By calculating the value of the unknown oxygen positional parameter x for a large number of A JiB 4 j 0 7 pyrochlores, it was determined that increasing the radius of the A cation in a series of constant B increased the value of x, and that increasing the radius of the B cation in a series of constant A decreased the value of x. The region of pyrochlore formation has a range of x values from 0.404 to 0.432 and contains B0 6 polyhedra that closely approach Oil symmetry and A 0 8 polyhedra that are highly deformed. It was shown that the characteristics of the B0 6 polyhedra were the most important in determining whether or not a pyrochlore would form.

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