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

The disordered structures and low temperature dielectric relaxation properties of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) and Bi_1.67Ni_0.75Nb_1.50O₇ (BNN) misplaced-displacive cubic pyrochlores found in the Bi₂O₃-M^IIO-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 (frac(1, 2), frac(1, 2), frac(1, 2)) to 96 h (frac(1, 2), frac(1, 2) - ε{lunate}_A, frac(1, 2) + ε{lunate}_A) while the O′ oxygen was shifted from position 8b (frac(3, 8), frac(3, 8), frac(3, 8)) to Wyckoff position 32e (frac(3, 8) + ε{lunate}_o^′, frac(3, 8) + ε{lunate}_o^′, frac(3, 8) + ε{lunate}_o^′). 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.