(358d) Displacement Pathways in Bismuth Pyrochlores

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. Using density functional theory (DFT), quantum mechanical calculations were performed to examine several Bi-containing pyrochlores with the Fd-3m (No. 227) space group to determine the role of chemical substitutions on the local geometric and electronic structure. We considered six simple bismuth pyrochlores (Bi₂B⁴⁺₂O₇ with B⁴⁺ = Ti, Ru, Rh, Os, Ir, and Pt) and four pyrochlores with cation substitution selected based on availability of experimental IR and Raman spectroscopy (Bi_3/2M²⁺Nb_3/2O₇ and Bi_3/2M²⁺Ta_3/2O₇ with M²⁺ = Zn, Mg). We will present DFT results on the structural properties of the simple pyrochlores including equilibrium lattice constants, oxygen positional parameters, and atomic displacement patterns. For the pyrochlores with cation substitution, energetics related to cation ordering will be presented, as well as atomic displacement magnitudes and pathways. Where possible, our DFT results will be compared to available experimental studies to understand the role of the atomic substitutions on the material properties.