An unexpected crystal-chemical principle for the pyrochlore structure

Phase equilibrium studies of the Bi-Zn-Nb-O system show that pyrochlore does not form at chemical compositions predicted by the traditional formula for this crystal structure, A(2)B(2)O(6)O', where A denotes large (8-coordinated, e.g. Bi3+) and B small (6-coordinated, e.g. Zn2+, Nb5+) cation sites. Instead, pyrochlore forms only at compositions with excess B cations which, surprisingly, occupy the large A-cation sites. Reports of similar behavior in other pyrochlores suggest a previously unrecognized inherent structural feature (displacive disorder) which allows the formation of a large family of cubic pyrochlores with small B cations occupying up to ca. 25% of the large A-cation sites. Many pyrochlores can now be synthesized by deliberately combining large and small metal ions on the A sites. (c) Wiley-VCH Verlag GmbH & Co.