Systematic structural change in selected rare earth oxide pyrochlores as determined by wide-angle CBED and a comparison with the results of atomistic computer simulation

An unknown oxygen atom fractional co-ordinate characteristic of the pyrochlore structure type has been determined for selected rare earth zirconate and titanate pyrochlores via a systematic row wide-angle CBED technique and shown to vary systematically with rare earth ion size. In the case of the titanate pyrochlore Gd₂Ti₂O₇, the obtained results contrast with previously published X-ray results. Atomistic computer simulation is used to predict the value of the same parameter for a wide range of oxide pyrochlores. Comparison of calculated values with experimentally determined values shows that the general trends are correctly predicted although there appears to be systematic underestimation of both the observed values (by approximately 0.007) as well as their rate of change with rare earth ion size. Cation anti-site disorder is proposed as the origin of these discrepancies. (C) 2000 Academic Press.