(212d) Dielectric Properties of Bismuth Pyrochlores From First Principles Calculations

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 (Bi_1.5ZnNb_1.5O₇, Ca_1.5Ti_1.5NbO₇) 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.