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

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