymorph, δ-Bi₂O₃, adopts the cubic defect fluorite structure, where 25% of the oxygen sites are vacant and the vacancies are distributed statistically.  Relative to fluorite, in a typical pyrochlore structure, A₂B₂O₇, only 12.5% of the oxygen sites are vacant, but the vacancies are ordered. 
It has been widely reported that the formation of the pyrochlore structure over the fluorite structure depends on the cation radius ratio, r_A/r_B, with the pyrochlore forming in the range 1.46-1.78 [1].  This ratio may be adjusted by aliovalent or isovalent doping of the cation sites, which can also induce anion disorder [2].  This is especially important in the pyrochlore structure, where the oxide ion vacancies are usually ordered, hence the creation of anion disorder should result in a greater oxide ion conductivity.
Our current research efforts are focused on probing the fluorite-pyrochlore structural phase boundary in (Bi,Zr)₄O₇ – Bi₂B₂O₇ (B = Ti, Sn, Hf) systems.  We have recently determined a fluorite – pyrochlore phase boundary in the (Bi,Zr)₄O₇ – Bi₂Ti₂O₇ system.  Here we present some results of our recent synthetic work and in-situ variable temperature powder XRD studies.

1.  M.A. Subramanian, G. Aravamundan & G.V. Subba Rao, (1983), Prog. Solid St. Chem. 15, 55-143
2.  B.J.Wuensch, K.W. Eberman, C. Heremans, E.M. Ku, P. Onnerud, E.M.E Yeo, S.M.Haile, J.K.Stalick, J.D. Jorgensen, (2000), Solid State Ionics, 129, 111-133

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