(212d) Dielectric Properties of Bismuth Pyrochlores From First Principles Calculations
AIChE Annual Meeting
Tuesday, October 18, 2011 - 9:33am to 9:54am
There is considerable interest in pyrochlore systems (A2B2O7) for use in high-permittivity dielectrics, capacitors, and high-frequency filter applications. The properties of these materials can be tuned through substitutions on the A and B cation sites, resulting in an extensive parameter space. Better understanding of the role of the local atomic structure and dynamics on the macroscopic properties will enable rational design within the vast number of possibilities. For example, the dielectric relaxation in Bi-based pyrochlores were linked to local atomic hopping events and it has been proposed that a highly polarizable cation, chemical substitution, atomic displacement, and the resulting fractional occupancy of equivalent sites, are necessary for dielectric relaxation in pyrochlores. Based on density functional theory (DFT) calculations, we have shown Bi2Ti2O7 is expected to have ionic displacements. Being a bismuth based pyrochlore without chemical substitution of multi-valence cations, Bi2Ti2O7 is an ideal compound to de-convolute these features and better understand the nature of the dielectric relaxation in pyrochlores. Quantum mechanical calculations were performed on Bi2Ti2O7 and other pyrochlores (Bi1.5ZnNb1.5O7, Ca1.5Ti1.5NbO7) with the Fd-3m (No. 227) space group to determine the role of a highly polarizable cation, chemical substitution, and atomic displacement. We will present DFT results on the structural properties of Bi2Ti2O7 and other Bi-containing pyrochlores including lattice stability, atomic displacement patterns, atomic hopping events, and the static dielectric permittivity. Based on these results, predictions will be made regarding the likelihood of dielectric relaxation in Bi2Ti2O7 which further illustrate the need to synthesize the Bi2Ti2O7 pyrochlore without the ferroelectric Bi4Ti3O12 secondary phase, which is expected to mask the intrinsic dielectric behavior of the pyrochlore. Where possible, these predictions are compared with available experimental results.