Systematics of the pyrochlore structure type, ideal A2B2X6Y

The geometry of the ideal pyrochlore structure type (8A26B24X64Y, Fd3m) has been examined using a data base of 440 synthetic samples. Mean ionic radii of the cubic and octahedral cations together account for 95% of the variation in the cell edge. Nonempirical expressions for calculating the cell edge are also derived, using that the interatomic distances are functions of the cell edge and a single positional parameter, x, of the X anion. This alternative method is equally successful for calculating the cell edge, and additionally provides for the calculation of x. Comparison of calculated x values with 27 experimentally measured values indicates that the stoichiometry, x values, or bond length predicting radii are in error for 20% of these pyrochlores. For 21 observed x values judged to be most accurate, the calculated anion coordinates are within 0.009. In addition, calculated x values are given for 400 pyrochlores for which no values have been previously determined.