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. Bi 3+ ) and B small (6‐coordinated, e.g. Zn 2+ , Nb 5+ ) 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)