The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3–MIIO–Nb2O5 (M=Mg, Ni) systems

The disordered structures and low temperature dielectric relaxation properties of Bi1.667Mg0.70Nb1.52O7 (BMN) and Bi1.67Ni0.75Nb1.50O7 (BNN) misplaced-displacive cubic pyrochlores found in the Bi2O3–MIIO–Nb2O5 (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B2O6 octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′A2 tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d   (12, 12, 12) to 96 h   (12, 12-εA, 12+εA) while the O′ oxygen was shifted from position 8b   (38, 38, 38) to Wyckoff position 32e   (38+εo′, 38+εo′, 38+εo′). The refined displacement magnitudes off the 16d and 8b sites for the A and O′ sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively.

The final displacively disordered average structure of Bi1.667Mg0.70Nb1.52O7 (BMN) projected along a close to 〈110〉 direction.Figure optionsDownload as PowerPoint slide