Local Structure of Displacively Disordered Pyrochlore Dielectrics

Local cation environments in the pyrochlore dielectrics A 2 B 2 O 6 O‘ = (Bi,M) 2 (M,Nb) 2 O 6 O‘ (M = Zn, Fe) were determined from the extended X-ray absorption fine structure. The A cations and O‘ ions are displaced from their ideal positions, yielding the effective (4 + 2 + 2) coordination for the A cations. The A-cation displacements are directed toward a pair of the framework O atoms; the specific displacement directions are determined by the nature of the A cations (i.e., Bi, Zn, Fe) and the local Nb/Zn (or Nb/Fe) B-site configurations. Local correlations among the atomic displacements and both [A 4 O‘] and [A 2 B 2 O] configurations provide near-ideal bond valence sums for all constituent species in these pyrochlore structures.