The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi 2O 3– MIIO–Nb 2O 5 ( M=Mg, Ni) systems

Abstract

The disordered structures and low temperature dielectric relaxation properties of Bi 1.667Mg 0.70Nb 1.52O 7 (BMN) and Bi 1.67Ni 0.75Nb 1.50O 7 (BNN) misplaced-displacive cubic pyrochlores found in the Bi 2O 3– M IIO–Nb 2O 5 ( 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 B 2O 6 octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′ A 2 tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16 d ( 1 2 , 1 2 , 1 2 ) to 96 h ( 1 2 , 1 2 - ε A , 1 2 + ε A ) while the O′ oxygen was shifted from position 8 b ( 3 8 , 3 8 , 3 8 ) to Wyckoff position 32 e ( 3 8 + ε o ′ , 3 8 + ε o ′ , 3 8 + ε o ′ ). The refined displacement magnitudes off the 16 d and 8 b 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.